Tec hniques of Water-Resources Investigations of the United States Geological Survey

Chapter EI 0 APPLICATION OF BOREHOLE GEOPHYSICS TO WATER-RESOURCESINVESTIGATIONS

By W. Scott Keys and L. M. MacCary

Book2 COLLECTION OF ENVIRONMENTAL DATA DEPARTMENT OF THE INTERIOR

DONALD PAUL HODEL, Secretary

U.S. GEOLOGICAL SURVEY

Dallas L. Peck, Director

First printing 1971

Second printing 1976

Third printing 1981

Fourth printing 1985

UNITED STATES GOVERNMENT PRINTING OFFICE: 1971

For sole by the Branch of Distribution, U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 PREFACE

The series of manuals on techniques describes procedures for planning and executing specialized work in water-resources investigations. The material is grouped under major subject headings called “Books” and further subdivided into sections and chapters. Section E of Book 2 is on subsurface geophysical methods. Provisional drafts of chapt,ers are distributed to field offices of the U.S. Geological Survey for their use. These drafts are subject to revision because of experience in use or becauseof advancement in knolvledge, techniques, or equipment. After the technique described in a chapter is sufficiently developed, the chapter is published and is sold by the U.S. Geological Survey, 1200 South Esds Street, A.rlington, VA 22202 (authorized agent of Superintendent of Documents, Government Printing Office).

l CONTENTS

Page Page Preface...... III Resistivity logging...... ____.______36 Glossary...... VII I Normal devices...... 38 Abstract...... 1 Principles and applications...... 38 Introduction...... -.... 1 Instrumentation ...... 44 Background...... -.... 1 Calibration and standardization ...... 46 Purpose and scope...... 2 Bed-thickness effects ...... 47 Why log?...... ~.... 2 Lateral devices...... 49 Limitations...... 3 Principles and applications...... 49 Lithologic parameters ...... 4 Instrumentation ...... 49 Resistivity ...... ~.... 5 Bed-thickness effects ...... 51 Formation factor...... 6 Wall-resistivity devices ...... 51 Permeability...... -.... 6 Focused devices ...... 53 Porosity...... 7 Induction device ...... 55 Specific yield...... 7 Principles and applications ._.______56 Grain size ...... 8 Instrumentation...... 57 Fluid parameters...... 8 Calibration and standardization .______58 Fluid conductivity ...... ~.... 8 Radius of investigation...... 58 Fluid temperature ...... 10 Extraneous effects...... 58 Fluid movement ...... ~.... 10 Nuclear logging ...... 58 Moisture content ...... 10 Fundamentals of nuclear geophysics ...... 59 Log interpretation...... 10 Characteristics of radiation ...... 59 Qualitative interpretation...... 10 Radiation statistics ...... __.___._59 Quantitative interpretation ...... 11 Radiation detection...... 61 Log calibration...... 12 Quantitative interpretation...... 61 Standardizaton and log accuracy ...... 13 Natural-gamma logging...... 64 Composite interpretation...... ~.... 15 Principles and applications...... 64 Geometric effects...... 18 Instrumentation ...... 66 Radius of investigation...... 18 Calibration and standardization _.______66 Bed-thickness effects...... 18 Radius of investigation...... ~.... 67 Borehole effects...... 19 Extraneous effects...... 67 Hole-diameter effects ...... ~.... 19 Gamma spectrometry...... 67 Casing effects...... -.... 19 Gamma-gamma logging ...... 70 Drilling effects...... 20 Principles and applications...... 70 Logging equipment ...... 21 Instrumentation ...... -...... - .... 72 Spontaneous-potential logging...... -.... 23 Calibration and standardization...... 72 Principles and applications ...... 24 Radius of investigation...... 73 Instrumentation...... -.... 28 Extraneous effects...... 73 Calibration and standardization ...... 28 Neutron logging ...... 74 Radius of investigation ...... 28 Principles and applications...... 74 Extraneous effects ...... 29 Instrumentation ...... 81 Resistance logging ...... 30 Calibration and standardization...... 82 Principles and applications...... 31 Radius of investigation...... 83 Instrumentation...... -.... 35 Extraneous effects ...... 84 Calibration and standardization ...... 35 The use of radioisotopes in well logging ______.___ 86 Radius of investigation...... -.... 35 Acoustic logging ...... _...... _.._....._.... 88 Extraneous effects...... 35 Principles and applications...... 88 V VI CONTENTS

Page Page Acoustic logging - Continued Temperature logging -- Continued Instrumentation _ 91 Calibration and extraneous effects _____._.___.._._..._105 Calibration and standardization _ 92 Fluid-conductivity_logging_106 Radius of investigation ._92 92 Principles and applications ______.______Extraneous effects . 93 93 Instrumentation ._..____.____._.._...... ~.... Caliper logging ______.___.______...... 94 94 Calibration and extraneous effects ______...... Principles and applications _ 94 Fluid-movement logging 109 Instrumentation ._.______.._._.._...... 97 97 Principles and applications .__._____.______109 Calibration and standardization ______.__...9898 Instrumentation ______._._._...... 114 Radius of investigation and extraneous Calibration and extraneous effects...... 118 effects ...... ______.__...... 9999 Caiing logs ..______.__.._...... 118 Temperature logging .___.______...... 9999 Principles and applications _ 99 Index...... 125 Instrumentation ______.____._...... ____104_

ILLUSTRATIONS

Page 1. Approximate resistivity of granular aquifers versus porosity for several water salinities ...... 6 2. Electrically equivalent concentrations of a sodium chloride solution as a function of resistivity or conductivity and temperature ...... 9 3. Neutron log and a plot of porosity measured in the laboratory ...... 14 4. Hypothetical response of nuclear logs in a mixture of quartz sand, clay, and water...... 15 5. The relationship of six different geophysical logs to lithology ...... 16 6. Lithology and corrected porosity from composite interpretation of gamma-gamma and neutron logs ...... 1’7 7. Schematic block diagram of geophysical well-logging equipment ...... 22 8. Geophysical logs, showing some common equipment problems ...... 23 9. Spontaneous-potential junctions and currents ...... 25 10. Electric log of oil-test well in western Kentucky ...... 26 11. Spontaneous-potential measuring circuit and the electrical equivalent ...... 28 12. Spontaneous-potential current flow in highly resistive beds ...... 29 13. Spontaneous-potential noise from electrolytic action of steel cable ...... 29 14. Geophysical logs of granite in drill hole CX ill...... 30 15. Conventional simultaneous single-electrode resistance and spontaneous-potential logging system ...... 31 16. Response of single-point log to lithology ...... 33 1’7. Differential resistance system, showing current distribution ...... 34 18. Examples of differential and conventional single-point resistance logs of fractured crystalline rocks ...... 34 19. Identification of fractures from caliper and differential single-point resistance logs...... 34 20. Response of laterolog and differential single-point logs to changes in hole diameter...... 36 21. Electrode arrangement for resistivity measurements in cores and in boreholes ...... 37 22. Electrode arrangements for the 16- and 64-inch normals ...... 39 23. Relative radii of investigation of point-resistance and normal devices ...... 40 24. Graph showing resistivity and specific conductance, and table of formation factors ...... 42 25. Relation between concentration, resistivity, and specific conductance and between specific conductance and dissolved solids ...... 43 26. Area1 plot of formation factors and electric log of a well penetrating the Wilcox Group...... 44 27. Electric log of water well, graph of permeability and F, and table of formation factors...... 45 28. Normal-device calibrator ...... 46 29. Response of a normal device opposite beds of various thicknesses ...... 48 CONTENTS l Page 30. Electrode arrangement for lateral device ...... 49 31. Response of a lateral device opposite beds of various thicknesses ...... 50 32. Electric log of well drilled in limestone ...... 52 33. Micronormal and microlateral device ...... 53 34. Comparison of unfocused and focused devices...... 53 35. Guard-logging system...... 54 36. Laterolog system, electrode arrangements, and comparison of current distribution for normal device and laterolog...... 55 37. Microlaterolog device ...... 56 38. Induction-logging system...... 56 39. Induction-electric log of a test hole ...... 67 40. Time constant and its effect on logging parameters ...... 60 41. Nuclear-logging parameters ...... 62 42. Radius of investigation of nuclear logs ...... 63 43. Gamma spectra-potassium, uranium, and thorium ...... 68 44. Pulse-energy discrimination ...... 69 45. The equipment, principles, and interpretation of gamma-gamma logs ...... 71 46. Gamma-gamma logs, used to interpret position of grout behind casing, and caliper logs, used to select depth for grouting, and to estimate volume required...... 72 47. Examples of logs made by commercial oil-well logging equipment and small water-well logging equipment ...... 74 48. The equipment, principles, and interpretation of neutron logs ...... 76 49. Correlation with gamma and neutron-N logs ...... 78 50. Interpretation of a neutron log and a gamma log indicate water is perched on a clay bed ...... 79 51. Location of the brine-fresh-water interface from geophysical logs ...... 80 52. Neutron logs, Yukon Services well, Anchorage, Alaska ...... 81 53. Neutron logs made with different spacing between source and detector and under different hole conditions ...... 82 A comparison of neutron logs made in the same hole uncased and through PVC and steel casing ...... 84 Neutron and reversed gamma-gamma logs, made through drill rods, and a single-point resistance log ...... 85 56. Uncompensated acoustic-velocity probes ...... 88 57. The principles and equipment for making borehole-adjusted acoustic-velocity logs...... 89 58. Chart based on time-average equation for the determination of porosity from acoustic velocity ...... 90 59. Three different porosity-related logs ...... 93 60. The effect of drilling technique on hole diameter and the effect of hole diameter on gamma- gamma logs ...... 95 61. Correlation of fracture zones between rotary hole T-5 and core hole C-l from caliper logs...... 96 62. Temperature logs ...... 100 63. Temperature logs during artificial recharge ...... 102 64. Relation of viscosity of water to temperature ...... 103 65. Relation of specific gravity of water to temperature...... 103 66. Principles, equipment, and examples of differential-temperature logging ...... 104 67. Field- and reconstructed-temperature logs ...... 105 68. Relation of electrical conductivity to temperature in dilute solutions of potassium chloride and sodium chloride ...... 107 69. A brine-brackish-water interface, shown by fluid-resistivity and neutron logs...... 108 70. Continuous flowmeter logs, used to locate the zone of flow and to determine stationary time-drive measurements across that interval...... 110 71. The movement of slugs of radioactive tracer under injection conditions and injectivity profiles and their relation to a caliper log ...... 111 72. Radioactive-tracejector logs ...... 112 73. Double-detector radioactive-tracejector and logs of iodine-131 injection...... 116 74. Brine ejector-detector probe and logs of three different salts, used’ as tracers...... 117 75. A typical CCL log, made with an event marker, and continuous logs, made at two different potential scales, which were used to locate perforations in casing ...... 119 VIII CONTENTS

TABLES

Page 1. Summary of log applications...... 6 2. Hydrologic applicability of electric logs ...... 24 3. Electric log cross reference ...... 39 4. Range of radioactivity of selected sedimentary rocks ...... 66 5. A comparison of neutron logging techniques ...... ~...... 75 6. Data on common nuclides ...... 80

GLOSSARY

[Commonly used symbols in borehole geophysics]

A Mass number. fpm Feet per minute. A Lower current electrode. G Geometric factor. AM Electrode spacing, normal device. g/cc Grams per cubic centimeter. A0 Electrode spacing, lateral device. mm Gallons per minute. API GR Unit American Petroleum Institute h (or e) Bed thickness. gamma-ray unit. h Mud cake thickness. API N Unit American Petroleum Institute H”,” Unit of frequency, one cycle neutron unit. per second. L3 Upper current electrode. HI Hydrogen index, fresh water C Capacitor. equals 1. CCL Casing-collar locator. Kev Thousand electron volts. CPS Counts per second. kHz Kilohertz. cpm Counts per minute. kn Kilohms [thousand ohms]. D, Or P,, Bulk density. LL Laterolog. 4 Borehole diameter. x Wave length. 4 Electrically equivalent diameter M Potential electrode, normal device. (lateral depth) of the Mev Million electron volts. invaded zone. m Cementation factor. D, or pf Density of fluid. ma Milliampere. D, 01 &na Grain density or matrix density. mc Millicurie. At Interval transit time (of acoustic Meg-ohms Million ohms. wave). mg/l Milligrams per liter. E (or emf) Electromotive force. mv Millivolt. EC Electrochemical component of Mr/hr Milliroentgens per hour. the SP. aa Microampere.

Ek Electrokinetic component of pg Ra-eq/ton Microgram radium-equivalent the SP. per ton. ES Electrical Survey-SP, 16” normal, Pmhos/cm Micromhos per centimeter. 64” normal and 18’8” lateral. N Resistivity reference electrode. E8 Shale potential. n Symbol for the neutron. El4 Environmental unit of neutron n Frequency, in hertz. deflection. ohm-m (or 0m) Unit of resistivity or specific e Symbol for the oribital electron. resistance. ev Electron volts. 6 Porosity. F Formation-resistivity factor, P Symbol for the proton. equals RJR,. mm Parts per million. Field-formation-resistivity factor. PR Point resistance. ‘CONTENTS IX

PVC casing Polyvinyl chloride casing. Rt True formation resistivity. r Resistance. Rto Formation water resistivity. R Resistivity of flushed zone. rf Equivalent resistance of formation. SF Spontaneous potential. r,h Resistance of shale. SSP Static spontaneous potential. r 88 Resistance of sandstone. Resistance of mud. &D Water saturation. Resistivity. T Tortuosity. Resistor. T Temperature. Apparent resistivity. TX Half life. Time constant. Invaded zone resistivity. tc V Acoustic velocity, in feet Mud resistivity. per second. Mud cake resistivity. Vf Velocity of sound in fluid, Mud filtrate resistivity. in feet per second. Resistivity of formation lOO- vm Velocity of sound in matrix, percent saturated with water in feet per second. of resistivity R,. V, Velocity of signal in rock, R, Resistivity of surrounding in feet per second. formations. VOM Volts and ohms test meter. R,h Shale resistivity. z Atomic number.

Commonly used symbols in borehol geophysics APPLICATION OF BOREHOLE GEOPHYSICS TO WATER-RESOURCES INVESTIGATIONS

By W. Scott Keys and L. M. MacCary

Abstract In the United States the first geophysical This manual is intended to be a guide for hydrol­ well logs may have been plotted in the 1890’s ogists using borehole geophysics in ground-water from temperature measurements made by studies. The emphasis is on the application and in­ W. B. Hallock (1897). C. E. Van Orstrand terpretation of geophysical well logs, and not on (1918) of the U.S. Geological Survey the operation of a logger. It describes in detail those described downhole-temperature equipment logging techniques that have been utilized within the Water Resources Division of the U.S. Geological with a sensitivity of O.Ol”C, which was used Survey, and those used in petroleum investigations to make “depth temperature curves.” These that have potential application to hydrologic prob­ curves showed anomalies caused by escaping lems. Most of the logs described can be made by gas and flowing water, and Van Orstrand commercial logging service companies, and many can speculated that temperature curves might be made with small water-well loggers. The general “afford a means of determining the relative principles of each technique and the rules of log interpretation are the same, regardless of differences water content of rocks in situ.” in instrumentation. Geophysical well logs can be The first resistivity curves were run in interpreted to determine the lithology, geometry, 1927 in an oil field in France (Schlumberger resistivity, formation factor, bulk density, porosity, and Schlumberger, 1929). These logs were permeability, moisture content, and specific yield of made by manually plotting the deflections water-bearing rocks, and to define the source, move­ of a galvanometer that responded to the ment, and chemical and physical characteristics of ground water. Numerous examples of logs are used resistivity of the rocks and their contained to illustrate applications and interpretation in vari­ fluids. A good example of the state of the ous ground-water environments. The interrelations art about 1927 is shown in the photographs between various types of logs are emphasized, and in the paper by Schlumberger and Schlum­ the following aspects are described for each of the important logging techniques : Principles and appli­ berger (1929). The method was called “elec­ cations, instrumentation, calibration and standardi­ trical coring” by the Schlumberger brothers, zation, radius of investigation, and extraneous because it gave lithologic information about effects. the rocks penetrated by wells. About 1931 Schlumberger engineers discovered the exis­ Introduction tence of a natural electric potential also re­ Background lated to lithology while they were measuring A log is defined by Webster as “a record resistivity in Rumania (Schlumberger and of sequential data* * * .” Geophysical well others, 1934). A log of these potentials was logging, also called borehole geophysics, in­ called the “porosity log” by Schlumberger. cludes all techniques of lowering sensing Deflections in potential were logged on either devices in a borehole and recording some side of a zero level, with positive potentials physical parameter that may be interpreted deflecting to the right and negative ones to in terms of the characteristics of the rocks, the left. Within a few years logging of spon­ the fluids contained in the rocks, and the taneous potential and resistivity was in gen­ construction of the well. eral use in oil fields worldwide, the methods B 1 2 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS being termed “electric logging.” As new geo- used to’ illustrate applications and interpre- 0 physical logging methods were discovered, tation of logs in various ground-water en­ they were for a time categorized under the vironments. Note, however, that these general name of electric logging, even though examples may not necessarily apply to the methods did not involve electrical mea­ a similar geohydrologic environment else- surements. By common agreement, logging where. The interrelation among various service companies today use the term “elec­ types of logs is emphasized in this manual. tric log” only for those geophysical methods Basic principles common to many types of which measure the spontaneous electric po­ logs are described only in the introductory tentials and electrical resistivities of natural sections, and the index provides cross-refer­ rock materials penetrated by the drill. encing information. In the years since the discoveries by the Schlumberger brothers, the following types of sensors have also been used for geophysi­ Why log? cal investigations in wells: Visual, mechani­ Exploratory drill holes or wells are the cal, nuclear, acoustic, magnetic, and thermal. only means of direct access to the subsurface. The first comprehensive description of “Sub- Drilling is an expensive means of access to surface Geophysical Methods in Ground- the lithosphere, and sampling of the rocks Water Hydrology” was written by P. H. and fluids penetrated and geophysical well Jones and H. E. Skibitzke in 1956, yet the logging are the only ways information can sophistication and use of geophysical logging be derived from these holes. Valid well logs, to solve problems of ground-water hydrology correctly interpreted, can be used to reduce is still many years behind that of the pe­ future drilling costs by guiding the location, troleum industry. proper drilling, and construction of test holes and production or disposal wells. Well log­ ging also enables the vertical and horizontal Purpose and scope extrapolation of data derived from drill holes. The purpose of this manual is to furnish Geophysical logs can be interpreted to de­ a guide to planning and implementing a pro- termine the lithology, geometry, resistivity, gram of borehole geophysics applicable to formation-resistivity factor, bulk density, ground-water studies. The manual is not in- porosity, permeability, moisture content, and tended to provide operational instructions specific yield of water-bearing rocks, and for borehole loggers; rather, the emphasis to define the source, movement, and chemical is on application and interpretation of geo­ and physical characteristics of water. Quan­ physical well logs. It describes in detail those titative interpretation of logs will provide logging techniques that have been utilized numerical values for some of the rock char­ within the Water Resources Division of the acteristics necessary to design analog or digi­ U.S. Geological Survey and those techniques tal models of ground-water systems. Log data used in petroleum work that have potential aids in the testing and economic development application to hydrologic problems. Most of of ground-water supplies and of recharge the logs described in detail can be made by and disposal systems and can be of consider- commercial logging service companies, and able value in the design and interpretation many can be made with small water-well of surface geophysical surveys. Stallman loggers. The general principles and rules of (1967) pointed out that if the following pre- log interpretation are the same, regardless test information is not obtained, failure of of differences in instrumentation. Basic prin­ pumping tests is invited : Hydraulic condi­ ciples of logging instrumentation are tions along the well bore ; storage character­ described because they are essential back- istics of the aquifer; and depth to, and ground to planning a logging program and thickness of, the aquifer being tested, as well interpreting logs. Numerous examples are as changes in either within the area of the APPLICATION OF BOREHOLE GEOPHYSICS 3 test. He also suggested that changes in trans­ logs in each new geologic environment. Side- missivity should be mapped. Estimates of wall-sampling techniques are available for pertinent hydraulic properties of the aquifer poorly consolidated sediments and can be util­ can be provided by borehole, geophysical ized after logs have been run, to provide studies. the most representative samples (Morrison, ,Geophysical well logging can provide con­ 1969). Sidewall samples can also be taken tinuous objective records with values that are in hard rocks by commercial logging service consistent from well to well and from time companies ; however, they are relatively ex- to time, if the equipment is properly cali­ pensive. One well, adequately sampled and brated and standardized. In contrast, the logged, can serve as a guide for the hori­ widely used geologist’s or driller’s log of cut­ zontal and vertical extrapolation of data tings is subjective, greatly dependent upon through borehole geophysics. Furthermore, personal skills and terminology, and is limited well logging provides the only means for to the characteristics being sought. Geo­ obtaining information from existing wells physical logs can be reinterpreted in a post- for which there is no data and from wells mortem investigation of some geologic or where casing prevents sampling. hydrologic factor that was not considered Geophysical logs can be digitized in the while the hole was being drilled. Serendipity field or office by commercial service com­ -the gift of finding agreeable or valuable panies and are then amenable to computer things accidentally-has resulted in the dis­ analysis and the collation of many logs. They covery of uranium, phosphate, potash, and can be very economically stored on, and re­ other minerals from the interpretation of trieved from, magnetic tape. Digitized geo­ well logs. Each year many more wells are physical logs of oil wells are transmitted by drilled for water than for petroleum. Al­ radio and telephone for interpretation by though most of the water wells are shallow, log analysts in response to the need for rapid each is a valuable sample of the geologic answers at the well site. environment, and logs of these holes can aid Logging techniques also permit time-lapse in the definition and development of water measurements to observe changes in a dy­ supplies, sand and gravel deposits, other non- namic system. Changes in both fluid and metallic and metallic mineral deposits, pe­ rock characteristics and well construction troleum, and waste storage or disposal and caused by pumping or injection can be deter- artificial-recharge sites and can provide the mined by periodic logging. Radiation logs engineering data necessary for construction. and, under some conditions, acoustic logs are In contrast to uninterrupted geophysical unique in providing data on aquifers through logs, samples of rock or fluid almost never casing. This permits logging at any time provide continuous data. Even if a hole is during or after the reestablishment of native entirely cored, with loo-percent recovery, fluids behind the pipe. laboratory analysis of the core involves the The graphic presentation of geophysical selection of point samples. Continuous coring logs allows rapid visual interpretation and and subsequent analysis of enough samples comparison at the well site. Decisions on to be statistically meaningful costs much where to set screen and on testing proce­ more than most geophysical-logging pro- dures can be made immediately, rather than grams. In addition, the volume of material after time-consuming sample study or lab- investigated by most logging sondes may be oratory analyses. more than 100 times as large as the volume of most core samples extracted from the Limitations hole. Although geophysical logging should part­ The single most important factor that has ly supplant routine sampling of every drill limited the use of geophysical logging in hole, some samples, properly taken and ana­ ground-water hydrology is the cost in rela­ lyzed, are essential to the interpretation of tion to the unit value of product. In 1966 oil- 4 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS well logging costs in Canada averaged $0.75 logs do not have a unique response-that is, per foot compared with drilling costs of a certain recorded deflection may be due to about $13 per foot (Canadian Well Logging one lithology in one area and due to another Society, 1967). This represents a drastic lithology elsewhere. For this reason, back- increase from 1952 logging costs of $0.10 ground information and experience in each per foot. During the same period, drilling geologic environment is essential to guide costs decreased from $15 per foot. The log­ log interpretation. Interpretation must also ging cost increase is due mostly to an in- be based on sound knowledge of the theory crease in the number of types of logs run of each logging technique. and in the number of runs per well. Com­ There is no text that gives unambiguous parable figures are not available for water rules for log interpretation. For example, wells in the United States; however, it is clay does not always exhibit a higher nat- known that logging costs are considerably Ural-gamma intensity than adjacent sands. lower and that the percentage of water wells Furthermore, extraneous effects, such as logged increases rapidly with depth and cost borehole parameters, must be recognized and of the well. If logging is to be widely used in corrected, generally by empirical methods. ground-water hydrology, a constant effort These limitations have not prevented the will have to be made to keep its cost low in nearly universal application of well logging relation to the cost of drilling and develop­ to petroleum exploration; almost every well ment. drilled for oil anywhere in the world is In order to improve the cost-benefit ratio logged. Ground-water hydrologists and pe­ of logging, interpretation must be provided troleum geologists are seeking the same basic along with logs. Inasmuch as few water-well information on the occurrence, character, drillers, hydraulic engineers, or hydrologists and movement of fluids underground in order have the experience necessary to interpret to guide subsurface fluid withdrawal or in­ geophysical logs, p,ersonnel training is essen­ jection. tial. Logger operation is another facet of Following are steps essential to obtaining training that is greatly needed. Although the the maximum benefit from a geophysical small loggers have become more dependable well-logging program : in recent years, they have also become more 1. Plan the logging program on the basis of complex because of the addition of new tools. the data needed. (See table 1.) The trend toward complexity will continue as 2. Carry out drilling operations in a manner more sophisticated logging techniques be- that produces the most uniform hole come available. Therefore, in order to provide and the least disturbance of the en­ accurate, dependable logging, operators must vironment. be properly trained in both equipment oper­ 3. Take representative formation and water ation and maintenance, and in log interpre­ samples where necessary, using logs tation. as a guide, if possible. One fundamental problem in the applica­ 4. Insist on quality logs made with calibrat­ tion of geophysical logs is that the interpre­ ed and standardized equipment. tation of many logs is more of an art than 5. Logs should be interpreted collectively, on a science. The numerous environmental fac­ the basis of a thorough understanding tors causing log response are difficult to ana­ of the principles and limitations of lyze quantitatively. Even when theoretically each type of logging technique, and derived equations are available, empirical some knowledge of the geohydrologic data are needed to determine the unknowns environment under study. in the equation; therefore, direct empirical methods may be more reliable. Empirical Lithologic Parameters methods are based on the relationship of log response to samples, core, models, or experi­ Most characteristics of rocks, the fluids mentally derived equations. Most geophysical they contain, and the borehole have an effect APPLICATION OF BOREHOLE GEOPHYSICS 5 on the response of geophysical-logging in­ would also be necessary in hydrology if we struments. Table 1 i,s a simplified tabulation made resistivity logs above the water table. of parameters that can be measured directly or interpreted from commonly available geo­ Table 1. - Summary of log applications physical logs. The most widespread uses of logs in ground-water hydrology at the pres­ Required information on the prop- Widely available logging erties of rocks. fluid. wells, or techniques which might be ent time are to define the lithology and geom­ the ground-water system utilized etry of aquifer systems and to estimate the Lithology and stratigraphic corre- Electric. sonic, or caliper quality of contained water. In order to prop­ lation of aquifers and associated logs made in open holes. rocks. Nuclear logs made in open erly interpret one log or a set of geophysical or cased holes. logs, the basic principles governing the re­ Total porosity or bulk density...... Calibrated sonic logs in open holes. calibrated neutron or sponse of logging devices to characteristics gamma-gamma logs in of the rock matrix must be understood, as open or cased boles. well as the interrelation among lithologic Effective porosity or true resistivity..Calibrated long-normal resistivity logs. parameters. The lithologic parameters de- Clay or shale content ...... _...... Gamma logs. scribed in this section are those of principal Permeability ...... _...... No direct measurement by logging. May be related to interest to ground-water hydrologists, and porosity. injectivity, sonic they will be used throughout this technical amplitude. Secondary permeability-fractures. Caliper. sonic, or borehole manual without further definition. solution openings. televiewer or television logs. Specific yield of unconfined aquifers..Calibrated neutron logs. Grain size ...... Possible relation to forma­ tion factor derived from Resistivity electric logs. Location of water level or saturated Electric. temperature or fluid The electrical resistivity of a rock depends *ones. conductivity in open hole or inside casing. Neutron on physical properties of the rock and the or gamma-gamma logs in fluids it contains. Most sedimentary rocks open hole or outside casing. are composed of particles having a very high Moisture content ...... Calibrated neutron logs. Infiltration ...... Time-interval neutron logs resistance to the flow of electrical current. under special circum­ When these rocks are saturated, the water stances or radioactive tracers. filling the pore spaces is relatively conductive Direction, velocity, and path of Single-well tracer techniques compared with the rock particles or ma­ ground-water flow. - point dilution and single-well pulse. Multiwell trix. The resistivity of a rock, therefore, is a tracer techniques. function of the amount of fluid contained in Dispersion. dilution, and movement Fluid conductivity and tem­ of waste. perature logs, gamma logs the pore spaces, the salinity of that fluid, for some radioactive and how the pore spaces are interconnected. wastes, fluid sampler. Source ,nd movement of water in Injectivity profile. Flowmeter The main factor in pore geometry is probably a well. or tracer logging during tortuosity, which is defined as the square of pumping or injection. Temperature logs. the ratio of the actual path length of the Chemical and physical character­ Calibrated fluid conductivity current to the length of the sample through istics of water. including salinity. and temperature in the which it flows. Resistivity is measured in temperature. density, and viscosity. well. Neutron chloride logging outside casing. ohm-meters, which is the resistance of a cube Multielectrode resistivity. of material that is one meter on a side, and Determining construction of exist­ ~a~~agamma, caliper, ing wells. diameter and position collar. and perforation the resistance of that cube, in ohms, is nu­ of casing, perforations. screens. locator, borehole television. merically equal to the resistivity, in ohm- Guide to screen setting...% ...... _...._....All. logs providing data on the lithology, water-bearing meters. characteristics. and corre­ The resistivity of a rock that is loo-per- lation and thickness of aquifers. cent saturated with formation water is R,, Cementing ...... Caliper, temperature. gamma- and the resistivity of the water is R,. True gamma. Acoustic for cement bond. resistivity, Rt, is distinguished from R,, be- Casing corrosion ..._...._._...... Under some conditions cause correction for partial saturation by caliper. or collar locator. Casing leaks and (or) plugged Tracer and flowmeter. hydrocarbons is necessary in petroleum ex­ SCITC?“. ploration. Correction for partial saturation .

6 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

Using R, from logs and F, it is simple to petroleum or ground-water geologist ; yet, calculate R,, which is a function of the tem­ there is no log- that can directly measure perature and quality of water in an aquifer: permeability. Intrinsic permeability is a mea­ sure of the relative ease with which a porous R+ medium can transmit a liquid under a poten­ tial gradient. Hydraulic conductivity is the quantity of water that will flow through a Formation factor unit cross-section area of rock per unit time, The formation resistivity factor (F) is de- at a specified temperature under a unit hy­ fined as the ratio of the electrical resistivity draulic gradient. Permeability has been re­ of a rock loo-percent saturated with water lated to the response of a number of types to the resistivity of the water with which it of geophysical logs and to other parameters is saturated, F = RJR, (Archie, 1942). Be- derived from logs. For example, an empirical cause most rock grains have a very high relationship between natural-gamma-log in- resistance relative to water, the formation tensity and permeability has been demon­ factor is always greater than 1. Formation strated by Rabe (1957) and Gaur and Singh factor is roughly related to effective poros­ (1965) for local areas. The relationship be- ity in poorly consolidated rocks as follows: tween permeability and porosity-sensitive F = 0.62@-*.15.Guyod (1966) showed how po­ logs, however, is generally dependent on de­ rosity and resistivity data from logs can be termining the porosity and permeability of a used to determine the salinity of the inter­ number of samples, as an intermediate step. stitial water (fig. 1). The figure is based on An exalmple of this type of study was re- average values for clay-free granular aqui­ ported by Bredehoeft (1964)) in which an fers and is, therefore, approximate when ap­ empirical relation between porosity and per­ plied to a particular aquifer. Thus, if all other meability from core analyses was used to factors are equal (fig. 1) , the higher the po­ estimate permeability from acoustic and neu­ rosity and salinity, the lower the aquifer tron logs. The relationship between perme­ resistivity. ability and formation-resistivity factor may be even more important in future work (Al- > ger, 1966). Jones and Buford (1951) showed Permeability that both permeability and formation-resis­ Permeability or hydraulic conductivity are tivity factor increase with grain size. Alger the lithologic parameters most needed by the used sorne of their data to show that the per­ meability of fresh-water sands increases with

3000 the formation factor, all other factors being the same, and he suggested the use of local data to establish empirical relations and guide log interpretation. Croft (1971) tested Al­ ger’s plot of permeability versus formation factor and found that permeabilities obtained from electric logs were in reasonable agree­ ment with those obtained by other methods. When attempting to derive permeability from logs, one must remember that perme­ ability is not only related to porosity, but also to the grain size distribution, shape, and orientation and to the type and distribution 1 of cementation. Another factor to be consid­ 2 5 IO 20 50 POROSITY. !3’ W ered when attempting to relate permeability

Figure 1. - Approximate resistivity of granular aquifers versus to logs is the possible error in laboratory porosity for several water salinities. From Guyed (1966). measurements of permeability of cores. In- , r APPLICATION OF BOREHOLE GEOPHYSICS 7 situ permeability measurements are needed ter in the crystal structure of minerals, such to evaluate core permeabilities: For these as gypsum, should not be considered to be reasons, empirical relationships between logs part of the total porosity. To further com­ and permeability should be used only in a plicate the interpretation of porosity from relatively consistent geohydrologic environ­ geophysical logs, there are several different ment and only after the relationships have laboratory techniques for measuring poros­ been clearly established. ity, all of which may give different results (Jenkins, R. J., 1966). Therefore, before any attempt is made to use laboratory values for Porosity calibrating log response, one must determine Porosity ($) may be derived from gamma- the kind of porosity that was measured on gamma, neutron, acoustic-velocity and elec­ the samples and how it was done. tric logs. Because porosity is related to the Porosity is also divided into primary po­ amount of water stored in rocks and has a rosity, or intergranular pore space, and local relation to permeability, it is an im­ secondary porosity. Primary porosity is char­ portant hydrologic parameter. Porosity is acteristic of detrital rocks, related to the simply defined as the ratio of the void volume original size and shape of the grains and of a porous medium to the total volume, and reduced by compaction and cementation. Pri­ is generally expressed as a percentage. This mary porosity tends to be homogeneously simple definition is complicated in much of distributed. Secondary pore spaces, which are the literature on well logging by use of the formed after the rock is deposited, tend to word “porosity,” without definition or with- be nonhomogeneous. Secondary porosity is out the modifying terms “total” or “effec­ caused by chemical solution, by fracturing tive.” Further, total porosity and effective and jointing, and by chemical alteration, porosity are not easily distinguished in many such as dolomitization, and the alteration of rocks. Because various types of logs respond anhydrite to gypsum. The acoustic-velocity differently to the two types of porosity, an log and some resistivity logs can provide introductory discussion is necessary. erroneous porosities where the voids are non- Total porosity, which will be called simply homogeneously distributed. The neutron probe porosity in this manual, includes all the pore tends to average the water-filled pore spaces spaces in a material, regardless of whether within the volume investigated. Primary po­ they are interconnected or not. Effective po­ rosity is generally related to a lithologic rosity encompasses only the pore spaces that character of the sediment, which may be are interconnected and, therefore, effective in interpreted from certain geophysical logs. In transmitting fluids. In most detrital sedimen­ contrast, secondary solution openings may be tary rocks, effective and total porosity are distributed within a limestone, despite its nearly the same; however, in volcanic rocks lithologic differences. To locate these voids and in many carbonate rocks, isolated voids within the rock, one must rely on direct are common. Unconnected pore spaces filled detection. with water do not lower the resistivity mea­ sured by an electric log, whereas connected Specific yield spaces do. An additional factor to consider in interpreting porosity from logs is intro­ Specific yield is very important in ground- duced by water held between layers in the water studies because, when gravity drainage clay minerals. Chemically bound water can- is complete, it is a means of comparing the not be distinguished from free water on a various capacities of different materials to neutron log. In some logging literature, the yield water. The specific yield of a rock is term “porosity” is used for effective porosity, defined as the ratio of (1) the volume of and “shaliness” factors are introduced for water that the rock, after being saturated, correction of logs that respond to uncon­ will yield by gravity to (2) its own volume nected voids or chemically bound water. Wa­ (Meinzer, 1923). Specific yield plus specific

441-003 CJ - 7, - 2 8 TECHNIQUESTECHNI OF WATER-RESOURCES INVESTIGATIONS

retention equals effective porosity. Meyer Fluid conductivity (1963) described the use of a neutron probe to determine the specific yield (or storage The conductivity or resistivity (R,) of coefficient) of an unconfined aquifer. Johnson fluid in a well bore or aquifer is one of the (1967) showed that specific yields are related most useful ground>water factors that can to grain size distribution and porosity. In be derived from geophysical logs. Fluid re­ general, the highest specific yield is found in sistivity, rather than the reciprocal conduc­ medium sand because of the more uniform tivity, is used for most oil-well logging. size distribution, and the lowest specific yields Figure 2 is a graph for NaCl solutions, con­ are found in silts and clays. Specific yield can siderably modified from one published by the be estimated from geophysical logs. Schlumberger Well Surveying Corp. (Alger, 1966). To make the chart more useful to ground-water hydrologists, it was extended Grain size to cover a wider range of salinities, as well The importance of grain size in analyzing as lower temperatures and conductivity, and the hydrologic characteristics of a rock can Celsius temperature scales were added. If be seen from its effect on permeability, po­ both the resistivity or conductivity of the rosity, and specific yield. No log is purported fluid and the fluid temperature are known, to give information that bears directly on then the total electrically equivalent NaCl, the distribution of grain size in a sediment. in milligrams per liter (mg/l), can be ap­ However, Jones and Buford (1951) showed proximated from this graph. (At the present a fairly consistent increase in both perme­ time all commercial logging equipment and ability and formation-resistivity factor with logging literature still use Fahrenheit and an increase in grain size. Alger (1966) pre­ parts per million.) If ot.her ions are present, sented both laboratory and log data showing the following multiplying factors can be used an increase in formation-resistivity factor for converting to electrically equivalent so­ with an increase in grain size. dium chloride concentrations : Ca+2 = 0.95, Mg+a = 2.00, K+‘= K.00, SO4-2 = 0.50, HC03-l = 0.27, and C03-2 = 1.26 (Lynch, 1962). Thus, if the chemical nature of water Fluid Parameters in an aquifer is known from chemical analy­ ses, and the ratios of ions are consistent, re­ In addition to the lithologic parameters, a sistivity or conductivity from logs can be second important category of information used to determine the approximate quantities that can be derived from well logs includes of those ions present. For example, a solution data on the character and movement of water containing 1,000 mg/l Ca-t2 and 2,400 mg/l in the borehole and the formation. Charac­ SOse2would be electrically equivalent to a teristics of fluids in the well bore can be solution with 2,150 mg/l NaCl, calculated measured directly, but the data on formation fluids must be inferred from logs. Logs of as follows: 1,000 X 0.95 + 2,400 x 0.50 = the conductivity and temperature of fluid in 2,150 mg/l equivalent NaC1 solution. Thus, the borehole can be related to formation-fluid according to figure 2, the resistivity of the characteristics, provided that the well has CaSO, water at 50” F (1O’C) would be about had time to attain chemical and thermal equi­ 3.5 ohm-meters (2,860 pmhos/cm) . Desai and librium with the ground-water reservoir, that Moore (1969) showed thiat the above conver­ an adequate hydraulic connection exists be- sion factors vary according to concentration, tween the hole and the rocks penetrated, and and they presented a maoreaccurate method that the inhole flow does not disturb these for dissolved solids in concentrations greater conditions. than 100,000 ppm. APPLICATION OF BOREHOLE GEOPHYSICS 9

RESISTIVITY - ohm-meters

110

120

130 140

CONDUCTIVITY -pmhos/cm

Figure 2. - Electrically equivalent concentrations of a sodium chloride solution as a function of resistivity or conductivity and temperature.

The conductivity of fluid in the hole and that a neutron probe measures. Therefore, a rock must also be known in order to correctly lower porosity, or moisture content, will be interpret other types of logs. The effect on indicated on the log. This becomes a problem resistivity logs is obvious, but it is not gen­ only for very concentrated solutions. Fluid- erally realized that high-conductivity fluids resistivity information can also be used to in the borehole environment can necessitate calculate fluid density for correction of wa­ the application of a correction factor for the ter-level data. quantitative interpretation of both neutron The relationship of water in the hole to and gamma-gamma logs. The effect of in- water in the surrounding rocks must be un­ creased salinity is to raise the fluid density, derstood in order to properly interpret a used to calculate porosity from the gamma- number of geophysical logs. The chemistry gamma log. An increase in total dissolved of fluid in the hole is not necessarily similar solids also lowers the amount of hydrogen ~ to the chemistry of fluid in the rock. After 10 TECHNIQUES OF WATER-RESOURCES 1NVESTIGATIO:NS

a hole is drilled, cemented, or pumped, it may fer artesian sequence are not likely to be be as much as several months before chemi­ meaningful. For these reasons, the selection cal and thermal equilibrium is reached. Even of observation wells and the depth chosen in an otherwise completely stagnant column for taking water samples should always be of water, convective mixing, due to tempera­ based on information on the relationship of ture gradient or fluid-density differences, the fluid column to fluid in the rock sequence. may occur. If the formation-resistivity fac­ Flow in a well bore open to several aqui­ tor is known and R, can be determined from fers or zones having different heads will resistivity logs, R, can be calculated and cause anomalies on fluid-conductivity and compared with values obtained from logs temperature logs. Properly interpreted fluid- of fluid conductivity. conductivity and temperature logs can be used to locate zones where water enters and Fluid temperature (or) leaves the well bore. Flowmeter or trac­ er logs will also define these zones. The temperature of water in a drill hole may approach the temperature in the rock Moisture content without hydraulic connection between the two, provided that inhole flow is not taking The percentage of moisture in the unsat­ place. Mud cake or casing that could prevent urated zone is an important hydrologic chemical equilibrium between the fluids in parameter that is related to evapotranspira­ the hole and in the formation does not neces­ tion and recharge, and can be derived from sarily prevent thermal equilibrium. Temper­ logging. The measurement of moisture ature logs can be used to determine the changes by neutron logging is also one of position of curing cement behind casing. Tem­ the most accurate methods for determining perature logs are also necessary to make the specific yield. Although neutron-logging correction of water viscosity, specific con­ devices are the only practical means of ob­ ductance measured by fluid-conductivity logs, taining in-situ moisture content, other logs and all logs sensitive to temperature changes are invaluable aids in interpreting neutron in the hole. Water from different aquifers logs. The short-spaced neutron-moisture me­ generally differs in temperature so that ters (spacing between the neutron source sources of water and movement in the hole and detector) are especially susceptible to may be identified from temperature logs. A borehole effects, and lithologic and porosity new use for sensitive temperature-logging information are needed to properly interpret equipment is tracing of injected water, which moisture measurements. is identified by its thermal contrast with the native fluids.

Fluid movement Log Interpretation

The direction and velocity of fluid moving Log interpretation refers to all processes within a borehole may be measured by log­ of obtaining qualitative or quantitative in- ging. Work with radioactive tracers at the formation from geophysical logs. Logs are National Reactor Testing Station in Idaho only useful if they can be interpreted to pro- has demonstrated that stagnant water can vide new information or extend information exist in a well within just a few feet of water already available. moving at a high velocity. If such conditions are not defined where periodic water samples are taken for monitoring ground-water con­ Qualitative interpretation tamination, stagnant water might be ob­ In ground-water studies most well logs are tained, thus providing erroneous data. Water used in a qualitative sense for identification levels measured in wells open to a multiaqui­ of lithology, borehole conditions, and for APPLICATION OF BOREHOLE GEOPHYSICS 11 stratigraphic correlation. To make correct logs will vary little in response to minor qualitative interpretations of geophysical changes in fluid resistivity. The natural- well logs, the log analyst must have a thor­ gamma log will often indicate the sands ough understanding of the principles of which have the highest clay content, and, each sensing technique and an understand­ therefore, a lower effective porosity and per­ ing of the geohydrologic environment being meability. Fluid-conductivity and tempera­ investigated. Therefore, only the general ture logs or a flowmeter log might indicate principles of interpretation are discussed in that the sand with fresher water was being this section, and descriptions of each logging contaminated with saline water moving up technique must be consulted for the specific or down the hole from another aquifer. logs to be used. Because few logging systems These are examples of the types of interpre­ have a unique response, log analysts must tation that can be made by rapid visual exam­ rely on background information on lithology, ination of logs, along with application of water quality, and hole conditions to sub­ some knowledge of the geohydrologic envi­ stantiate their log interpretation. The amount ronment. of information that can be derived from logs Interpretation is more apt to be correct is generally a function of the background as logs of more wells in a single geohydro­ information available, the number of differ­ logic environment become available and can ent types of logs run, the number of wells be related to whatever is known about the logged in a geologic environment, and the wells. Incorrect analysis of log response in experience of the log analyst. Fortunately, one well due to some extraneous borehole the novice in borehole geophysics can make effect is not likely to be repeated throughout simple interpretations and stratigraphic the area. If a response does repeat consis­ correlations. tently, then it is probably due to a lithologic The common technique of collecting and characteristic. Gradual changes in log re­ analyzing a few lithologic samples from each sponse due to changes of bed thickness or hole drilled generally does not provide ade­ facies changes become apparent when a num­ quate data to guide log interpretation. A few ber of logs are available in a depositional nonrepresentative samples of any kind can basin. be very misleading. More information of use for quantitative log analysis is obtained ei­ Quantitative interpretation ther by continuous coring in one hole or by sidewall sampling after logging, or by drill­ The most important and, to date, little ing a second hole for selective coring after used function of water-well logs is to quan­ logging. The lithologic factors causing log tify geohydrologic parameters. Quality con­ response can then be extrapolated to other trol of logs is most important to this aspect holes in the area on the basis of logs. of borehole geophysics (Wyllie, 1963 ; Lynch, Qualitative log interpretation is more like­ 1962). To be most useful quantitatively, logs ly to be correct when several different types should be recorded at the highest sensitivity of logs are available. On the other hand, it is that is consistent with a minimum of off- expensive and nonproductive to run every scale deflections; scales and other pertinent kind of log available in every well because data must be recorded on each log, and the many types of logs are only informative equipment must be properly calibrated and under certain conditions. Caliper logs are standardized. Vertical scales should be se­ essential for determining hole-diameter ef­ lected on the basis of the detail required. fects. Two different porosity-dependent logs For example, if in doubt, select 10 feet per will often provide more information than inch rather than 20 for shallow wells. Seldom one. For example, resistivity logs will give is 50 feet per inch justified for shallow water a different response in sands of the same wells, and when used for deep wells, a second porosity that are filled with water of differ­ log at 20 feet per inch is often useful. Verti­ ent quality, whereas certain types of neutron cal detail that is lost cannot be regained by 12 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

photoenlargement. Logs should always’be in­ units based on an inferred relationship to the spected for equipment malfunction. Repeat quantity measured. Thus, it is preferable logging is the best way to check a question- to use microseconds per foot for acoustic able log. Most of the logging tools described logs and counts per second for neutron logs, in this manual do not directly measure the rather than to use environmental scales of lithologic or fluid parameter that is desired. porosity, in percent.. Porosity scales should Instead, they measure a physical property be superimposed on these logs only after the from which the required parameter may be log response has been related to standards calculated or inferred, mostly by means of and has been corrected for extraneous effects. empirical relationships. To make quantita­ If possible, it is desirable to use units that tive analysis of logs, the equipment must are widely accepted in the industry, so that first have been properly calibrated and stan­ logs made by different companies can be com­ dardized, the extraneous effects of petrophys­ pared. Inches of hole diameter for caliper its and fluid characteristics must be consid­ logs and ohm-meters for resistivity logs are ered, and all corrections for borehole and examples of units that alre universal. In con­ geometric effects must be made. These fac­ trast, the following unit.s have been used for tors are common to the interpretation of natural-gamma logs: Counts per second or most types of geophysical logs and are a minute, milliroentgens or microroentgens per prerequisite to quantitative log analysis. hour, micrograms of radium-equivalent per Environmental scales displayed on well-log ton, inches of deflection, and API gamma-ray headings, such as g/cc (grams per cubic units. There is no accurate method of quan­ centimeter) or percent porosity, should never titatively relating gamma logs made with be accepted at face value, unless the equip­ different equipment and with different scale ment has been calibrated and (or) standard­ units unless the response of instruments has ized. Extraneous effects, such as borehole been compared in an environmental cali­ diameter, must also be considered before brator. these environmental scales become meaning­ Calibration of logging equipment should ful. be done at sufficient points on each scale to establish the linearity of equipment response. Log calibration Calibrators or environmental models should Calibration of logging equipment refers be considerably larger than the radius of to the process of making sure the values on investigation of the logging device-that is, the curve scales used are correct. Too often, they should be large enough so that a further the scales on log headings signify nothing increase in the radius of investigation would more than the position of the scale-selector not alter the response of the logging system. switch. This is particularly true for the sin­ Calibration devices must be stable, or vary gle-point resistance and gamma logs. Cali­ only in a readily predictable way. Calibrators bration defines the numbers on log scales, should be so constructed that the geometrical such as 0, 10, 100, and 1,000 ohm-meters. relationship of the logging sonde and the en­ Standardization is a method for comparing vironment modeled can always be duplicated these values in time. Are 0, 10, 100, and exactly. 1,000 ohm-meters still in the same place on Although two different sondes may have the chart, even though they may not be cor­ the same response in one simulated environ­ rect? Calibration is generally accomplished ment, they may not have the same response in models that simulate inhole environmental in all environments. For example, two neu­ conditions or by measuring physical proper- tron probes may give the same porosity values ties of core samples from a logged hole. in limestone but may give different values in Prior to calibration, the scale units for the a shaly sandstone. Most companies that make log must be selected. It is preferable to label the same general type’ of tool use different log scales with units that are actually related signal sources, detectors, and electronic cir­ to a measured phenomenoii, rather than to use cuits, which will modify tool response. For a. APPLICATION OF BOREHOLE GEOPHYSICS 13 this reason, where quantitative measurements the usual technique is to plot the laboratory are desired, it is best to calibrate logging data on the same depth scale as the log and equipment in models that closely simulate then slide the plot up and down on the log the actual environment. A flowmeter to be until the best fit is obtained. This technique used in a $-inch hole should be calibrated in can also lead to significant errors in indi­ a 4-inch, not an 8-inch, pipe ; and a neutron vidual values. A recent paper by Jeffries probe to be used for measuring the porosity (1966) describes the use of correlation func­ of sand will probably not provide the correct tions to derive the best relationship between values if it is calibrated in the American samples and logs. Figure 3 shows a neutron Petroleum Institute limestone pits. (See sec­ log and a plot of more than 200 values of tion on “Neutron Logging.“) However, cor­ porosity measured on core samples in the rection factors for borehole, fluid, or lithologic laboratory. Note the wide variation in core effects can be applied. values between depths of 200 and 300 feet. Laboratory core analyses are widely used If only a few of the samples in this depth for placing values on logs. This is generally interval had been analyzed and used for cali­ an interpretive postmortem process that at- brating the neutron log, the results could tempts to relate a parameter, which was not have been very inaccurate. Also note how measured directly, to log response. When com­ great the porosity error would be in figure 3 paring laboratory data to logs, remember if the core locations were plotted in slight that cores represent point values. In con­ error vertically between depths of 360 and trast, some logs represent an average value 420 feet. of a physical parameter for a volume of material more than 100 times larger than Standardization and log accuracy the core sample. A large number of core Most of the environmental calibrators or samples must be taken and analyzed in order models are too large for transportation in to be statistically representative of the usual logging vehicles ; yet, it is highly desirable nonhomogeneous geologic environment. If to run checks on the reproducibility of logs the core samples are representative, the next in the field. Devices used to check the re­ problem is relating the point values to the sponse and stability of logging equipment in continuously varying geophysical log. The the field are referred to in this manual as error introduced at this point can be most standards. They provide a means of com­ significant. If the trace of most geophysical paring the response of geophysical-logging logs sensitive to lithologic parameters is ana­ equipment from time to time and from place lyzed, the pen deflections between 45” and to place. If a standard check is not recorded horizontal will be predominant. A small error on a geophysical log, the accuracy of the in the vertical placement of a laboratory scale shown is open to question. Because value on this type of log can result in a very temperature drift is such a common problem large error in the related log value unless in certain logging equipment, it is desirable the vertical scale of the log is greatly ampli­ to record standard checks both before and fied. The exact depth of a sample is subject after logging if a high degree of accuracy to an error which may be very large in a is required. Some logging sondes have built- single-coring run with high core loss, if the in standards that provide a reference signal driller cannot determine which interval was which may be recorded at any time as a retained. Likewise, geophysical logs are sub­ means of checking system drift. Other types ject to footage discrepancies due to cable of standards are resistance-decade boxes for stretch, equipment malfunction, and human conductivity or resistivity tools, plastic cyl­ error. In addition, the measuring datum for inders for neutron tools, and radioactive drilling and logging may not be the same. sources for gamma tools. At least two stan­ The kelly bushing on the drill rig is com­ dard values should be available so that both monly used as zero datum on commercial positioning and scale span, or sensitivity, can logs. To ameliorate some of these difficulties, be checked. 14 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

DEPTH, FEET

- 100

-200

- 300

- 400

NEUTRON-EPITHERMAL NEUTRON CORE POROSITY MEASURED LOG IN CASED HOLE IN THE LABORATORY T-14 C-l

Figure 3. - Neutron log and a plot of porosity measured in the laboratory, upper Brazes River basin, Texas. Standards may or may not display unit will not be true if the contact resistance of values that are the same as the units in which the electrodes in the tool changes. the equipment is calibrated. Most logging If logs are to be interpreted quantitatively, standards used in the field do not accurately calibration, standardization, and corroborat­ simulate the environment being logged. ing data are essential to establish the accuracy Therefore, values placed on logs by the use of the respons,e values. This applies to com­ of field standards may not be accurate guides mercial logs made with oil-well equipment, to log interpretation. The resistance-decade as well as to those logs made with small box, commonly used for standardizing fluid- water-well equipment. An understanding of conductivity tools, can be related to actual the principles of each logging technique used fluid resistivities ; however, decade-box values will permit an evaluation of the methods of APPLICATION OF BOREHOLE GEOPHYSICS 15

determining log accuracy. Although most ( questions of log accuracy relate to the re- corder deflection of the logging equipment, errors in depth or footage do occur, and, therefore, cross-checks with other logs or data on the hole should be made if depth measurements are suspect.

Composite interpretation As a general rule, the more types of geo­ physical logs that are available for a single well and the more wells that are logged with- DUARTZ INCREASINGNATURALGAMMA COUNTRAlE - in a given geohydrologic environment, the CLAV greater the benefits that can be expected SAN0 - INCREASING?ERYEABILIlV ASSUMINGEQUALPOROW”

from logging. The synergistic character of Figure 4. - Hypothetical response of nuclear logs in a mixture logs is due to the fact that each type of log of quartz sand, clay, and water. actually measures a different parameter, and when several are analyzed together, each relate directly to porosity. The neutron and will tend to support or contradict conclusions gamma-gamma logs are both related to po­ drawn from the others. Similarly, a large rosity, even though they measure different number of wells logged in one ground-water parameters, and will therefore not respond environment will provide a statistically mean­ the same to the different lithologies. ingful sample of the environment and reduce Figure 5 shows how the relationship of the chance of interpretive errors. various logs can be utilized to determine hy­ Three nuclear logs, natural-gamma, gam­ drologic characteristics. These six different ma-gamma, and neutron logs will be used to logs were run in a rotary-drilled test hole illustrate the synergistic nature of logs. Fig­ adjacent to a core hole on the upper Brazos ure 4 is a hypothetical diagram of nuclear- River in Texas. The lithologic description on log response in saturated elastic sediments. the left was derived from the cores. For illustration purposes, these sediments The simplified response of these six logs are considered to be a mixture of water, is listed below : quartz sand, and clay, in various proportions, which may change the porosity and perme­ Type of Parameters measured and Parameters inferred log direction of log deflection and direction of log ability of the hypothetical aquifer as de- deflection scribed on the outside of the triangle. The Caliper...... Hole diameter (increases Cementation and effect following generalities can then be made to right). of drilling. about nuclear-log response, which is shown Gamma- Scattered and attenuated Bulk density (decreases gamma. gamma photons (radia- to right) and hole on the inside of the triangle: tion increases to right). diameter (increases 1. The natural-gamma count rate will in- to right).

crease with increasing clay content. Natural- Natural-gamma radiation Clay content (increases 2. The neutron count rate will increase with gamma. (increases to right). to right). decreasing porosity. Spontaneous Natural electrical poten- Clay or shale content. 3. The gamma-gamma count rate will in- potential. tials (positive to right). crease with increasing porosity. Single-point Electrical resistance of hole Hole diameter and resistance. and adjacent rocks water salinity A sample of this hypothetical sediment (increases to right). (decreases to right) : will fall somewhere within the triangle illus­ effective porosity trated and can be characterized by relative (increases to right). percentages of the three components. Natu­ Neutron- Neutrons slowed and scat- Saturated porosity neutron. tered by hydrogen (decreases to right). ral-gamma logs indicate sand-clay ratios and (radiation increases relative permeabilities, but do not necessarily to right). 16 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

DEPTH DEPTH - FEET FEET Poorly cemented very fins sandstone 1 ID0

Very fins sandstone wth shola 1

Hord holit* cemented rdtstono

1 and sdtrton.

Siltstone prodmg down mto mudslone wath onhydrtte ond halite rams end nodules

Mudrtons wh nnhydres noduless--

I Mudrlono - - LITHOLOGY CALIPER GAMMA-GAMMA GAMMA SP RESISTANCE NEUTRON-N

Figure 5.-The relationship of six different geophysical logs to lithology, upper Brazes River basin, Texas. The significant increase in total porosity This more permeable za’ne shows the highest (deflections to the left) shown on the neu­ total porosity on the neutron log, the lowest tron log below a depth of 300 feet does resistivity on the single-point resistance log not indicate an increase in effective porosity and, as indicated by the natural-gamma log, or permeability as might be expected. Instead the lowest clay content. The neutron log the gamma log suggests that the increase measures total porosity in saturated rocks, in total porosity is due to an increase in which in some areas can be related to perme­ clay content. (See section on “Gamma ability. Shales and clay-bearing sedimentary Logging.“) Laboratory analyses of particle- rocks tend to have a high total porosity but size distribution showed a higher percentage a low permeability, and, therefore, they pro­ of clay particles, in agreement with the high­ duce water slowly. Composite interpretation er gamma log response. From these logs the of gamma and neutron logs helps to distin­ zone above 100 feet was selected as having guish these sediments and also identifies the highest permeability in the well and this zones where chemically bound water may was borne out by packer and pumping tests. cause an error on neutron logs. APPLICATION OF BOREHOLE GEOPHYSICS 17

A very useful technique of composite log available from commercial logging service interpretation has been pioneered by Schlum­ companies. berger Well Services (Raymer and Biggs, Another example of the synergistic nature 1963). Figure 6 is an example of a Schlum­ of logs is furnished by use of the Archie berger plot of apparent limestone porosity equation F = R,/R,, where F = formation- from neutron-log response versus bulk den­ resistivity factor, and R, = resistivity of a sity from gamma-gamma log response. The formation saturated with water of resistivity basis for this chart is the difference in re­ R,, (Archie, 1942). He also established that sponse of the two types of porosity devices F is related to porosity and permeability, due to the effects of rock matrix characteris­ and expressed this relationship as F = @-“‘, tics. Neutron and gamma-gamma tools will where m is a cementation factor that varies not give the same count rate in limestone and between 1.3 and 2.6. Both F and m tend to be dolomite of the same porosity. With properly consistent within a lithologic unit in a given calibrated gamma-gamma and neutron equip­ depositional basin. The Humble Oil Co. for­ ment, a plot like this may be used to deter- mula F = 0.62~-~J” is widely used for sand- mine the lithology of the rock matrix and to stones (Winsauer and others, 1952). By com­ correct porosity for matrix effects. For exam­ bining either of the preceding formulas with ple, an apparent neutron porosity G iV of 10 F = RJR,,, R,, derived from a multiple-elec­ percent on the abscissa and a gamma-gamma trode resistivity log, and @,derived from a bulk density Pbof 2.76 on the ordinate would neutron, gamma-gamma, or acoustic log, indicate that the rock was actually a dolomite would permit the calculation of R,,, a quan­ with g-percent porosity. If the gamma-gam­ tity which is related to the salinity of water. ma-log bulk density was 2.44 g/cc and the (See fig 2.) An empirical approach to obtain apparent neutron porosity was 10 percent, F and R,, was devised by Turcan (1966). the rock would be interpreted as a sandstone (See section on “Normal Devices.“) with a porosity of 13 percent. Similar charts Computer collation and interpretation of relating acoustic-log response to either geophysical logs is now available from some neutron or gamma-gamma log response are commercial well logging companies. The pro- grams were written for oil-field applications ; however, factors useful in ground-water hy­ drology may also be computed from logs recorded digitally on magnetic tape. These include corrected total porosity, effective porosity, grain density, shaliness, true resis­ tivity, and formation-fluid resistivity. Com­ puter output may be printed numerically or plotted continuously as a graphic record, with the presentation of data preselected to fit the problem. Logs either can be digitized in the field as they are run or existing graphic geo­ physical logs can be digitized and recorded on magnetic tape, making them amenable to computer analysis. Computer interpretation of geophysical logs is also used in uranium ore reserve and bulk-density calculations by the U.S. Atomic Energy Commission (Scott, 1963 ; Dodd and Droullard, 1966). For programing and NEUTRON INDEX - f&N (perteni npporent limsstons porosity\ computer time to be economically justified

Figure 6. - Lithology and corrected porosity from composite for ground-water applications, a large num­ interpretation of gamma-gamma and neutron logs. ber of digitized logs must be available. One 18 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS meaningful use of computers that does not Bed-thickness effects require a large number of logs for justifica­ The concept of a volume of material being tion is the use of correlation functions to fit measured by a downhole sensor also serves laboratory data to geophysical logs or to cor­ to explain the effects of bed thickness on logs. relate several different logs (Jeffries, 1966). For a thin bed, the volume being investigated may include part of the adjacent rocks, so Geom’etric effects that the logging sonde does not measure the Anomalies in logs due to variations in the same value that it would in a thick bed of size and shape of the environment that is the same lithology. (See fig. 42.) If a logging sensed by the logging device are called geo­ device has a volume of investigation that metric effects and must be considered in the extends a considerable distance up and down interpretation of all geophysical logs. the hole from the detector, a lithologic change will begin to affect the signal transmitted Radius of investigation from the sensor before it actually reaches the The interpretation of many well logs de­ depth at which the cha.nge occurs. The ef­ pends on an understanding of the radius of fect of the bed will also be seen on the log investigation or volume of influence. These after the sensor has passed beyond the bed terms refer to the material in and around boundary. This is the ch.aracteristic of radia­ the borehole that contributes to the varying tion-logging devices that; produces a gradual signal measured by the logging probes. For change in log response even though the litho­ sake of simplicity, the radius of investigation logic change may be sharp. Thus, the com­ is here considered to include that part of the mon technique of drawing lithologic contacts formation contributing 90 percent of the through the half-amplitude point on radiation measured signal. The radius of investigation logs can be misleading, particularly for thin ranges from fractions of an inch for micro- beds. resistivity logging devices to many feet for A serious problem is encountered when focused resistivity sondes. Radius of investi­ interpreting some multiple-electrode resistiv­ gation must be considered for all logging ity logs in thin-bedded sediments. These logs devices measuring lithologic characteristics. may actually exhibit a cratering effect or If a logging sensor is visualized to be reversed response in t.hin beds of higher within a volume of material that is contrib­ resistivity rocks between layers of lower re­ uting to the recorded signal, the drill hole, sistivity rocks. (See section on “Normal the fluids in the hole, and the wall of the hole Devices.“) The only resistivity curves that that has been invaded and caked with drilling do not exhibit reversals are made with the materials are also within the volume investi­ single-point device. It is, therefore, one of the gated. Changes in any of these factors will best tools for logging lithology in a thin- alter log response even though the character­ bedded sequence. However, because of its istics of the surrounding rocks are constant. sensitivity to borehole effects, it cannot be Furthermore, sondes measuring similar pa­ used quantitatively for measurement of re­ rameters but having different volumes of sistivity and is more correctly called a resis­ investigation will record dissimilar logs. tance log. The spontaneous-potential log is Therefore, several kinds of resistivity sondes effective for determining the true position of having different volumes of investigation can bed boundaries because contacts are gener­ be used to measure the thickness of the in­ ally at the point of maximum inflection on vaded zone which is related to formation per­ the curve. A sensitive caliper log is also use­ meability. Some resistivity sondes have a ful for locating bed boundaries; however, zone of maximum response that is some dis­ logs made with a feeler-type sonde must be tance from the pickup electrodes. For most corrected for the different measuring depths logging devices, however, the materials clos­ caused by major changes in hole diameter. est to the sonde contribute most of the re- In summary, the accurate measurement of corded signal. bed thickness and the location of bed bound- \ APPLICATION OF BOREHOLE GEOPHYSICS 19 aries is best accomplished by using several fractures, and solution openings are respon­ types of logs with proper recognition of the sible for most borehole rugosity. Correction volume of material investigated by the var­ of logs for rugosity where the tool spans thin ious logging sondes. washed-out zones generally is not possible. These zones should be eliminated from quan­ D titative log analysis because, even with the Borehole effects aid of caliper logs, corrections cannot be made. A hole cannot be drilled without disturb­ Decreases in hole diameter to less than bit ing the environment to be measured. Most size are due either to mud cake build-up or borehole effects are extraneous to the logging to clay squeezing into the hole. Clay squeezes measurement desired and must, therefore, be will generally continue to decrease diameter considered in log interpretation. until the hole is closed. Mud cake or clay squeezes will affect the response of some log­ Hole-diameter effects ging devices. Scale and mineral deposits may One of the most common extraneous effects decrease the hole diameter or increase the present on logs is caused by changes in the casing or screen thickness in pumped wells. diameter of the hole. Diameter changes are Any change in the nature or thickness of divided into two types that may affect logs material between the logging sensor and the in different ways - changes in average hole rock being measured is likely to affect the diameter and changes in borehole rugosity. log response. Therefore, the sensitivity of Differences in average hole diameter are gen­ each logging device to hole diameter should erally due to changes in bit size or drilling be considered, and where necessary, a caliper technique, or to thick units of different lithol­ log should be made. ogy. Changes in average diameter that ex- tend over a depth range of many feet do not Casing effects greatly affect the signal from decentralized Only the nuclear logs are routinely used logging tools. In contrast, borehole rugosity, for logging through casing, although it is or rapid fluctuations in hole diameter, will possible to use acoustic-velocity logs if the cause erroneous values on logs from decen­ casing is bonded to the wall of the hole by tralized and axially symmetric logging de- cement. Casing and cement introduced be- vices. Hole diameter, measured with caliper tween a radiation detector and the wall of the probes, is essential for the quantitative inter­ borehole change the radiation detected. Cas­ pretation of most logs. ing of consistent thickness and type intro­ Few holes drilled for ground water have duces a constant error that will shift or smooth walls or are of constant diameter. suppress log response, but it generally does The most irregular bores are in poorly con­ not change the character of the log. solidated or thin-bedded sediments, and the Qualitative nuclear logs can even be made smoothest bores are generally in igneous through two thicknesses of casing or through rocks. In sedimentary rocks the drilling tech­ drill stem, but each change in size or type nique is a major factor in hole diameter; for of pipe will cause a baseline shift on the log. example, a rapidly drilled hole generally has Furthermore, as casing thickness increases, the smoothest bore. The longer the drilling the signal-to-noise ratio decreases, and log and sampling operations are continued in a character is subdued. The increase in metal hole drilled in poorly consolidated sediments, thickness at casing collars or joints is easily the larger and more irregular the bore be- apparent on some gamma-gamma logs, but comes. Circulating drilling mud or water for generally is not discernible on neutron or long periods also tends to affect the diameter. natural-gamma logs. Also, as the radius of Drilling techniques and major changes in investigation increases, the effect of casing lithology are the common reasons for changes decreases. The short-spaced neutron-mois­ in average hole diameter. Thin-bedded units, ture tool does not provide accurate values in 20 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS casing that is more than 2 inches in diame­ ed to the hydraulic properties of the aquifer ter, whereas the conventional long-spaced system. In addition to the type of mud, neutron-logging tool will make excellent logs the differential pressure, and the length of in lo-inch casing. In general, however, better time an aquifer has keen exposed to mud, the Iogs can be obtained in small.diameter cas­ porosity and permeability of sediments will ing. A caliper log prior to the installation of also determine the depth of invasion. In most pipe is the only way to accurately measure oil fields, if all other factors are equal, the a borehole diameter. As an alternative, radi­ low-porosity sediments will generally be ation devices having different radii of inves­ invaded deeper than the high-porosity sedi­ tigation can be used to derive qualitative ments. The most obvious reason for this information on hole-diameter changes behind apparently anomalous relationship is that casing. there is a greater volume to fill in high- porosity .formations. Further, the permeabil­ Drilling effects ity of the mud cake is generally lower than Drilling a hole generally disturbs the fluids the permeability of the rock, so that the mud and pore spaces in the environment to be cake and the differential pressure become the measured. Rotary drilling with mud probably factors that control the rate of filtration. In causes the greatest disturbance in the en­ contrast, thicker invaded zones were found vironment near the borehole. Augering prob­ to occur in aquifers, rather than in confining ably produces the least disturbance in the beds in shallow, poorly consolidated sedi­ environment. Vibration caused by driving ments. A complete discussion of invasion casing in cable-tool drilling may compact characteristics is beyond the scope of this unconsolidated sediments and reduce the po­ manual, and reference should be made to Doll rosity and permeability. Drilling operations (1955). can also produce changes in the temperature Inasmuch as both the mud cake and the and resistivity of fluids in the borehole and zone of filtrate invasio:n vary in thickness, adjacent rocks, reduce the movement of fluid logging devices having different volumes between the hole and the rocks, and change (radii) of investigation provide a means of the porosity and permeability of the rocks. measuring these factors. A sensitive caliper In rotary drilling, a natural or artificial log can provide data on mud cake build-up, mud is circulated down the drill stem to and various multielectrode-resistivity devices bring the cuttings back to the surface; the are designed to investigate the mud cake, the mud also keeps the hole open, cools the bit, invaded zone, and the native fluids. Nuclear lubricates the drill stem, coats the wall of the logs provide a means of investigating the hole to reduce fluid loss, and serves as an .elec­ changes that occur behind casing as drilling trical-coupling medium necessary for many mud and filtrate are gradually dissipated logging operations. Because the pressure by native fluids and na.tural conditions are of the mud column exceeds the hydro- approached. static pressure in the formation being pene­ Thus, if the drilling process can alter con­ trated by the bit, the mud filtrate invades the ditions in and near a drill hole, it follows, rock adjacent to the borehole and displaces then, that pumping and development opera­ the native fluids away from the hole. During tions can remove mud cake and invading this process, many of the particles suspended fluids and increase the porosity near the hole. in the mud are filtered out and form a mud Procedures used for the development of wa­ cake, or filter cake, on the wall of the hole. ter wells are intended to increase porosity The invaded zone and mudcake may intro­ and permeability near the hole to greater duce unknown and, in general, undesirable than natural undisturbed values. This is factors in log interpretation. achieved in granular sediments by removing The effects of drilling also have a positive fine-grained material from between the larg­ aspect. Thickness of the filter cake and thick­ er grains. Logs related to porosity, such as ness of the invaded zone are sometimes relat­ the neutron log, can provide data on the APPLICATION OF BOREHOLE GEOPHYSICS 21

relative effectiveness of well development and ponents of geophysical well-logging equip­ the location of developed zones. (See fig. 52.) ment. In a well-logging system the sensor is This can be done by running logs at inter- contained in a watertight probe or sonde, r vals during development. The same proce­ which generally receives power from the sur­ dure can be used with resistivity logs to face and transmits a signal to the surface observe the return toward normal fluid con­ through logging cable. The cable also serves ditions in and near uncased drill holes, or to position the probe in the hole by means through some plastic well screen. (See sec­ of a winch. Controls on all recent loggers are tion on “Normal Devices.“) used for regulating (1) logging speed and Drilling and sampling operations should direction, (2) power to surface and down- always be designed to cause the least pos­ hole electronics, (3) signal conditioning, and sible disturbance in the environment if log­ (4) recorder response. ging is to be done. Changes caused by these Chart paper or photographic film is moved operations, and subsequent changes with through the recorder as a function of the time, must always be assessed in the in­ position of the sonde in the hole. This is terpretation of geophysical logs. For this accomplished either by a mechanical con­ reason, a two-hole drilling, logging, and nection between the cable sheave or by self- sampling operation may provide more reli­ synchronous motors (selsyns). Vertical able data for the drilling dollar. This is scales, in feet per inch, are generally selected accomplished by rapidly drilling one hole to by changing a gear ratio in the recorder. The the desired total depth and making a full depth indicator is preset when the probe is suite of geophysical logs. On the basis of the at the surface, and is then turned by the geophysical logs, sample depths are selected cable-measuring sheave. that will provide the most representative and The signal transmitted by the cable varies meaningful core and water samples. If a in response to lithologic, fluid, or borehole sidewall sampler is available, cores can be parameters, which change as the probe moves 3 I taken subsequent to logging the first hole; down or up the hole. This signal is processed if not, a second hole can be drilled close to by electronic equipment at the surface and, the first hole and cored at the intervals pre- if necessary, changed to a varying direct- determined from log analysis. Another ad- current voltage that moves the recorder pens vantage to drilling a second hole is that or optical galvanometers horizontally. Oper­ aquifers selected from logs can be developed ator adjustments necessary for all logs are and tested before they become greatly affect­ made at this point. They include zero posi­ ed by drilling mud. tioning, or basing, the pen on the paper and sensitivity, or scale selection, which controls the amount of pen deflection for a given change detected in the hole. Many other Logging Equipment operator adjustments, such as time constant, pulse-height discrimination, frequency, and Geophysical-logging equipment appears so forth, are made both in the tool and at complex, and the variety of instruments in the surface in some types of logging oper­ use is very large. Fortunately, any well log­ ations. ger-from the single-conductor water-well The basic logging equipment described is unit to the seven-conductor oil-well unit-­ extremely dependable when properly built, can be divided into the same basic compo­ tested, maintained, and operated. To derive nents. Minor differences in operating con­ the maximum amount of accurate data from trols and how a function is performed are logging equipment, operators must under- not fundamental to an understanding of log­ stand basic log interpretation, and log ana­ ging. Any basic measuring system consists of lysts must understand the principles of a sensor, signal conditioners, and recorders logging-equipment operation. The operator or indicators. Figure 7 shows the basic com­ should be able to recognize equipment mal- 22 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

RECORDER DRIVE - I I

CABLE-MEASURING SHE,\ 1 , w m

--

L------J------i----J LOGGING CONTROLS

Figure 7. -Schematic block diagram of geophysical well-logging equipment. APPLICATION OF BOREHOLE GEOPHYSICS 23

NATURAL SINGLE-POINT GAMMA RESISTANCE LOG SINGLE-POINT SPONTANEOUS SPONTANEOUS loG SPONTANEOUS SINGLE-POINT RESISTANCE POTENTIAL POTENTIAL POTENTIAL RESISTANCE LOG LOG LOG LOG LOG

Drift eliminator Different logs on the Simultaneous logs Simultaneous logs not operating some recorder amplifier. Regular noise due to Intermittent norse probably properly Pen drive sticking or 60-cycle AC caused by drilling equipment omplifier gain too low at the well

Figure 8. - Geophysical logs, showing some common equipment problems.

functions from a log in the field, where it is described will operate in the basic logging often possible to correct the problem and system shown in figure 7. rerun a log. If the log analyst is completely familiar with logging equipment and pro­ cedures, he will not make lithologic interpre­ tations of instrumental problems shown on Spontaneous-Potential the logs. Figure 8 illustrates several obvious Logging extraneous or instrumental effects on actual logs. These and many other problems are Spontaneous-potential logs are records of very common on geophysical logs run with the natural potentials developed between the all types of equipment both in the petroleum borehole fluid and the surrounding rock ma­ field and in ground-water hydrology. Un­ terials. The spontaneous potential is used recognized equipment malfunctions and op­ chiefly for geologic correlation, determina­ erator errors are responsible for most tion of bed thickness, and separating non- poor-quality logs. porous from porous rocks in shale-sandstone Because such a wide variety of logging and shale-carbonate sequences. equipment is utilized in ground-water hy­ Because the electric log is a measure of drology, the equipment used to make each natural potentials and resistivities, it can type of log will not be described in detail. be run only in open (uncased) holes that In this report the basic principles of each are filled with a conducting fluid, such as logging device and the major differences be- mud or water. The chart paper commonly tween types of sondes are discussed in the used for the electric log is divided into two sections on instrumentation. All the sondes vertical columns, called tracks. Under the 24 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

API (American Petroleum Institute) sys­ tacts between the borehole fluid, the shale or tem, the left-hand track is 21/2 inches wide, clay, and the water in the aquifer. Two with four divisions per inch and major divi­ sources of potential are recognized. The first sions at llh-inch increments across the width source, and least important to the magnitude of the track. The right-hand track is 5 inches of SP, is the streaming potential caused by wide, with four divisions per inch, and also electrokinetic phenomena. This electromotive has major divisions at l?Lb-inch intervals force (emf) develops when an electrolyte across the width of the track. The footage moves through a permea.ble medium. The emf scale is divided into 10 divisions per inch, appears in the borehole at places where mud with major divisions at l&inch and 21/s-inch is being forced into permeable beds, although intervals. The API chart paper allows for in water wells, streaming potentials may be a wide choice in SP, resistivity, and foot- generated in zones gaining or losing water. age scales. The electric log usually includes Streaming potentials can sometimes be de­ the spontaneous potential in the left-hand tected on the SP curve by sudden oscillations track and one or more resistivity curves in or by departures from the more typical re­ the right-hand track. Some of the commonly sponse in a particular environment. (See sec­ used resistivity curves are the single point, tion on “Extraneous Effects.“) A discussion short normal, long normal, lateral devices, of the streaming potential and its effects on microlog, microfocused log, and the guard, SP was given by Gondouin and Scala (1958). or laterolog. Each of these devices has its The second and most important source of application, depending on the lithology, SP arises in the. electrochemical emf pro­ depth of mud invasion, and other borehole duced at the junction of dissimilar materials conditions. Table 2 shows the applicability in the borehole. The junctions are between of various electric logging methods to the the following materials : Mud-mud filtrate, solution of typical hydrologic problems. mud filtrate-formation water, formation wa­ ter-shale, and shale-mud. Because the mud Principles and applications filtrate is derived from the mud, it generally has a similar electroc’hemical activity, and The spontaneous-potential log is a graphic any emf developed across this junction will plot of the small differences in voltage, mea­ be minimal and can be disregarded. The PO­ sured in millivolts, that develop at the con­ tential developed across the junction from

Table 2. - Hydrologic applicability of electric logs

Type of electric log - Properties to be WSll investigated Single Short ,.tF$l $$$$ resistivity guardFocused and Micro- Induetion sP point normal (nonfocused) laterolog foc”sed Lithologic correlation ...... X X ...... X ...... x x Bed thickness ...... x X ...... X X X X X Formation resistivity (low R muds) ...... X X ...... X ...... X ...... Formation resistivity (freshmud) ...... x x ...... X ...... Invaded zone resistivity ...... X ...... Flushed zone resistivity ...... X ...... X ...... Mud resistivity’ (in place in hole) ____.___ ...... X ...... Formation water resistivity...... x x ...... x ...... ‘Use mud kit for pit samples. APPLICATION OF BOREHOLE GEOPHYSICS 25

formation water to shale to mud is called As the SP electrode moves beyond the mid- the membrane potential, and the potential point of the sandstone, the curve recorded developed across the junction from mud fil­ is a mirror image of the lower half if the trate to formation water is called the liquid- beds are uniform. The SP log, therefore, is junction potential. The potentials arising a measure of the potential drop that occurs from these junctions cause a current to flow in the mud, and only approaches the static near shale-aquifer boundaries in the mud spontaneous potential (SSP) under favorable column in the borehole. Figure 9 is a sche­ conditions. (See “Glossary” for definitions.) matic diagram of the circulation of current If, on the other hand, the formation water is across the various junctions and through the fresh compared with the mud, the polarity borehole (Doll, 1948). When the formation of the SP curve is reversed, and the recip­ water is much more saline than the mud, the rocal of the log in figure 9 is produced. current follows the paths shown by the ar­ The SP is, therefore, more positive opposite rows, entering the mud column from the the sands and is more negative opposite the shale and moving into the sandstone. As the shales. This condition occurs in hydrologic SP electrode moves upward through the bot­ regimes where ground water contains very tom shale (fig. 9), it senses a decreasing few dissolved solids and results in an electric potential because the current flows parallel log on which both the SP and the resistivity to the well bore. At the bed boundary the deflect in the same direction, opposite the current density is maximum, and the SP sand and shale beds. An example of this is curve exhibits an inflection at this level. As illustrated in figure 10. The water in the for­ the SP electrode moves toward the midpoint mations above 500 feet contain dissolved of the sandstone bed, the current density de- solids of the drilling mud lie somewhere creases, the curvature of the SP curve is tions below 800 feet have a dissolved-solids reversed, and the SP attains its maximum content of about 250 mg/l. The dissolved negative potential at midpoint in the sand. solids of the drilling mud lie somewhere

-

Static SP Diagram

SP Curve

Figure 9. - Spontaneous-potential junctions and currents. From Doll (1948). 26 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

SP RESISTIVITY LITHOLOGY between these two extremes. The resistivities (MILLIVOLTSI (OHMS-MtlERSl above 450 feet are all off-scale to the right, DEPTH, FEE1 0 and lie between 100 and 200 ohm-meters. 0 100 Spontaneous-potential deflectkns are re­ zorded on the left-hand track of the electric SAND log, with deflections to the left considered as negative and those to the right as positive. In a sand-shale sequence containing forma­ CLAY tion water that is more saline than the mud, the greatest positive deflections can be ex­ pected opposite the shales, and the greatest negative deflections cart be expected opposite the sands. A shale line can be constructed

SAND to fit through as many of the extreme posi­ tive deflections as possible, and a sand line through as many of the extreme negative deflections as possible.. If the ionic concen­ tration of the borehole fluid and the aquifer water are constant throughout the length of tL*Y the borehole, the shale line and the sand line SAND will generally be parallel to the vertical axis of the log. The shale line is considered to be the base line, and all potential measurements CLAY are made perpendicular to this line, using the scale of SP, in millivolts, on the log head­ SAND 600 ing. When carefully applied, this method can be used to estimate sand-shale ratios and is applicable to aquifers with water of high salinity and, in some cases, to fresh-water aquifers. Unfortunately, the method fails in CLAY many water wells in sand-shale formations because the SP deflections are not necessarily negative opposite the sands. If the borehole fluid has a very low resistivity compared with the sand beds, the SP deflections opposite the sands may actually be more positive than the SP deflections opposite the shales. Accord­ ingly, the sand-shale ratios become meaning- less for formations that do not contain shale or clay beds, even though large positive SP deflections can be seen. (See Guyod, 1966, SAND fig. 11.) Perhaps the most significant, but often OFF-SCALE I DEFLECTIONS misapplied, use of the SP in ground-water FROM hydrology is the deter:mination of water qual­ loo TO 2000tlM-M ity from SP deflections. In petroleum explo­ ration, where NaCl is dominant, the following equation is useful to ,calculate the quality of the formation water : SP=-,K log +‘Y Figure 10. - Electric log of oil-test well in western Kentucky. 1” APPLICATION OF BOREHOLE GEOPHYSICS 27

where linity-resistivity charts, he found it necessary SP = log deflection, in millivolts; to convert all anions and cations in the water K = 60 +O.l33T; to an equivalent sodium chloride solution. T = borehole temperature, in degrees Alger assumed that, within one ground-water Fahrenheit; regime, the relative ionic tincentrations and R,, = resistivity of borehole fluid, in ratios are approximately constant. The rela­ ohm-meters ; and tions between SP, R,, and total dissolved R,, = resistivity of formation water, in solids - once determined from a borehole ohm-meters. having an electric log and water analyses - In actual practice, the SP deflection opposite were extrapolated by Alger to other bore- a sand bed is read from the log, and the R, holes in the regime exclusively from the SP is measured with a mud kit or a fluid-resis­ deflections. tivity tool. Inserting these values in the equa­ In an unpublished paper by Hubert Guyod, tion determines the formation fluid, R,, Consultant, entitled “Fundamentals of Elec­ which can be related back to milligrams per trical Logging and their Application to Wa­ liter of NaCl from salinity-resistivity charts. ter Wells,” presented at the Second Advanced The equation is predicated on the following Seminar on Borehole Geophysics, U.S. Geo­ important assumptions, which may or may logical Survey, Denver, Colo., December 1968, not hold true in water wells: (1) Both the Guyod pointed out that the formula borehole fluid and the formation water are SP=--log+ sodium chloride solutions; (2) the shale for­ 141 mations are ideal ion-selective membranes, can be used only when the following condi­ and the sand formations have no ion-sieving tions are simultaneously satisfied : (1) The properties (no clayey sand or sandy clays in formation water is very saline, (2) NaCl is the zone of investigation) ; and (3) the bore- the predominant salt, and (3) the mud is hole fluid has a much greater resistivity than relatively fresh and contains no unusual ad­ D the combined resistivity of the sand and the ditives. Guyod further stated that “the three shale. In petroleum logging the sand forma­ conditions specified above are probably never tions are generally saturated with brines of met by fresh water aquifers. Therefore, it is low resistivity, and the conditions in assump­ not possible to determine, not even estimate, tion 3, above, are generally satisfied. In hy­ from the SP curve the resistivity of forma­ drologic logging where the sand formations tion water, that is, the total dissolved solids are saturated with fresh water, the resistiv­ in fresh water sands.” ity of the sands may be many times that of Patten and Bennett (1963) also emphasized the borehole fluids, and the conditions in the unreliability of the SP formula for de­ assumption 3, above, may no longer be valid. termining dissolved solids. Considering the However, the combined resistivity of the unreliability of the method, especially for sands and shales can be less than that of the dissolved solids of less than about 10,000 borehole fluid if the fresh water used in the mg/l, the method probably should not be drilling fluid has a lower ionic concentration used in the analyses of fresh-water-bearing than the formation water. aquifers. The divalent ions Ca+2 and Mg+z com­ Vonhoff (1966)) however, found that a monly found in fresh water have a different “workable empirical relationship exists be- effect on the SP than Na+l. The calcium and tween the spontaneous-potential deflection on magnesium solutions affect the SP as though the electric log and the water quality in the the water were saltier than its resistivity glacial aquifers.” His conclusions are based indicates. Alger (1966) described a method on a study of test wells in Saskatchewan, for finding the R,, of fresh waters by use of Canada, in which the ionic composition of the the equation SP = - K log (RJR,,). In drilling fluid and formation water are simi­ order to correlate SP deflections with water lar, and the resistivity of the drilling fluid resistivity and ionic concentration from sa­ is much greater than that of the formation D 28 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS water. The actual dissolved solids in the test Calibration and standardization wells ranged from 1,191 to 3,700 mg/l. Spontaneous potential is measured in milli­ Instrumentation volts, and the millivolts per horizontal chart inch, or full scale of the chart paper (sensi­ In its simplest form, an SP logging device tivity), is shown on the log heading of the consists of a movable lead electrode, which left track. The small single-conductor cable traverses the borehole on an insulated wire; loggers used in ground-water studies com­ a ground electrode, also made of lead; and a monly have SP sensitivities of 10, 25, 100, device for measuring potential, such as a mil­ and 200 mv per inch. Some loggers are fur­ livolt meter. The movable electrode senses nished with an auxiliary SP calibrator. The the ohmic-potential drop caused by the cur- main purpose of the calibrator is to check rents flowing into the mud column near the logger function on all ranges of SP sensitiv­ shale-aquifer boundaries. Because potential ity. The calibrator contains a battery and at the ground electrode remains constant, the resistors so as to provide a predetermined potential read from the meter represents the potential, in millivolts, which is applied to change in potential in the mud between the the SP electrodes of the logger. Each range shales and the permeable formations. Figure on the calibrator should produce a l-inch 11 shows the SP measuring circuit and, also, deflection of the recordser pens when the SP the electrical equivalent, as depicted by Lynch sensitivity control on the logger panel is set (1962). In this diagram the upper E, repre­ in the corresponding range. Calibrators sup- sents the shale streaming potential, the lower plied with the single-oonductor loggers are Ek is the streaming potential developed across not precision devices, and deflections on cor­ the sand, Eb is electrochemical potential de­ responding recorder ranges may or may not veloped at the liquid junction (mud filtrate- exactly equal 1 inch (or one division). Ac­ formation water), and E, is the membrane curacies are generally on the order of +lO or shale potential. The terms rsh, rss, and r,,, percent. are the resistances of the shale, sandstone, and mud, respectiv’ely. Radius of investigation The potential drop along the mud column results from currents that originate away from the borehole along formation bounda­ ries. For example, in a lithologic section, such as that in figure 12 (Schlumberger Well Sur­ veying Corp., 1958), the currents tend to flow from the borehole into the permeable beds until a sufficient cross-sectional area of the compact (very resistive) formation is

SHALE = encountered to carry the current. The current E­ then flows across the large area of resistive formation until an impervious conductive shale bed is intersected. The currents then travel with greater density along the con­ ductive beds until the borehole is intersected again. Therefore, the radius of investigation is highly variable. Also, the SP may not re- A late directly to the permeable beds because the effect of these beds spreads the SP above

Figure 11. - Spontaneous-potential measuring circuit (A) and and below the bed boundaries, as shown by the electrical equivalent (El. From lynch (1962). the SP curve on the left-hand side in figure 12. APPLICATION OF BOREHOLE GEOPHYSICS 29

Noise from the part of the armored cable near the surface in the borehole can also be coupled into the SP ground electrode. If the well is cased to considerable depth, the cas­ ing may act as a shield around the wetted upper part of the cable to effectively screen the SP ground electrode from cable noise. This type of noise is most troublesome where surface formations have high resistivities. Corrective procedures to lessen cable noise caused from battery effect are based on the electrical isolation of the SP electrode and the SP ground electrode from the cable. Wrap- ping the armored cable with insulating tape for 10 feet or more above the SP electrode may displace the battery effect far enough uphole to reduce the magnitude of this source of potential. Moving the SP ground electrode as far as possible from the well head may also help. Most noise-reduction procedures Figure 12. -Spontaneous-potential current flow in highly re­ are largely trial-and-error processes because sistive beds. From Schlumberger Well Surveying Corp. (1958). the source of noise generally is not evident. Other sources of noise and anomolous SP Extraneous effects are magnetization of armored cable, currents SP noise can be defined as any spurious B or unwanted signals that are not correlated with the actual SP in the borehole. Noise and anomalous potentials are relatively common SP problems in SP logs. Some of the early model loggers used insulated cable with a single conductor, and the SP was relatively free from noise. With the introduction of steel- armored cable, noise became troublesome. Steel is electrochemically active, and when the cable is immersed in an electrolyte (water or drilling mud), a battery effect develops along its entire wetted length. If the cable remains motionless in the drill hole, the bat­ teries become polarized, and their output re- mains constant. This small current impresses on the SP electrode a potential that merely shifts the SP curve left or right. When the cable begins to move in the hole, however, CABLE ARMOR the polarization film is wiped off intermit­ tently, and the current output from the bat­ tery effect is therefore varied. The varying potentials resulting from cable motion are thus impressed on the SP to produce noise. The source and effect of this noise is shown

in figure 13 (Electra-Technical Laboratories, Figure 13. - Spontaneous-potential noise from electrolytic action 1959). of steel cable. From Electra-Technical laboratories (1959). 30 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS set up by casing corrosion in the Iogged well portional to the potential difference, E, caus­ or in nearby wells, magnetic storms from ing the flow. Another piarameter, resistance, solar flares, flow of ground water through r, determines the rate of flow and enters the well bore, and manmade effects. In this Ohm’s law as a constant to proportionality, last category fall such influences as circulat­ expressed as the formula ing ground currents near electrical switch E = Ir, yards and transformer stations, electrochemi­ where cal action along buried pipelines, cathodic E is in volts, protection of buried pipelines, and potentials Z is in amperes, and set up along large or long metallic objects, r is in ohms. such as railroad tracks. If the SP ground The resistance of a conductor depends not electrode is in the mud pit, even the operation only on the nature of the conductor, but also of the rig mud pumps, or generator, or the on the cross-sectional area and length of this dragline cleaning of the pits may disturb the conductor. Resistance-logging devices mea­ SP (fig. 8). Water moving into the hole may sure the resistance, in ohms, of the earth ma­ be located by SP noise. The oscillating SP terials lying between an inhole electrode and between 700-750 feet in figure 14 is probably a surface electrode, or between two inhole due to streaming potentials caused by water electrodes. Actually, a potential difference in moving into the well bore. volts or millivolts is measured between the electrodes, and this is converted to resistance Resistance Logging by Ohm’s law because a constant current is maintained. These qualitative-logging One of the basic concepts of electric theory methods are known as single-point, point- is Ohm’s law, which states that the rate of resistance, or single-electrode systems to current flow, I, through a conductor is pro- distinguish them from the two- and three-

SPONTANEOUS. ESTIM ‘ED CLAY CALIPER LOG POTENTIAL SINGLE-POINI RESISTANCE NEUTRON.EPITHERMAL ALlI ATION LOG FRO DEPTI FEET 300

600 -600 3

r -700

s

800

50 0 INCHES CENT Figure 14. - Geophysical logs of granite in drill hole CX 111, Clear Creek County, Cob. c APPLICATION OF BOREHOLE GEOPHYSICS 31 electrode systems used for quantitative mea­ surements of resistivity. (See section on “Resistivity.“) The main uses of point-resis­ tance logs is for geologic correlation, such as determining bed boundaries and changes in lithology and identifying fractures in resis­ tive rocks.

Principles and applications

The point-resistance log, as it is commonly CABLE SHEATH called, is the simplest electric-logging system. In its simplest form, using insulated cable, a \ single lead electrode is lowered into the hole and the return path for the current flow is furnished by the ground electrode, which is also made of an inert metal, such as lead. \I/ This method is herein termed the “conven­ -A’ lb\ A- tional” single-point system. Another single- point resistance system currently in use \ employs the tool shell as the return electrode for the current. This method is herein termed Figure 15. - Conventional simultaneous single-electrode resistance the “differential” single-point system. and spontaneous-potential logging system. From Guyed (1952). The electrode arrangement and current flow through a homogeneous isotropic media ohms of resistance between 10 and 20 ohms for the conventional point-resistance system on the meter face is much larger than the is shown in figure 15 (Guyod, 1952). The percentage of deflection for the lo-ohms in­ electric current from the AC generator (fig. crement between 100 and 110 ohms on the 15) travels down the cable to electrode A, meter face. Thus, the single-point logging moves radially out through the surrounding system exhibits a nonlinear response that is rock, and returns to ground electrode B. Each compressed at the higher resistances. current electrode, A and B, however, does With application of a constant current double duty and functions as a current elec­ from the AC generator (fig. 15), the poten­ trode and as a potential-sensing electrode. tial variations read on the meter are inversely Current electrode A and potential electrode A proportional to the resistance between the A are physically the same piece of metal, and electrode and ground electrode B. Because surface-current electrode B and potential current and potential points A are on the electrode B are also physically the same. same electrode, the radius of investigation is Because the current and potential electrodes small, from about 5 to 10 times the electrode are coincident, the single-point device func­ diameter. We can assume that the resistance tions in a manner similar to a VOM switched measured by the point resistance to consist to the ohms position. The unknown resistance of three resistances, in series according to an is, in effect, connected in series with a battery unpublished study by Hubert Guyod, Con­ and the meter movement. When the unknown sultant (written commun., 1968) : (1) The resistance is relatively small, a large current resistance of a spherical volume of borehole flows to deflect the meter movement, but fluid and rock surrounding the point electrode when the unknown resistance is relatively to a radius of 10 electrode diameters, as men­ large, only a very small current flows to de­ tioned above ; (2) the resistance of a surface- flect the meter because the battery voltage electrode hemisphere, the counterpart of the remains constant. For example, the percent- borehole-electrode sphere; and (3) the resis­ age of full-scale meter deflection for the 10 tance of the volume of rock lying between the 32 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS resistance sphere in the borehole and the circuit, current leaves electrode A and re- resistance hemisphere at the surface. Item 3 turns to electrode B, which is the shell of the above includes such a large volume of rock tool, separated from the A electrode by less that its resistance is very small, and can be than 1 inch of insulation. In the differential regarded as a negli,gible constant. system, the shell and the cable armor act as Because the radius of investigation is the return path for resistance, and the con­ small, the single-electrode log is strongly af­ ventional ground electrode is used only for fected by conductivity of the borehole fluid SP. Because the electrodes are so closely and by changes in hole diameter. If a bed to spaced, the current configuration is torus be logged has a higher resistance than the shaped, with the great,est current density borehole fluid, only a small amount of current parallel to the plane of the insulation between will flow in the rock, and most of the current electrodes. Due to the focusing of the current, will flow in the borehole fluid. As a result, the differential-resistance system is very sus­ thin resistive formations do not significantly ceptible to changes in b,orehole diameter. In affect the flow of current and may be difficult fact, the method can be used to locate frac­ to distinguish if the single-point method is tures with greater accuracy than with a used in salty mud or brine-filled holes. At caliper log because the differential-resistance points of hole enlargement (caving, wash- method detects nearly closed fractures not outs, fractures) the single electrode mainly delineated by the caliper tool. Figure 18 com­ measures the resistance of the borehole fluid, pares the response of the conventional resis­ which results in a decrease in resistance on tance log with that of the differential system the log. Despite these apparent shortcomings, in a borehole penetrating fractured crystal- the single-electrode method is a very useful line rocks. Some of the sharp spikes on the tool, inasmuch as any increase in formation left of the differential system at fluid-filled resistance produces a corresponding increase fractures are identified by the letters A in resistance on the log, and the deflections through H. As seen in figure 18, the type of on the single-point logs are interpreted to log response can be altered considerably by be due to changes in lithology. Reversed log choosing one point resistance in preference response due to the effects of thin beds and to the other. If a smooth, more or less inte­ adjacent beds are common on multielectrode grated response is preferred, the conven­ logs, but are not found on single-point logs. tional point resistance should be used. If, Figure 16 shows a typical response to lithol­ however, a sharply defle,cting log responding ogy for a point-resistance device. The resis­ to minor fractures and incipient fractures is tance increases next to the lignites, sands, required, the differential system should be and sandstones, as seen at letters A, C, D, F, used. H and I. The resistance decreases next to the Figure 18 also illustrates the qualitative shales, siltstones, and clays, as seen at letters nature of point-resistance logs. Both logs B, E, and G. The single-point logs are highly were run in the same borehole and, except desirable for geologic correlation because of for extreme peaks, were adjusted to give their unique response to changes in lithology approximately the same amplitude of deflec­ and the good vertical detail obtained in for­ tions. The log on the left shows a resistance mations of low to moderate resistance. In- scale of 10 ohms per unit, whereas the log creases in hole diameter appear as excursions on the right shows 100 ohms for the same to the left on the single-point log. In frac­ horizontal unit. The 10 to 1 scale difference tured rock the single-point log commonly between two single-point logs of the same appears as a mirror image of the caliper log. borehole points out the advisability of not The other type of single-point system uses attributing quantitative values to point-resis­ a differential-resistance measurement. A dia­ tance measurements made in the usual way. gram of the circuit of the differential-resis­ Quantitative values cart be obtained from tance system is shown in figure 17. In this conventional single-point loggers by using r- APPLICATION OF BOREHOLE GEOPHYSICS 33

“F’E’ETT”LITHOLOGY. SINGLE-POINT RESISTANCE LOG DEPTH, FEET 400 -400 Shale and sandy clay lignite and some shale A

Shale and siltstone layers t?

lignite and some clay e

Clay, silty

Silty, fine sand and some clay

500 500 Silt, sandy clay, becoming very sandy E Shale and lignite T- F Clay, silty, some lignite

600 600 l Sond, very fine Sondstone -- Sand, very fine = _ - -.---y7 Sand and cloy -=. .: ‘.‘.’ Sondstone -T-Z­_-- xc- - Sand ond clay I -- Sandstone

Clay and some lignite

Cloy ond some fine sand 700

Clay ond some lignite ond sand

Sondstone

Sand and cloy

Figure 16. - Response of single-point log to lithology, test hole 3,433, Mercer County, N. Dak. two ground electrodes, making a series of tionary in the hole, then measuring the resis measurements from each, in turn, to the tance between the two ground electrodes, and logging electrode, while the latter is held sta- placing the values in a suitable equation. The B 34 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS .

method is somewhat involved and gives val­ ues for discrete points in the hole rather than a continuous log. To examine the response of the differential single-point resistance to borehole fractures more closely, figure 19 was prepared by plot­ ting the caliper log on the right side and the mirror image of the point resistance on the left (compare with left side of fig. 18). Some of the fluid-filled fractures on the sensitive \ CABLE ARMOR caliper log show up to good advantage on the reversed differential single-point log, shown by the letters A through H. Another feature of the single-point method that works to advanta.ge is the nonlinear scale. At low resistances the response is am­ plified with resulting detail, whereas at high resistances the response is flattened. In other D words, a very large range in resistance can be portrayed on the log without off-scale Figure 17. - Differential-resistance system, showing current deflections at’the high-resistance end. distribution. A comparison of a commercial laterolog and a differential single-point log made in the

CON”RNT,ONAL SINGLR-POINT DE,11 DEFT”, “RSISTANCE LOG IESISTAWCE LOG REVElSED SINGLE-,O,NT RESISTANCE LOG SENSITIVL CALIFER LOG FEET FEE1

100 0 -E

100

Figure 18. - Examples of differential and conventional singlw point resistance logs of fractured crystalline rocks. Golden Figure 19. - Identification of fractures from caliper ond differen. Gate Canyon well, Jefferson County, Cola. tiol single-point Iresistance logs. APPLICATION OF BOREHOLE GEOPHYSICS 35 same hole (fig. 20) illustrates the sensitivi­ the logger when the sensitivity control on ties of the single-point log over a wide range the logger panel is set in the corresponding of resistivity, as well as hole-diameter ef­ range. fects, the major enlargements of which are shown by letters A through D. The somewhat Radius of investigation focused character of the current around the differential single-point sonde produces a Single-electrode systems have very little log response which is markedly similar to the radius of investigation and actually measure strongly focused current in the laterolog. the resistance of the borehole fluid as it is affected by the resistance of the surrounding volume of rock. Instrumentation The equipment for single-point logging is Extraneous effects similar to that used for the SP, and includes the sonde, logging cable, winch, uphole cir­ Single-point loggers used are susceptible cuits, and recorder. Single-conductor loggers to several of the same kinds of noise that af­ currently in use by the Water Resources fect the SP logs. Noise generated by dirty Division will log SP and resistance simul­ brushes or by worn slip rings on the winch taneously. This is possible because the SP can cause a periodic sharp deflection to be is a measure of direct current, and the resis­ superimposed on the resistance curve. This tance is measured with alternating current. type of noise can sometimes be spotted be- Typical logging circuits for measuring SP cause it occurs at regular intervals, related and resistance are shown in figures 15 to the winch revolutions. Another type of and 17. anomalous effect is caused by ground cur- rents from power-lines or other electrical sources beating against the alternating cur- Calibration and standardization rent used in the resistivity electrode. The The point-resistance method measures re­ beats appear as a sine wave superimposed sistance in ohms and should be calibrated in on the resistance curve. Some loggers have a ohms. The number of ohms per inch of span provision to eliminate beats by adjusting the (sensitivity) should be indicated at the log frequency of the AC current used in the re­ heading on the right-hand part of the record­ sistance electrodes. Another way to mini­ er chart. The single-conductor loggers com­ mize these beats is to adjust the frequency monly used in water-resources studies have of the llO-volt alternator (which supplies resistance scales on the order of 20, 50, 100, the logger) so as to lower or raise the nom­ 200, and 500 ohms per inch (or full scale on inal line frequency of 60 Hz. some makes of loggers). The SP calibrator Another type of erratic behavior of the furnished with conventional single-point log­ resistance log occurs occasionally with the gers can also be used to calibrate resistance differential system. The cable armor is by switching off the internal battery and grounded to the truck, but is isolated from using the resistors to supply a predeter­ the well casing by an insulated sheave. The mined resistance across the electrodes of the SP ground electrode in this system is isolated logger. This device allows the operator to from both the cable armor and the truck determine that each scale is functioning, and ground. If auto tires were perfect insulators, permits calibration of the logger, in ohms the circuits would remain isolated ; however, per unit of chart. Erroneously, many of the tires contain a conducting medium (carbon), older loggers had scales calibrated in ohm- and mud and water add to conduction. The meters. Each range on the calibrator should resistance of the earth material at the sur­ produce a l-inch or full-scale deflection on face also affects this problem. The result is 36 T%OHNIQUES 0F WATER-RE~O~RCE~ INVESTIGATIOIW

DEPT LATEROLOG SINGLE-POINT RESISTANCE LOG CALIPER LOG FEE1

1100

1200

1300

17 *IO- -11 6 10 20 80125 500 OHMS 8 OHMS HOLE DIAMETER, INCHES

Figure 20. - Response of laterolog and differential single-point logs to changes in hole diameter. Core hole No. 2, Rio Blanco County, Cola. a finite resistance from the truck frame to extremely high resistance, such as in crystal- the ground. In effect, the truck ground (cable line rocks saturated with fresh water. armor and resistance ground points) are connected to the earth (SP ground rod) through a large but finite resistance. Part of Resistivity Logging the current now follows this path, and up The linear and cross-sectional dimensions sets both resistance and SP logs. This situ- of a conductor can be included mathemat­ ation generally causes difficulty in obtaining ically with its resistance to obtain a property useful logs if the materials in the hole have that depends only on the material nature of APPLICATION OF BOREHOLE GEOPHYSICS -’ 37

the conductor, and not on its dimensions. a known section and length of the sample This property is called resistivity, or specific (fig. 21). The current flows from electrode A resistance, and is defined b ‘the formula: of higher potential to electrode B of lower P potential. Ordinarily, the potential drop, in R = r Xg=‘;zhrns x volts or millivolts, between electrodes M and meters X meters N is measured, and the current between = ohms x meters, meters electrodes A and B is held constant by suit- abbreviated ohm-m or am, where able circuits. The more resistant the sample, R = resistivity, in ohm-meters ; the greater the voltage drop between elec­ r = resistance, in ohms ; trodes M and N. Both in laboratory mea­ S = cross section, in square meters ; and surements and in actual logging, alternating L = length, in meters. current is used as the source of potential. The units of resistivity, therefore, are resis­ Direct current is unsatisfactory because a tance times length, conventionally in ohm- DC voltage applied to the natural electro­ meters. Resistivity, in ohm-meters, can also lytes in rocks would cause the migration of be defined as the resistance, in ohms, between anions and cations and, hence, introduce opposite faces of a cube of conductor that polarization effects. measures one meter on each side. Except for conductive minerals like graph­ Resistivity-logging devices measure the ite, metallic sulfides like pyrite and galena, electrical resistivity of a known or assumed and native metals, such as silver, most min­ volume of earth materials under the direct erals are good insulators when they are dry. application of an electric current or an in­ Completely dry rocks rarely occur in bore- duced electric current. Depending on the holes, and subsurface formations have mea­ device employed, resistivity logs can be used surable resistivities due to formation water for geologic correlation, although this is not in rock pores, solution channels, and ad- the most important application. Resistivity sorbed water on clay particles. The resis­ devices are generally used to determine the tivity of a rock therefore depends on the formation resistivity, formation porosity, composition of the contained water, on the mud cake resistivity, invaded zone resistivity amount of water, and on the shape and and porosity, hydrocarbon and water satura­ tion, fluid resistivity, and formation factor. Measuring resistivities in a borehole is E similar to the technique for measuring resis­ Q- tivities in the laboratory or in surface-resis­ tivity investigations. Figure 21 shows the similarity of electrode arrangement and nomenclature for resistivity measurements ) in both cores and boreholes. In practical well 7’L f logging, however, only two or three of the i electrodes actually influence the measure­ Con~"r,we ment, as the remaining electrodes are located 1G 1 Mud remotely from the sphere of influence. \ By convention, the current electrodes are designated by the letters A and B, and the potential electrodes are designated by the

letters A4 and N (fig. 21). Two outer elec­ CORE MEASUREMENlS BOREHOLE MEASUREMENTS trodes, A and B, supply current to the sam­

ple under study, and two inner electrodes, Figure 21. - Electrode arrangement for resistivity measurements M and N, measure the potential drop across in cores and in boreholes. 38 TECHNIQUES OF WATER-RESOURCEd INVESTIGATIONS length of the interconnected pores. True re­ Normal devices sistivity, Rt, is given by the formula Normal logs measure tk apparent resis­ tivity of a volume of rock surrounding the electrodes. The sho?t normals give good ver­ and is based on precise control of the tical detail and record the apparent resistiv­ ity of the invaded zone. The long normals current, I, through the rock sample and on record the apparent resistivity beyond the the accurate measurement of the voltage invaded ‘zone. The normals give poor results drop, E, between the potential electrodes. The cross-sectional area, S, and the length, in highly resistive rocks. The tool used by L, are measured directly on the sample. commercial services also logs the lateral Usually, the samples are in the shape of a curve. C:ommercial service tools are about cylindrical core of predetermined diameter 8 feet long and 35/8 inches in diameter; they and length, thus simplifying the geometric weigh up to 125 pounds, and require a heavy factor S/L (fig. 21). rubber-covered bridle more than 50 feet long In the field, resistivity is generally mea­ that carries the current and reference elec­ sured in a borehole that penetrates the rocks. trodes. Generally, a tower, an A frame, or a derrick over the hole is needed for com­ Although the current and potential electrodes mercial service electric logging. on the sonde may occupy the same relative positions as they do in core measurements, the current flow lines are no longer confined Principles and applications to a cylindrical shape as they would be in Multilple-electrode resistivity measure­ the core sample; thus, the geometric factor, ments include such curves as the short and G, is no longer a simple ratio of area to long normal, lateral, microlog, and focused length. logs (guard log and laterolog). These are The combined effect of the new geometric resistivity devices, and their curves are cali­ factor, G, and the current-distorting prop­ brated in ohm-meters. Table 3 cross refer­ erties of the mud column results in an ap­ ences the generic log types with the numerous parent resistivity measurement, which is an trade names applied to these logs by the average of the resistivities of the rocks sur­ commercial service companies. rounding the sonde. This apparent resistivity, The normal curves are derived from a R,, is the value recorded by the electric logging four-electrode system, using two current equipment and is expressed by the formula electrodes, A and B, and two potential elec­ R, = E/Z x G (fig. 21). Values for G must trodes, &l and N. Because only two electrodes, be calculated from the electrode arrange­ A and iW’,are effective in measuring apparent ment and from the electrode spacing for each resistivity, the normals are sometimes called type of electric logging device. the two-electrode method. Spacings between In petroleum logging, the value Rt repre­ electrodes A and M gave rise to the names sents the actual resistivity of the formation of the various normal curves; for example, which may contain both water and oil (or the g-inch, lo-inch, 16-inch, l&inch, 32-inch, gas), and R, represents the original resistiv­ 39-inch, 40-inch, 63-inch, 64-inch, and 84- ity of the formation where it is loo-percent inch normals. Most logging companies have saturated with water. Because hydrocarbons standardized on the short normal, where AM are electrical insulators, R, is greater than = 16 inches (some use 18 in.) and the long R, for the same formation, and empirical normal, where AM = 64 inches. These two relations between them are used in petroleum normal curves are generally run at the same work to determine water saturation in the oil time. reservoir. Generally, in hydrologic investiga­ The electrode arrangements for normal tions, only the water-saturated zones are logging are shown in figure 22. The diagram logged, and the value of R, is obtained from shows that electrodes A and M are relatively the apparent resistivities. close together, whereas B and N are not APPLICATION OF BOREHOLE GEOPHYSICS 39

Table 3. - Electric log cross reference

Generic IOP- tvm__ Service Co. Wall spontaneous Electric log’ Focused Microfocused Induction log potential (nonfocused) electric log resistivity

Birdwell ____.______Spontaneous Electrical log . Guard log Micro Contact ______...... Induction-electric potential. log. log. Dresser Atlas.. .__...... do .___._____Electrolog _____.Laterolog. Minilog ______.Microlaterolog Induction electrolog. ______do ____.______. Induction electrical log. Schlumberger.. .______do ______Electrical log .______do______Microlog .___. Proximity log.. Dual-induction 1 laterolog. Welex ____..____..___,.._._.____ do ______Electric log... Guard log Contact log _____FoRXO log _.____Induction electrical log.

‘Includes short normal. long normal. and lateral. ZIncludes 5FF40 induction log, 6FF40 induction log, and short normal or later&g.

only far from each other, but are also distant This value is substituted in the formula from the electrode group AM. In practice, the distance from the group AM to the B or iV electrode is about 50 feet. This means that in order to convert the potential measured the apparent resistivity is determined pri­ into an apparent resistivity. marily by the potential of the measuring A hole is generally logged from bottom to electrode M. Figure 22 also shows that the top, so that the long normal electrode, which physical location of electrodes B and N has is near the top of the sonde, is the first elec­ no bearing on the normal curves, provided trode to enter a given bed or formation. The they are distant from each other and from long-normal pen deflects first; then, when the AM electrode group. The geometric fac­ the sonde has moved 2 feet (the space be- tor, G, for the normal device is equal to tween reference points), the short normal 4nAM, where the distance AM is in meters. begins to “see” the same bed. Both normals are calibrated at zero on the A.C tan left edge of the resistivity side of the record­

E er chart. To distinguish one from the other 0 when the curves cross or run nearly the same, the long normal is shown as a dashed trace. The spacing of the short normal (AM = 16 in.) was selected by the commercial log­ ging companies because this spacing gives good vertical detail and measures an ap­ parent resistivity that is related primarily to the invaded zone resistivity in petroleum logging. The long-normal curve (AM = 64 in.) was selected by commercial logging com­ panies to investigate the average resistivity Rslsrencs point-_. 61” Normal beyond the invaded zone for a radius of ap­ 6” proximately twice the AM spacing. The short- and long-normal logs are generally used in conjunction with departure graphs

Figure 22. - Electrode arrangements for the 16- and 64-inch or other interpretative charts (Schlumberger normals. Well Surveying Corp., 1966)) so that invaded 40 TECHNIQUES OF WATER-RESOURCE$ INVESTIGATIONS

zone resistivity (Ri), true formation resis­ tivity (Rt), and depth of invasion (di) can be completely and accurately determined. The true resistivity of beds that appear infinitely thick to the sonde (10 ft for short normal ; 25 ft for the long normal) can be determined from the normal logs by using departure dia­ grams, Figure 23 shows the relative radii of investigation of the point-resistance device, .* as well as the 16-inch and 64-inch normals in Point Resistant* a homogeneous medium. The volume of inves­ O \ tigation has gradational limits and depends, in part, on the resistivities of rock and mud. U-inch Normal Therefore, the volume sampled constantly varies as the sonde traverses the borehole. Figure 23. -- Relative radii of investigation of point-resistance Nevertheless, for the type of formations for and normal devices. which the normal devices are most efficient would work satisfactorily, also. Periodic log­ (formations of low to intermediate resistivi­ ging through plastic screen permits the mea­ ties), the region that contributes the major surement of changes in porosity and fluid part of the resistivity signal is, on the aver- resistivity during pumping or injection. age, a sphere centered at the midpoint be- One important application of the normal tween A and M and having a diameter equal curves is in the estimation of water quality to about 2 times the AM spacing. in elastic rocks from electric logs. A relation Using 4-inch and 16-inch normals, Water between ionic concentration of formation Resources Division researchers have succeed­ water and the long-normal resistivity read­ ed in making valid logs in well bores lined ing can be established empirically. Early with short lengths of plastic well screen. A work by .Jones and Buford (1951) on labora­ test hole was drilled and logged using the tory samples and later work done under field 4-, 16-, 32-, and 64-inch normals. After log­ conditions by Turcan (1966) have shown ging, a lo-foot length of plastic well screen how the ratio can be determined for a specific was placed in the hole opposite a resistive ground-water environment. Once determined, sand bed. The plastic screen has 20-foot the relationship can be used to approximate lengths of blank plastic pipe both above and water quality from electric logs. below. After screening, the hole was logged Hubert Guyod (written commun., 1968), again, using the same equipment settings. mentioned that three tacit assumptions are When the sonde entered the plastic pipe be- made when the method is applied : (1) the low the screen, the pens moved rapidly to aquifer is elastic, (2) the aquifer has a rela­ the right, showing the sudden jump to nearly tively constant porosity and clay content infinite resistivity. With the sonde in the both vertically and laterally, and (3) the screened part, it was found that adjustment apparent resistivities, measured by the 64- of the zero-suppression control on the equip­ inch normal, approximate the undisturbed ment would bring the 4- and 16-inch normals formation resistivities. back on-scale. After some experimentation, The aquifer should be composed of elastic it was possible to log the screened part and rocks or, at least, should behave electrically obtain nearly the same normal curve as was the same as elastic rocks because pore geom­ run in the open hole. With only 10 feet of etry exerts a great influence on resistivity. screen, only the shortest of the normals would According to Keller and Frischknecht (1966)) have both the A and M electrodes within the rock can be categorized into three groups on screened part for a sufficient distance to the basis of pore geometry-intergranular develop a meaningful curve. With long porosity, joint porosity, and vugular PO­ lengths of plastic screen, the longer normals rosity. The porosity of elastic aquifers con- APPLICATION OF BOREHOLE GEOPHYSICS 41 D sists mainly of intergranular pores saturated (3) specific conductance and dissolved solids. with water, and the bulk resistivity is pro­ The rationale of the method is predicated on portional to the formation-water resistivity. first establishing the field-formation factor’ Not only sand and sandstone, but also many for an aquifer or for a hydrologic regime porous limestones and dolomites behave elec­ from preexisting electric logs and water anal­ trically the same as elastic rocks. The yses. When a new well is drilled into this porosity of dense igneous rocks, such as gran­ aquifer and logged, the long-normal-curve ite, may exist almost entirely in joints, with resistivity is used to determine fluid resis­ very little intergranular porosity. Because tivity in the aquifer. The value thus found only the water-saturated joints carry the cur- is converted to specific conductance at stan­ rent, the resistivity of dense rock is dependent dard temperature from tables or formula. on joint geometry and may attain values of The specific conductance is then converted to several thousand ohm-meters. Some carbon- dissolved solids and chloride content by em­ ate rocks have vugular porosity, consisting pirical relationships previously determined of large irregular cavities linked together for the aquifer. By these determinations, by smaller connecting pores. Rock with a reasonable estimates of total dissolved solids high ratio of vugular porosity to intercon­ netting porosity will have a higher bulk and chloride content can be made over broad ! resistivity than a rock of the same porosity areas, solely on the basis of electric logs in that has a low ratio. Formation factors elastic rocks. computed from resistivity measurements of Using the Wilcox Group as an example jointed igneous or vugular carbonate rocks will best illustrate the three steps required will give misleading results when used to to apply the method described by Turcan. compute the water quality. 1. Determine F, from preexisting logs and The clay content of the aquifer is a signifi­ analyses. cant factor because ion exchange by clay a. Find fluid resistivity from the l minerals makes water in the rock pores con­ relationship ductive, even though the water after extrac­ 10,000 R,= tion from the rock is very resistive. If the Specific conductance ’ saturating water is highly saline, the increase or use graph shown in figure 24 in conductivity produced by ion exchange (from Turcan, 1966). is relatively minor, but in fresh-water aqui­ b. Find resistivity of the aquifer, fers, ion exchange from the rocks may dras­ R,, at borehole temperature tically alter the resistivity of the water (while from the long-normal curve. it is in intimate contact in the rock pores). Correct R, to standard temper­ According to Keller and Frischknecht (1966, ature by use of the Schlumberg­ p. 25)) “even in rock with very low exchange er chart (fig. 2, this manual), capacities, pore water resistivity rarely ex­ ceeds 10 ohm-m.” or use figure 10 of Jones and When applied to the fresh-water aquifer, Buford (1951) or figure 12 of the departure of apparent resistivity of the Olmsted (1962). (Also see R, long normal from true aquifer resistivity columns in the table in fig. 24, will be approximately the same, provided the this manual.) beds are relatively uniform both vertically c. Calculate Ff from the relationship and laterally and are more than 20 feet thick. F,=R,, Turcan’s (1966) method for estimating water quality from electric logs makes use and see last table column in of mathematical expressions which relate the figure 24. following parameters : (1) Field-formation- 2. Determine relationship between specific resistivity factor and fluid resistivity, (2) conductance and total dissolved solids fluid resistivity and specific conductance, and \ 1 See section on “Formation Factor.” fluidresistivity and specific conductance, and 42 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

ship: log X = a log Y, or X a = Y”. As parts per million is shown on the X axis, and specific con­ ductance is shown on the Y axis, the relationship for the Wilcox Group becomes: Dis­ solved solids (in ppm) = X0.93, where K = specific conductance, in pmhos/cm. (See fig. 25B.) 3. Determine water quality from the newly made electric log. a. Read R, value from the long-nor­ mal curve in a water-bearing bed (R, = 23 ohm-meters, as 0.5 1.0 5 10 shown in fig. 26B). RESISTIVITY,ohms-mz/m b. Correct the 23-ohm-meter value at 69°F to 21.2 ohm-meters at 7’7°F (Standard temperature). Field formation resistivity factors for aquifers of the Wilcox Group, northwestern Louisiana c. Enter corrected R, value and Ff value determined in Step 1, Specific RO R,’ R”= Well conductance (ohms- (ohms- above, in formula F, = R,/R,o, (~mhos/cm (ohms- nP/m Ff3 NO. m=/m m2/m ) at 71°F) at 77°F) at 77°F) and solve for R,. R, = 21.2/ Bo-137 ______605 16.5 35 34 2.0 2.4 = 8.8 ohm-meters for the 173 ____....2,470 4.0 12 11 2.7 Wilcox group. 180 ___..... 1,360 7.4 25 22 2.9 d. Find 8.8 ohm-meters on upper X 200 ...... 1,510 6.6 18 17 2.6 axis in figure 25A. Read 205 ______1,080 9.3 17 15 1.6 straight down to find specific Cd-360 _.______953 10.5 20 19 1.8 430 . . 1,580 6.3 14 13 2.1 conductance of 1,100 Pmhos/ Sa-292 ____.... 905 11.0 30 28.5 2.5 cm. Na-54 ______1,280 7.8 17 16.1 2.1 e: Still using figure 25A, find inter- 55 .___...... 1,040 9.6 25 31.4 3.2 section of the 1,100 Pmhos/cm 58 ______.2,270 4.4 15 13.5 3.1 line with the two curves, and ‘R, = lO’/specific conductance (K). read dissolved-solids content *Resistivity reading from long-normal curve on electrical log at field temperature. of 650 ppm and a chloride con- V, (field formation factor) = R,IR,. tent of 140 ppm.

Figure 24. - Graph showing resistivity and specific conductance, Turcan showed that the dissolved-solids and table of formation factors. From Turcan (1966). content from a chemical analysis of water from this well is actually 586 ppm, and the chloride content from analysis is actually 130 and chloride content, both in parts per ppm. Figure 26A shows the type of area1 million.* map of field formation factor that can be a. Use graphical plot, as shown in drawn on the basis of electric logs and water figure 25A (from Turcan, analyses. Such a map makes spot estimation 1966). of water quality from electric logs relatively b. Or, derive empirical formula for quick and accurate over wide areas, a large dissolved solids from relation- part of ;a State in this example. 2The U.S. Geological Survey has reported water-quality data Another potentially useful application of in milligrams per liter. in preference to parts per million, since 1968. If the dissolved-solids content is less than 7.000 mg/l, or if the normal-resistivity curves to hydrologic the specific conductance is less than 10,000 hmhos/cm. then parts problems involves the calculation of the per­ per million values and the corresponding milligrams per liter meability of elastic aquifers. Croft (1971) values can be considered equivalent. a, APPLICATION OF BOREHOLE GEOPHYSICS 43 l RESISTIVITY, ohms-mz/m

SPECIFIC CONDUCTANCE, kmhos/cm DISSOLVED SOLIDS, ppm

A. B. l Figure 25. - Relation of dissolved-solids and chloride concentration, in ppm, to resistivity and specific conductance (A), and relation between specific conductance and dissolved solids (8). From Turcan (1966).

described a method he used for determining various porosities, sorting coefficients, and permeability in North Dakota. His method clay content can only be determined by fur­ makes use of a graph by Alger (1966) which ther laboratory analyses. However, Croft shows the relationship between permeability showed that his permeability values from and formation factor. Alger’s data were taken electric logs agree well with those obtained from original research by Jones and Buford by conventional methods. Inasmuch as the (1951). Alger’s figure 12 shows that there formation factor is the key to these perme­ is a systematic increase in permeability with ability values, the inhomogeneities of aquifer increase in grain size for graded sand sam­ sands may not be too restrictive on applica­ ples. His figure 12 also shows an increase in tion of the method because the formation formation factor with increase in grain size factor also accounts for tortuosity and ce­ for each of the nine sand samples. By recom­ mentation. In order to apply the method, we bining the two graphs, he derived figure 13, must determine the values of R, and R, for which shows an increase in F with an in- the particular aquifer. The electric log, (fig. crease in permeability. Because Croft was 27, left; from Croft, 1971) shows the five dealing with F values of less than 3, he had zones selected for analysis. Zone C, for ex- to extend Alger’s curve across one additional ample, has a field R, value of about 11.7 ohm- log cycle. meters from the long normal. The water The porosities of the graded-sand samples temperature is reported to be 56°F; hence, in Alger’s paper ranged from 40.0 percent adjusting the log value to 77”F, with to 45.3 percent. Whether or not the ,plots Schlumberger Chart A-6 (Schlumberger for laboratory graded-sand samples will ade­ Well Surveying Corp., 1966) results in an R, quately duplicate natural sand deposits of value of 8.5 ohm-meters. The R, value was 44 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

agree with those determined by aquifer tests. Because anomalous formation factors result from resistivity measurements of rocks with only joint or vugular porosity, the method should be applied only to elastic rocks or to rocks that behave like elastics.

Instrumentation The normal logs are generally run on mul­ tiple-conductor cable, so that two normals and the SP can be logged simultaneously. Commercial logging service companies often run a long-lateral curve along with the nor­ mals. The Water Resources Division four- conductor logger uses two conductors to carry the current down to the A and B electrodes and two1 conductors to carry the 16- and 64- inch potentials to the surface, with the SP picked off the .16-inch electrode. The two nor­ SPONTANEOUS mal curves and the SP are displayed on a POTENTIAL three-pen recorder. The Water Resources Division multielectrode-research logger em- ploys the system shown on the left side of figure 22. Electrode A is at the bottom end -- of a rubber-insulated steel mandrel, about ‘7 feet long, with two M electrodes located 16 a inches and 64 inches above A. These M elec- I trodes pick up the voltage changes for the NO NORMAL 16- and 64-inch normal curves. The cable head fastens to the top of the mandrel with a watertight threaded connection that is B. wrapped with insulated electrical tape. b Figure 26. -Areal plot of formation factors (A) and electric The armored cable is insulated with a rub- log (13) of Q well penetrating the Wilcox Group. From Turcan ber sleeve for 50 feet above the cable L (1966). head. Since the cable armor is used as the current-return electrode, B, the taped determined from the Schlumberger chart cable head and rubber sleeve effectively move after converting the chemical analysis of B more than 50 feet above the AM electrode formation water into an equivalent NaCl group. The voltage-return electrode, N, is a i solution by use of the multipliers on page lead sleeve on the end of a rubber-covered A-5 of the Schlumberger document. The R, wire and is placed in the mud pit. Electrode i value is 3.5 ohm-meters. Solving for forma­ N also is the ground return for the SP, which tion factor : is measured on the lower M (16-inch) elec­ F=R,=8.5=24 trode. The reference point of the short-nor­ R, 3.5 . * mal measurement is the midpoint between Formation factor is converted to perme­ the electrode A and the 1Binch M electrode. ability by use of the graph in figure 27 Because the spacing AM is different for the (upper right), and gives a value of 60 gpd/ short and long normals, the AM midpoints ft3 (gallons per day per square foot). The for the two curves do not coincide. The mid- permeability values of the other zones in the point for the long normal is 24 inches above well are given in the table in figure 27 and the short-normal midpoint. This difference d

e APPLICATION OF BOREHOLE GEOPHYSICS 45

LONG NORMAL

1 2 3 4 5 F (AVERAGE POROSITY = 41.5)

Formation factors and values for permeability at selected depths from the electric log shown.

Permeability Depth RO Formation factor R,

A ..._.___1,205 F5 1.9 20

B .___._._1,340 F5 2.9 150

c 1,395 -g 2.4 60

lb00 D ______1,600 $j 1.7 11

E ______1,655 3 1.1 5

Ygure 27. - Electric log of water well (left), graph showing permeability and F (upper right), and table of formation factors. From Croft (1971). is compensated at the surface by adjusting inclusive-require different pen separations. the long-normal recorder pen to read 2 feet Some commercial logging service companies higher than the short-normal pen. Other use optical galvanometers to record the var combinations of normals-4, 8, or 32 inches, ious traces. Generally, sufficient channels are 46 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS available so that a backup trace can be uti­ Calibration and standardization lized to record off-scale deflections. In figure The normal logging systems, like other 10, for example, the resistivities from the apparent resistivity devices, are calibrated in bottom of the hole to a depth of 510 feet are ohm-meters. A calibration device similar to less than about 100 ohm-meters. Between 510 the one used with single-point resistivity and 480 feet, the resistivity went off-scale, systems can be employed to establish a ref­ and the backup galvanometer (cross-hatched erence resistance for the normal curves. A pattern) recorded the resistivity from 100 calibrator that simulates both the contact to 175 ohm-meters on the long normal and resistance and the formation resistivity when a little less on the short normal. From 480 attached to the normal electrodes is shown to 450 feet, both traces are back on the 0- to in figure 28. The equivalent resistance for the lOO-ohm-meter galvanometer. From 450 to various resistivities is calculated from the 100 feet, the long normal is off-scale again, formula and is recorded on the backup galvanometer between 100 and 200 ohm-meters. The short R, = ;4=AM. normal is also off-scale from 450 to 100 feet, Because E = T, the formula is rewritten except for a small deflection back on the O- I to lOO-ohm-meter scale from 230 to 200 feet. rf = 4rrAM Part of the instrumentation of a normal R ’ logging system is some provision to maintain where 1~~= equivalent resistance of forma­ constant current into the formation through tion ; the current electrodes for a given resistivity 4=AFM = geometric factor for normal de- range and the option of selecting greater or vice ; and lesser current for each resistivity range. .R = resistivity, in ohm-meters. Constant current can be maintained by put­ ting a sufficiently high resistance between the power source and the current electrodes, so + To cable that, in effect, the same current will pass [n through the series circuit, regardless of the formation resistivity, within a limited range. B An alternate method is to use a constant- current generator, generally of solid-state Fh circuitry, to maintain current stability at the A and B electrodes. Several systems have been designed to M’64” Mudpit electrode (N) make normal curves with single-conductor loggers, but, to date, they have not been operational over a wide range of resistivities. One experimental system tried by the Water Resources Division used a local oscillator to generate current and used the potential from 100 the 16- and 64-inch electrodes to program M 16” ohms voltage-controlled oscillators. The resultant pulses were sent to the surface, where they were discriminated, integrated, and made into an analog signal. The inherent difficulty / , Normal sonde in this system, as with any single-conductor 1000 196 49.0 system, is the inability to select different currents for different resistivities in order 10,000 1960 490 to get an optimum potential from the pickup electrodes. Figure 28. - Normal-device calibrator. APPLICATION OF BOREHOLE GEOPHYSICS 47

For example, the value of 4rAM for the 16- across ri and will saturate the recorder in- inch normal is approximately 5.11. By using puts. this value and substituting various values of R, one can derive values of rf for each Bed-thickness effects resistivity. For 10 ohm-meters, rt equals 1.96 ohms ; for 100 ohm-meters, rl equals 19.6 The curves produced by the normal devices ohms ; for 1,000 ohm-meters, rl equals 196 are affected by bed thickness and resistivity. ohms ; and for 10,000 ohm-meters, rf is 1,960 Figure 29 (Lynch, 1962) shows four fairly ohms. The research logger uses a 300-volt typical examples of resistive beds: bed DC supply, which is mechanically chopped thickness (e) = > 6 AM ; bed thickness into a square wave. This square wave is = 11/2 AM; bed thickness = AM; and applied to current electrodes A and B (fig. bed thickness

0123456

e=GAM

e= I+A,M

---AM R*=l &-,=I

(a) e > AM

(b) 8 = I+AM 123456 I I 1 1 1

I+,=1 %- II23456 M I 1 I I I I Shale LS Rstl .I 4 e=AM A AM/2 L­

-F-

Sh

(cl 0=AM (d) e

- Theoretical Resistivity (Hole diameter infinitely small) ---- Actual Measured Resistivity e Bed Thickness

Figure 29. - Response of a normal device opposite beds of various thicknesses. From Lynch (1962). c APPLICATION OF BOREHOLE GEOPHYSICS 49 l recorded as conductive beds if their thickness Instrumentation is equal to or less than the AM spacing. Commercial service companies run the lat­ eral device simultaneously with the short and long normals. The lateral curve generally Lateral device appears in the third track of the recorder, The lateral log measures the formation resistivity beyond the invaded zone by use en. of widely spaced electrodes. The lateral is a deep-looking device and gives best results in beds whose thickness exceeds twice the electrode spacing. Its efficiency is poor in highly resistive rocks. Like the normal de- vice, commercial lateral-logging devices are long and heavy, and necessitate a derrick over the hole.

Principles and applications The lateral devices consist of three effec­ tive electrodes actually present in the bore- hole. The potential-measuring electrodes M and N are positioned below the current elec­ trode A. The electrode spacings on the lateral device are measured from current electrode A to the midpoint between electrodes M and N. This distance is called the A0 spacing and has been standardized at 18 feet, 8 inches, although older logs may be seen with spac­ ings of 4 feet, 8 inches; 6 feet; 9 feet; 13 feet; 15 feet; 19 feet; and 24 feet. The elec­ trode B is’located a remote distance from the group AMN and, therefore, has substan­ tially no effect on the curve. Figure 30 shows the electrode arrangement for the lateral device. The geometric factor, G, for the lat­ eral device is equal to 4~ (A0)2 MN, where distances A0 and MN are in meters. The prime objective of the lateral log is to measure true formation resistivity beyond the invaded zone by use of an electrode spac­ ing large enough to insure that the invaded zone has little or no effect on the response. The lateral device has its best response in Reference point beds that are more than twice the thickness of the A0 spacing (more than 40 ft) and 18’-8” lateral where the thick bed has contrasting beds above and below that are at least 1 A0 space thick. Figure 30. - Electrode arrangement for lateral device. 50 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

Sh

Ls \ -7 e C 2

e-5A0 I’1

c

Sh (b) e P $A0

(a) e > Ao \

Sh 6

Ls

e=AO

t Sh \ nb

I

I Shadow Peak (cl e = A0 (d) e < A0

Theoretical Resistivity (Hole diameter infinitely small) ---- Actual Measured Resistivity e Bed Thickness

Figure 31. - Response of a lateral device opposite beds of various thicknesses. From Lynch (1962). APPLICATION OF BOREHOLE GEOPHYSICS 51

although on some logs it appears in the sec­ wall-resistivity devices are about 5 feet long l ond along with the short normal. Like the and more than 4 inches in diameter, and they normal curves, the lateral requires multicon­ weigh nearly 100 pounds. A rig tower or A ductor logging cable. frame generally mu& be used due to the As with the normal devices,. the lateral great tension on the line caused by friction curves, are calibrated in ohm-meters, and as the device is pushed against the wall of external calibrators can be used to establish the hole. tool and instrumental response to various Wall-resistivity devices are very short resistivities. spaced multiple-electrode arrangements mounted on pads that hold the electrodes Bed-thickness effects against the wall of the hole. They give fine The most obvious characteristic of the lithologic detail and are also used to delineate lateral curve is its lack of symmetry about porous versus nonporous formations on the a given bed. Figure 31 (Lynch, 1962) shows basis of the presence or absence of a mud four fairly typical examples of resistive beds. cake. Figure 33 (Schlumberger Well Sur­ The curve is badly distorted by adjacent beds veying Corp., 1958) shows the electrode and by thin-bed effects. Not only do the arrangement and current lines for a micro- apparent resistivities depart from the actual normal and microlateral device. The pad value, but shadow peaks and dead zones show holds three dime-sized electrodes, 1 inch up near thin resistive beds whose thickness apart, designated A, M, and M2, from bottom is less than the distance A0 (fig. 31d). De­ to top. The electrode group AM2 provides a parture curves are necessary to correct lat­ micronormal (two active electrodes) with eral curves for thin-bed and adjacent-bed AM equal to 2 inches, and the electrode group effects. Very salty muds and changes in hole AMMz provides a microlateral (three active diameter also affect the response of the electrodes) with an A0 equal to 11/z inches. lateral device and must be corrected using The microlateral (called microinverse on departure curves for quantitative-log inter­ some logs) measurement is affected mainly pretation. by the mud cake because it “sees” only 11/z Figure 32 is an electric log of a water well inches into the wall of the hole. The micro- in Texas that penetrates thick limestone. The normal measures the resistivities just beyond log is fairly typical of the conventional re­ the mud cake and, thus, gives a resistivity sistivity survey because it records SP, 16- that is mostly influenced by formation resis­ and 64-inch normals, and the 13’3” lateral. tivities. The response of the lateral to thin resistive When the 2-inch normal continuously re- beds is exhibited by the sharp deflections at cords a higher resistivity than the microlat­ 709 feet and at 791 feet. The 16-inch normal eral, it is indicative of a uniformly thick mud also shows sharp resistive peaks at these cake. In general, the uniform mud cake oc­ points. The two beds must be 64 inches or curs on the more porous formations. In shale, less in thickness because the long normal there is no mud cake, and both microcurves exhibits cratering at both of these beds. (See read the same resistivity. fig. 29c and d.) The utility of wall-resistivity devices is limited by the minimum diameter hole they will fit, generally about 6 inches, and the Wall-resistivity devices maximum diameter which the bow springs The unfocused wall-resistivity devices or presser arms will accommodate, generally measure mainly the resistivity of the mud about 16 inches. Mud cake thickness, mud cake. They are used mostly to detect the pres­ cake resistivity, and depth of invasion all ence or absence of a mud cake. Commercial exert such a profound effect on the perfor- 52 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

iPONTANEOUSPOTENTIAI IEPTH, RESISTIVITY RESISTIVITY (MILLIVOLTS) FEET (OHMS, m2/m) (OHMS, m2/m)

SHORT NORMAL,,, o 18’8” LATERAL 5oo -A+ 5000 0 5000 o LONG NORMiAL,,, ------5000 I

I I I I 1 -- ‘-\

600

800

Figure 32. - Electric log of well drilled in limhone in Texas. APPLICATION OF BOREHOLE GEOPHYSICS 53

mance of this tool that it is sometimes re­ formation resistivity and the mud column, l garded mainly as a mud cake detector. so little current enters the formation that the Wall-resistivity logs appear under such resistivity curves lack character, bed bound­ trade names as Microlog, Minilog, Contact aries cannot be determined, and the ap­ log, and Microsurvey log. parent resistivities depart so radically from true resistivity that the latter cannot be de­ termined. The focused-resistivity devices were devel­ oped to overcome some of the limitations of the conventional resistivity systems. Focused devices include such electrode arrays as the guarded-electrode, laterlog, and the micro- focused devices. Figure 34 (Hubert Guyod, written com­ mun., 1968) shows the design philosophy of the guarded-electrode system and the basic difference between it and the unfocused de- vices. In figure 34, the borehole is assumed to have penetrated a formation that is much more resistive than the mud column. Dia­ Pad gram A represents the current pattern about a short electrode, such as is used for the normals. The current tends to follow the conductive mud for a considerable distance / relative to the length of the electrode. Where Mud-filled borehole

Figure 33. - Micronormal and microlateral device. From Schlum­ Mud-falled borehole / berger Well Surveying Corp. (1958).

Focused devices ‘i-i Focused-current devices are used to mea­ sure relatively high formation resistivities through a conductive mud. Focused devices give good vertical resolution and great pene­ c:1 tration, although their curves are seemingly distorted due to a nonlinear response at high­ er resistivities. (See fig. 20.) Commercial guard-log devices are 11 feet or more in length and 35/ inches in diameter, and weigh 150 pounds. A derrick or tower is required to handle this tool over the well bore. All the conventional resistivity devices dis­ cussed so far yield anomalous results when they are used to log formations that are much more resistive than the borehole mud column. Electric current tends to follow the path of A. least resistance, and under the above condi­

tions most of the current will Mow in the mud Figure 34. -Comparison of unfocused (A) and focused (8) de- column. For very high contrasts between the D vices (Hubert Guyed, written commun., 1968). 54 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

the resistivity contrast between formation electrode M and, in commercial logging sys­ and mud is very great, most of the current terns, ranges from 3 to 12 inches. These remains in the mud column. Diagram B focused systems are especially useful for log­ shows the current pattern around the elec­ ging thtin beds and for use in highly conduc­ trode if it were stretched out into a long tive mud. The length of the guard sections is cylinder. Near the center of the cylinder, the generally 5 or 6 feet ; the longer these guards, current is constrained to move at nearly right the flatter the current beam, and the greater angles to the surface of the electrode, and the depth of penetration. it is only at the ends that the current has the In general, the depth of penetration is tendency to fan up and down the borehole. about 3 times the length of one guard; thus, If a measurement of resistivity were made for a (j-foot guard (overall tool length is at point M, the relatively flat beam of cur- about ‘13 feet), the depth of investigation rent could be utilized to penetrate deeply will be about 18 feet. One failing of the into the formation. The current focusing of guard system is that no SP reading can be the various guard systems is achieved in obtained within about 25 feet of the top of this manner by the expedient of cutting the the uppermost guard when R,/R, is very cylinder in two sections, called guards, and high. For this reason, it is impossible to mea­ inserting a measuring electrode at point M. sure SE’ to the bottom of the hole when using Figure 35 (Doll, 1951) shows a typical guard- guard aystems. electrode system. The thickness of the cur- Another focusing system is the laterolog rent beam is approximately the length of method. Instead of the solid conductors of the guard method, the laterolog system employs a series of conventional electrodes A, AZ, lK1 Mt2, and MIMa to focus the current into a beam about 32 inches thick, the distance between O1 and O2 in figure 36 (from Doll, 1951). The depth of investigation is about the same as that of a guard tool whose guards are 5-6 feet long each. Owing to a thicker current beam than that of the guard tool, the bed definition by the laterolog is not so good as, that by the guarded-electrode meth­ od. A log from a laterolog device is shown on the left in figure 20. With the laterolog system, SP can be mea­ sured at the potential electrode almost to the bottom of the borehole because the metal­ lic guards are eliminated. Other types of focused equipment are cur­ rently in use in the petroleum industry, such as the microfocused devices. These devices are designed primarily to measure the resis­ tivity of the flushed zone (R,,) with the objective of obtaining the formation factor. The microfocused logs are wall-resistivity de- vices; they consist of small disk-shaped elec­ G Upper Guard Electrode trodes, less than 1 inch in diameter, which d Lower Guard Electrode are pressed against the walls of the borehole. M Meoourmg Electrode Figure 37 (Schlumberger Well Surveying Figure 35. - Guard-logging system. From Doll (1951). Corp., 1958) shows the electrode arrange- c APPLICATION OF BOREHOLE GEOPHYSICS 55

--

-..e. ..~--

-- -- -

Non-Focussing System Focussmg System - c (Normal Dewce) (Loterolog 1

Rt Reslstwty of bed

RS Reslstlwty of odjocent formohon -Az Rm Reslstwty of mud

A B

Figure 36. - Later&g system, electrode arrangements, and comparison of current distribution for normal device and later&g. From Doll (1951). ment and current lines for the microlatero­ Induction device log device. Compare this figure with figure 33 to see the effects of focusing on the cur- The induction device records the conduc­ rent lines. These electrodes behave much as tivity (reciprocal of resistivity) of rocks by the point-electrode focusing technique used inducing a current to flow in the rocks. in the laterolog, but they have a depth of Where invasion is shallow, the induction log investigation of only about 5 inches; thus, measures true resistivity. Also, the device the measurement is primarily related to the has the further advantage of working in resistivity of the flushed zone. either air- or oil-filled holes. The large resis- 56 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

the formation, and the other for receiving the signal returned from the formations, The transmitting coil is driven by an oscillator (20 kHz), and the signal penetrates the for­ mation in a torous-shaped pattern about the coil. Figure 38 (Schlumberger Well Survey­ ing Corp., 1958) shows the arrangement for a single-transmitter single-receiver induction logging sonde. The electromagnetic field in­ duces ground currents in the formation that flow along circular loops perpendicular to the axis of the borehole. One of these ground- current loops is shown in figure 38. The Pad ground currents have an attendant magnetic field, the strength of which is dependent on the magnitude of the ground currents. The magnitude of the ground currents is propor­ Mud-filled borehole tional to the conductivity of the formation Figure 37. - Microloterolog device. From Schlumberger Well carrying this current. By instrumenting the Surveying Corp. (1958). receiving coil to respond only to the secon­ tivity contrast between formation water and dary magnetic field, which is due to the borehole mud in fresh-water systems seri­ ously limits the applicability of induction logs to hydrology. A further limitation is in the great size of the tool. A typical induction device is 3vs inches in diameter and more than 15 feet long, and weighs nearly 200 RECEIVER COIL pounds. Tools of this size require a large mast, or drill rig, to get in and out of the hole.

Principles and applications Conventional electrode-resistivity devices require a conductive medium in the borehole to carry the current into the formations; thus, they cannot be used in air-filled holes

or in wells drilled with an oil-base mud. To Measuring point overcome this problem, Schlumberger intro­ cross-sectional orea duced induction logging in 1948. This method is based on the idea of using electromagnetic induction to couple a signal from the logging sonde into the formation and back to the sonde (Doll, 1949). This method has also proven extremely successful in fresh-water muds and is generally the best resistivity TRANSMITTER COIL logging tool in fresh mud, except where for­ OSCILLATOR mation resistivities are much higher than those of the adjacent shale beds. In its simplest form, the induction sonde contains two coils of wire, one for transmit­ Figure 38. - Induction-logging system. From Schlumberger Well ting an alternating current by induction into Surveying Corp. (1958). APPLICATION OF BOREHOLE GEOPHYSICS 57

ground currents, a curve proportional to for- SPONTANEOUS ' DEPTH, RESISlIVIlY CONDUCTIVITY POlENTIAL FEET MILuY"oS,n : l!aL D mation conductivity is developed. Because pAILL1"OLTS, IOHMS..~,., OhS.d,. 0 resistivity is the reciprocal of conductivity, A.,b".M 'O 6 FF40 it is customary to electronically invert the -h+ conductivity log and display both resistivity and conductivity curves. In actual logging practice, focusing coils are also incorporated in the sonde to eliminate or minimize effects of variations in hole diameter, adjacent beds, and borehole-fluid invasion. For many situations, in oil-base mud, fresh mud, and in empty holes (air or gas filled), the induction log virtually measures true for­ mation resistivity. The unit of electrical conductivity is called the “mho/meter.” Conductivity equals l/re­ sistivity, and it would seem logical to use l/ohm-meter (mho/meter) as the scale for induction logging. As this would result in a scale of fractions for all resistivity values greater than 1 ohm-meter, the induction-log scale is commonly calibrated in one-thou­ sandth of a mho, or millimhos/meter. Using this scale, formation resistivities of 10, 100, and 1,000 ohm-meters would have conduc­

tivities of 100, 10, and 1 millimhos/m, respec- Figure 39. - Induction-electric log of a test hole in northwestern B tively. The conductivity curve is displayed on Colorado. the log, with zero on the right, which corre- inefficient electrical transformers, the induc­ sponds to an infinite resistivity. As conduc- tion log is insensitive to large changes in tivity increases, the curve trace moves to the resistivity in a high-resistivity environment. left. In terms of resistivity, the curve is non- The difference between 400 and 500 ohm- linear, with the low resistivities emphasized meters probably cannot be detected, and, and the high resistivities attenuated. therefore, the induction log should only be Figure 39, part of the log of a test hole used where true resistivities are less than in northwestern Colorado, shows the conduc- 100 ohm-meters or, better still, less than 50 tivity and resistivity curves for the 6FF40 ohm-meters. induction device, as well as the 16-inch nor­ mal and SP curves. The conversion from Instrumentation conductivity to resistivity can best be illus- Induction logs are customarily run by the trated by an example. The conductive peak logging service companies in the second and at 2,850 feet reads 175 millimhos/meter third recorder tracks, along with the 16- from the right-hand curve of figure 39. Using inch normal or a laterolog (Tixier and the relationship millimhos/meter = l,OOO/ others, 1963). The induction log is displayed ohm-meters, we insert our value to get 175 as resistivity, in ohm-meters, in the normal =l,OOO/ohm-meters. Solving for ohm-meters channel, and as conductivity, in millimhos/ gives 1,000/175 = 5.7 ohm-meters. Reading meters, in the third track. Ordinarily, SP the electrically reciprocated curve (the dashed or natural gamma, or both, are logged si­ line in the center track of the log) at 2,850 multaneously in the first recorder track. feet gives a resistivity value of about 6 ohm- Multiple-conductor logging cable plus both meters. downhole and uphole electronics are neces- Because low-conductivity formations are sary for induction logging. 58 TECHNIQUESOFWATER-RESOURCESlNVESTIGATIONS

Calibration and standardization resistivity. Actually, it is the ratio of mud resistivity to formation-water resistivity that The zero conductivity in the third recorder determines the applicability of induction track is determined by suspending the induc­ tools. For example, assume we are logging tion sonde in air above the ground and away a porous sandstone more than 10 feet thick. from all conductors. If the receiving coil If the value of R,, does not exceed about 3 picks up no signal, the zero-conductivity to 5 times R,, the departure of apparent re­ point is established on the chart; however, in sistivity from true resistivity will be small areas of high humidity, this procedure may for the induction tool. These are the condi­ not result in an accurate zero. The resulting tions that generally occur in permeable for­ error of a few millimhos in zero conductivity mations containing petroleum. In fresh-water is not too critical in petroleum logging but formations, however, the value of R, may can be very serious in fresh-water zones. often exceed 5 R,, and the departure of R, Resistivity points are determined by suspend­ and R, becomes excessive. For this reason, ing a copper hoop of known dimensions the induction tool is generally not suitable around the sonde (still in free air) and by for logging fresh-water aquifers. adjusting the log display to agree with this Vertical resolution of the induction tool is known resistivity. adversely affected primarily when the bed being logged is less than about 6 feet thick Radius of investigation and is several times as resistive as the adja­ The radius of investigation of the induc­ cent beds. The induction-log response is sus­ tion log is a function of coil spacing. In the ceptible to hole eccentricity, and for this 5FF40 (service company nomenclature) reason,, the sonde should be positioned in the sonde, the main coils are 40 inches apart, borehole by centralizers. and the volume of investigation is a cylinder, In order to determine the depth of inva­ 40 inches in diameter, surrounding the well sion, the resistivity of the invaded zone, and bore. In the 6FF40 sonde, the main coils are the true formation resistivity, a resistivity also 40 inches apart, but the volume of in­ curve (such as a long normal) would be vestigation is a cylinder whose diameter is needed in addition to the short normal and 11/z times the spacing, or 60 inches. For the the induction log. Commonly, the service 6FF40 tool, the effect of material within companies run what is known as the “dual- about 30 inches (borehole plus some of the induction logs.” These consist of the 5FF40 invaded zone) is minimal. For this reason, and the 6FF40 induction logs plus either a where invasion is shallow, the 6FF40 induc­ 16-inch normal or a short laterolog. The use tion log virtually responds only to true for­ of cross-plot charts is required to solve for mation resistivity. If invasion is very deep, the correct values of resistivity and depth of one or two supplemental resistivity curves invasion. are needed to obtain true formation re­ sistivity. Extraneous effects Nuclear Logging Modern induction logs are relatively un­ affected by changes in hole diameter. Early Nuclear or radiation logs are all related tools were adversely affected, and the logs to the measurement of fundamental particles had to be corrected by special charts. High or radiations from the nucleus of an atom. to medium resistivity muds or an empty hole The commonly used nuclear logs are natural are necessary if the induction log is to have gamma, gamma-gamma, and neutron. Nu- satisfactory response. If very conductive clear logs have a fundamental advantage over muds are used, the response from the mud most other logs-they may be made in either and invaded zone will be relatively large, cased or open holes that are filled with any and the induction reading departs from true type of fluid. c APPLICATION OF BOREHOLE GEOPHYSICS 59

Fundamentals of nuclear geophysics Any of the processes involved in the trans- formation of unstable isotopes may leave the An understanding of some of the basic resultant nucleus with an excess of energy principles of nuclear geophysics is a pre- which may be emitted as high-energy elec­ requisite to the proper use of all types of tromagnetic radiation, called gamma rays radiation logs. Such basic information is not or photons. Since gamma rays possess some only a great aid to log interpretation but is characteristics of both particles and high- necessary to appreciate the limitations and frequency waves, the term “photon” is also errors inherent in nuclear logs. A general used to describe this bundle of energy. The lack of this knowledge is partly responsible energies of emitted gamma rays are charac­ for the limited use of nuclear logs in ground- teristic of the nucleus and, therefore, of the water hydrology to date and for the misin­ isotope emitting them. It is this characteris­ terpretation of logs. tic that is utilized in neutron-activation anal­ Characteristics of radiation ysis and in natural-gamma spectrometry for Basically, an atom consists of neutrons the identification of elements. Energy of radi­ (n) , with a mass of 1 and no charge ; pro- ation is measured in electron volts (ev), tons (p), with a mass of 1 and a positive thousands of electron volts (Kev) , and mil- charge ; and orbital electrons (e-) , with a lions of electron volts (Mev) . The amplitude mass of l/1,840 of a proton and a negative of the pulse emitted by a scintillation detec­ charge. The mass number, A, is the number tor is a function of the energy of the imping­ of protons and neutrons in the nucleus, and ing radiation. Radiation intensity is related the atomic number, 2, is the number of to the number of pulses detected per unit protons, ordinarily the same as the number time. of electrons. Isotopes are different states of Radiation statistics an element, where A changes due to a change in the number of .neutrons, but 2 remains An important factor in the interpretation l constant. For example, natural uranium con­ of all nuclear logs is the statistical nature sists of three isotopes with atomic weights of of radioactive emission. Half life is the time 234, 235, and 238. The term “nuclide” refers required for one-half of the atoms in a group to each of the possible combinations of neu­ or source to decay to a lower state and varies trons and protons. from fractions of a second to millions of Some isotopes are stable-that is, they years. Although this time is known very do not change structure or energy. Unstable accurately, it is impossible to predict how radioactive isotopes spontaneously change many atoms will decay or how many gamma structure, emit radiation, and become differ­ photons will be emitted during a short period ent isotopes. Of almost 1,400 nuclides recog­ of time. Photon emission follows a Poisson nized, 1,130 are unstable, although only 65 distribution-that is, the standard deviation occur naturally. Most radiation is emitted is equal to the square root of the number of from the nucleus of an atom, but X-rays are disintegrations observed ; therefore, experi­ derived from shell transitions by orbital elec­ mental accuracy can be calculated. Time con­ trons. The radiation emitted during radio- stant is the time, in seconds, over which active decay consists of alpha particles, radiation pulses are averaged, and is an positive and negative beta particles, and important adjustment on all radiation-log­ gamma photons or rays. Of this group, only ging equipment. Pulse averaging is achieved gamma rays are measured in well logging by a capacitor (C) in series with a resistor because of their unique ability to penetrate (R) . The time constant (tc) is equal to RX C high-density materials. Neutrons produced and is defined as the time for the DC voltage, by an artificial source are also used for well which drives the recorder pen, to rise to logging. They have the ability to penetrate approximately 63 percent of the final value, dense material, but are slowed (thermalized) or to fall to 37 percent of the initial value. in hydrogenous materials. From a practical standpoint, however, the 60 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

t- IO SECONDS - 4

II I~wullI~~~~11111 111WIII IUlIIIIlIIIIlIllI~~IIIlIllUllI11Ml MllMulllll 100 GAMMA PULSES DETECTED

9.8 COUNTS PER SECOND ) 7 CPS ) 4 II CPS 1 # S-SECOND TIME CONSTANT ONE-SECOND ONE- SECOND TIME CONSTANT 4 IO COUNTS PER SECOND 4 IO-SECOND TIME CONSTANT

6112 PULSES DETECTED IN ONE MINUTE. OR 10.2 COUNTS PER SECOND

- RADIATION INTENSITY -

5ONE-SECOND IO-SECOND TIME CONSTANT TlME CONSTANT GRAPHIC TIME- DRIVE RECORDS

Figure 40. - Time constant and it!I effect on bagging parameters. time constant includes the averaging effects of Water Resources Division logs made by of the entire logging system-from pulse J. H. :Dyck of the Saskatchewan Research detection to pen travel. The upper part of Council (written commun., 1969). The fig­ figure 40 shows how the measured count rate ures, in order, are time constant and mean can vary as the time constant is reduced from and standard deviation, in counts per sec­ 10 seconds to 1 second. A larger time con­ ond. Natural gamma log: 3 set, 83.Ok4.7 stant tends to reduce the magnitude of fluc­ cps ; 10 set, 83.1?1.5 cps ; and 20 set, 83.0? tuations on a log, as shown at the bottom of 0.8 cps. Neutron Log: 1 set, 1818.7~?133.1cps; figure 40. However, if the time constant is 3 set, 1817.6k12.8 cps; and 10 set, 1820.8t too long with respect to the logging speed, 7.4 cps. Time constant of the system and de­ the pen may not have time to record true tector efficiency vary among loggers, and, radiation intensity before a rock unit with because the requirements for statistical ac­ a different intensity enters the volume of curacy and bed definition are not the same, investigation. Some random fluctuations are rules for logging speed and time-constant present on all radiation logs, and the logs settings cannot be given. Logging speed can will not repeat exactly. Repeat logging runs be increased and (or) time constant can be are a positive means of separating random decreased when a higher count rate is mea­ changes from deflections related to lithology. sured. The time constant and logging speed The relationship of time constant and count are usually determined as a result of experi­ rate to the standard deviation are well illus­ ence with the equipment, on the basis of trated by the following computer analyses the amount of statistical fluctuation that can APPLICATION OF BOREHOLE GEOPHYSICS 61

be tolerated and the thickness of the beds and is transparent to these flashes or scintil­ D that are to be measured. For example, a lations. The sodium iodide or lithium iodide logging speed of 60 feet per minute, using crystal is optically coupled to a photomulti­ a time constant of 10 seconds, would mean plier tube, where a pulse of electrical current, that a nuclear log would theoretically record amplified about 1 million times, is produced. 63 percent of the correct value for a bed This pulse is further amplified electronically 10 feet thick. If a nuclear log is to be used in the probe, and then sent to the surface quantitatively, these factors must be very equipment. In the radiation-control module carefully considered, so that log values will on a logger, the pulses are integrated over approach true values, and statistical errors a preset time constant, and the DC-voltage will be minimized. output is used to drive the recorder. The The following nuclear logging parameters sensitivity of a scintillation detector is main­ are illustrated in figure 41: Logging direc­ ly a function of crystal size and shape. More tion and speed, time constant, and radiation pulses are measured in a given radiation intensity. The two neutron-gamma logs on field if the crystal is larger. the left in figure 41 were made with a 3-curie An important feature of scintillation de­ americium-beryllium (AmBe) source, and tectors is that the intensity of each flash all other factors were constant while logs of light in the detector is nearly proportional were made moving down and up the hole. to the energy of the gamma photon that pro­ The apparent differences between the logs duced it, With the proper amplifying circuits, are due to the random nature of radiation the height of each pulse sent up the cable and, possibly, to a different probe position is proportional to the energy of the detected in the hole. The log on the right was made radiation. This fact makes it possible to with a smaller plutonium-beryllium (PuBe) identify radiation according to its energy, ,’ source, a longer time constant, and a higher by using scintillation equipment and a multi- sensitivity at the same logging speed. Fluc- channel pulse-height analyzer. A single-chan­ D tuations in signal were recorded on time nel analyzer, which discriminates against all drive at the bottom of the log. The rounded pulses not within a preselected energy range, character of the deflections on the right-hand makes it possible to record a continuous log log was caused by an unsatisfactory ratio of of a part of the energy spectrum, or to elimi­ time constant and logging speed. Lithologic nate unwanted energy levels from a log. contacts would be difficult to identify from this log. The logs on the left clearly distin­ Quantitative interpretation guish thin beds that are not obvious on the Most laboratory measurements of nuclear right-hand log, and the bed contacts are radiation are made in terms of observed dis­ more sharply defined. Although the lower integrations, expressed in counts per second. emission of the PuBe source necessitated a Only a fraction of the total radioactive disin­ longer time constant, a lo-second time con­ tegrations per second from a source are stant was too long for the logging speed observed, the size of the fraction depending selected. on the efficiency of the detecting and counting system. The basic unit of radioactivity is the Radiation detection curie, which represents any source that is In the past, geiger tubes have been used decaying at a rate of 3.7 x lOlo disintegra­ extensively for gamma logging, but in re- tions per second. The roentgen is an exposure cent years they have been almost completely unit that is based on the ionizing effect of replaced by the more efficient scintillation gamma or X-rays on the air through which crystal for both gamma and neutron detec­ they pass. A radiation detector in a given tion at borehole temperatures below 150°F. field of radiation will detect a percentage The most commonly used phosphor-thal­ of the gamma-ray photons or particles pres­ lium-activated sodium iodide-emits flashes ent, depending on the geometric relation be- of light when exposed to nuclear radiation tween the source and detector and on the D 62 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

NEUTRON-GAMMA LOGS 200

300

\ LOGGED UP LOGGED DOWN LOGGED UP ., a

DEPTH, FEET TIME CONSTANT 3 SECONDS TIME CONSTANT IO SECONDS LOGGING SPEED 40 FEET/MINUTE LOGGING SPEED 40 FEET/MINUTE

STATISTICAL FLUCTUATIONS STATISTICAL FLUCTUATIONS I-- 1004 m--so--i COUNTS PER SECOND COUNTS PER SECOND 3-Curie AmBe Source 2-Curie PIBe Source

Figure 41. - Nuclear-logging parameters. efficiency of the detector and associated elec- ation. However, they can be standardized to tronics. Probably no two detecting systems provide similar recorder deflections, whether will measure exactly the same number of the units are milliroentgens (%oooof a roent­ pulses per unit time in the same field of radi- gen), percent porosity, bulk density, in APPLICATION OF BOREHOLE GEOPHYSICS 63 B grams per cubic centimeter (g/cc), or API ing recorded (fig. 42). The center of the gamma-ray units. Nuclear logs, recorded in volume of influence is at the detector for a counts per second, have the advantage that natural-gamma sonde, and between the source each pulse sent up the cable represents a and the detector for neutron and gamma- detected neutron or gamma photon, and elec­ gamma logs. The volume of influence does tronic counters and pulse generators reading not have sharp boundari& as illustrated; directly incounts per second can be used for rather, there is a zone of maximum contribu­ checking the linearity of response of the tion to the total radiation measured and a logging equipment and for correcting for gradual decrease in influence with distance dead time. Counts per second can then be away from the detector. The radius of inves­ converted to environmental units, such as tigation is here defined as including all the porosity or bulk density, from calibration material that produces 90 percent of the curves. signal recorded on the log. Inasmuch as the An important instrumental factor for volume of influence extends for some distance which correction must be made on all nuclear above and below the detector, it follows that logs is “dead time,” or “resolving time.” Co­ a rock layer begins to influence the log incidence error is caused by two pulses occur- response before the detector enters the bed, ring in a time interval smaller than the and continues to influence response after the resolving time of the equipment, so that only detector leaves the bed. Figure 42 shows one pulse is recorded. This is the most sig­ that unless the bed fills the volume of influ­ nificant cause of nonlinear response of radia­ ence, the neutron-log response will not reach tion-logging systems at higher count rates. a level representative of the lithology near Count rates on logs may be corrected by the detector because part of the radiation means of a graph, a log overlay, or the for­ recorded is due to the adjacent Ethology. mula: N = n/( 1-nt ), where N is the corrected The true log response for a porosity of 1 counting rate, n is the observed counting D rate, and tis the dead time of the instrument, 0= POROSITY R = RAmUS OF lN”ESTlGATlON A= LOG AMPLITUDE in seconds. The significance and calculation T= ED THICKNESS n= APPARENT WICKNESS of dead time are described in detail in an article by Crew and Berkoff (1970). Quantitative analysis of nuclear logs can be made only if the logging equipment is calibrated by using laboratory analyses of core samples from a logged hole or by using a model that is infinite with respect to probe response. Correction must also be made for the various borehole and instrumental effects present on all logs. Where nuclear logs are used for stratigraphic correlation, such cor­ rections are not important. In quantitative applications of nuclear logs, calibration, stan­ dardization, and correction for extraneous effects are essential. 1 THfORfTICAL NfUTRON LOG..ASSUMINGHOLf The radius of investigation, or volume of EFFECTSCONSTANT R DfCRfASfS WITH HIGHfR , OR FLUID CONTENT AND INCREASES influence, is a concept essential to both inter­ WITH LONGERSPACING THIS GENERAL THEOR ALSO APPLIfS TO NATURAL GAMMA AND pretation and quantitative analysis of nu- GAMMA-GAMMA LOGS. clear logs. The detector for natural-gamma logs and the source and detector for neutron and gamma-gamma logs are located within

a roughly spherical shaped volume of mate- Figure 42. - Relationship of the radius of investigation and bed rial that is contributing to the radiation be­ thickness to the quantitative interpretation of nuclear logs. 64 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

percent is only recorded for the thicker bed logging equipment will hinder the quantita­ in figure 42. This same principle applies to tive comparison of logs made in any other natural-gamma logging or bulk density mea­ mixture of radioisotopes. Until calibration sured by gamma-gamma logging. models simulating other hydrogeological en­ A widely used technique for the determi­ vironments are available, core analyses must nation of bed thickness from nuclear logs be used as a basis for the quantitative inter­ is the measurement of thickness of the anom­ pretation of nuclear logs. Field standards aly at one-half the maximum amplitude (fig. are also necessary to determine whether re­ 42); Because the volume of influence is not sponse changes with time or temperature and filled by thin beds, their thicknesses mea­ standards should be related quantitatively to sured from logs by this technique are gen­ response in calibration models. erally too great. An important concept in nuclear-log inter­ pretation is emphasized here-that the area Natural-gamma logging under the radiation curve is proportional to Natural-gamma logs are records of the the product of the bed-thickness times, Q amount, of natural-gamma radiation that is (quantity). Q is proportional to either the emitted1 by all rocks. The chief use of natural- amount of radioisotopes present in natural- gamma logs is for the identification of lithol­ gamma logging or to the bulk density or po­ ogy and stratigraphic correlation in open or rosity in gamma-gamma or neutron logging. cased, liquid- or air-filled holes. At present, conventional quantitative-radia­ tion log analysis utilizes only the relationship between maximum amplitude of the curve Principles and applications and the desired lithologic parameter. The gamma-emitting radioisotopes nor­ The “area-under-the-curve” approach was mally f’ound in rocks are potassium-40 and described by Scott, Dodd, Droullard, and daughter products of the uranium- and tho­ Mudra (1961). Their work dealt with the rium-decay series. The natural-gamma log quantitative interpretation of natural-gamma run by most equipment does not employ en­ logs in terms of grade and thickness of beds ergy discrimination to distinguish the various of uranium ore; however, the technique can radioisotopes, and only gross gamma activity and should be applied to the quantitative above a.detection threshold is recorded. Gam­ evaluation of other types of radiation logs. ma spectrometry is described on page 67. As nuclear logs become more widely ap­ Some of the important characteristics of the plied to ground-water investigations, the need three most important naturally occurring for the standardization of log scales and for radioactive materials (Belknap and others, the calibration of equipment response will 1959) are as follows : increase. It is hoped that a calibration facil­ Energy of Numberof Average Gamma ity for water-well loggers can be built; how- charac- photons per copnt intensity teristic second in shale ever, until such time, we recommend that the peaks per gram 20018?lale samples American Petroleum Institute neutron and (Mev) of element samples (percent) Potassium-40.. 1.46 3.4 2% 19 gamma pits be used to facilitate comparison Uranium-238 of water-well logs with commercial geophysi­ series in equilibrium.. 1.76 2.8~104 6 ppm 47 cal logs. These pits are located at the Univer­ Thorium-232 sity of Houston, Tex., and rental is charged series in on a time-used basis. The neutron pit is con­ equilibrium.. 2.62 1.0x104 12ppm 34 structed of limestone blocks of three different The actinium series is also present in rocks, porosities and is only valid for limestone but it starts with uranium-235, which has a porosity (Belknap and others, 1959). The concentration of only 1: 139 parts of urani- API gamma pit is constructed of an artificial um-238 in natural uranium, and, therefore, mixture of uranium, thorium, and potassium ; its contribution to natural radiation is minus­ hence, differences in the energy response of cule. APPLICATION OF BOREHOLE GEOPHYSICS 65

Even though the common gamma probe important application in ground-water hy­ detects the several radioactive elements with- drology is in identification of clay- or shale- out distinguishing them, it does provide bearing sediments. Clay tends to reduce the diagnostic lithologic information. Potassium, effective porosity and permeability of aqui­ which contains about 0.012-percent potas- fers, and the gamma log can be used to sium-40, is abundant in feldspars and micas, empirically determine the shale or clay con- which decompose readily to clay. Clays also tent in some sediments. Figure 5 demon­ concentrate the heavy radioelements through strates how this relationship can be utilized. the processes of ion-exchange and adsorp­ The neutron log and core analyses of this tion. In general, the natural gamma activity hole show a significant increase in average of clay-bearing sediments is much higher total porosity below a depth of 300 feet. The than that of quartz sands and carbonates. gamma log indicates that this zone has a Russell and Steinhoff (1961) showed that higher percentage of clay and, therefore, inclusions of volcanic material can increase does not have a higher effective porosity and the radioactivity of sandstones and that some permeability than the rocks above 300 feet. bentonite beds may be more radioactive than Laboratory analyses of particle-size distribu­ shales. Table 4, from Wood (1966)) shows the tion gave an average of less than 10 percent relative gamma intensities of some sedimen­ of clay-size particles ( <0.004 mm) above tary rocks. 300 feet. Below 300 feet, the average content From the range of gamma intensities of clay-size particles is greater than 20 per- shown in table 4, it can be seen that the cent, and two samples exceeded 50 percent. interpretation of gamma logs depends to a Rabe (1957) demonstrated an empirical large degree on experience in a restricted relationship between natural-gamma radia­ geologic environment. Probably the most tion and permeability for clay-bonded sands

Table 4. - Range of radioactivity of selected sedimentary rocks

API CAHIU RAY UNITS

Anhydrite ---__-__--

Salt ---__------

Dolomite -----____-_

Limestone -___------

Sandstone -_--_---_

Shaly sandstone-----

Sandy shale ______-

Shale ______

Organic marine shale- - - -

Potash beds ______------66 TECHNIQUES OF WATER-RESOURCE,S INVESTIGATIONS

of the Denverdulesburg basin. Rapolova averaging time, logging speed, recorder sen- a (1961) reported his studies in Russia that sitivity or span, and zero positioning or indicated a relationship between the “spe­ basing. For natural-gamma logs, zero radia­ cific yield” [permeability] of granular aqui­ tion is generally placed at the left margin of fers and the natural-gamma intensity from the paper. Changes in energy discrimination logs. Gaur and Singh (1965) showed a rela­ and high-voltage supply should not be neces­ tion between the area under gamma curves sary on routine logs. and permeability from hydraulic tests. The data were for an Oligocene sand in India that contained an average of 26 percent of cal­ Calibration and standardization careous material, in addition to interstitial Most of the scale settings recorded on clay. natural-gamma logs of water wells are mean­ In summary, the natural-gamma log does ingless. Furthermore, an unpublished study not have a unique response to lithology. The by Carl Bunker of the U.S. Geological Sur­ response is generally consistent within a vey (written commun., 1960) indicated that single geohydrologic environment. To date no quantitative correlation between gamma (1971)) the widest use of gamma logs is for logs made by various commercial service the identification of lithology, chiefly in de­ companies prior to 1960 can be accomplished trital sediments, where the fine-grained units with validity. Even though the API gamma have the highest gamma intensity. pit (Belknap and others, 1959) has some shortcomings, the API gamma-ray unit is Instrumentation now being used by all the large service com­ Most of the newer equipment utilized for panies, and it does offer a semiquantitative gamma logging of water wells employs thal­ means of relating gamma-log response. The lium-activated sodium iodide crystals to de­ only other pits available for calibration of tect gamma radiation. Older equipment and gamma probes are maintained by the U.S. most, oil-well sondes required to log in bore- Atomic Energy Commission, and they relate hole temperatures greater than 150°F use gamma response to radiometric equivalent- gas-filled Geiger-Mueller (G-M) tubes as uranium oxide (U308). In the past, the fol­ detectors. G-M tubes are considerably less lowing units have been used on gamma logs: efficient radiation detectors than scintillation “Inches of deflection,” “Standard units,” phosphors and do not emit pulses with ampli­ counts per minute, counts per second, micro- tudes related to the energy of the impinging roentgens per hour, milliroentgens per hour, radiation. Basically, the electronics in gam­ and micrograms of equivalent-radium per ma probes consist of a high-voltage supply ton. So long as the energy-dependence factor and pulse-amplification and shaping circuits. is recognized, the API gamma-ray unit is Power to operate the probes is either sup- certainly an improvement over the multi­ plied by batteries in the sonde or transmitted plicity of terms used in the past. The follow­ from the surface. Four important controls at ing relationship of gamma units used by the surface affect the gamma log, and the various logging service companies should settings of these controls should be recorded only be used for the semiquantitative com­ on every log-the time constant or pulse- parison of logs (Desbrandes, 1968).

Service Company Units Detector API units

Schlumberger .______._._._.._...... 1 microgram radium- ...... 16.5 (11.7, GNT-J equivalent/ton. sonde) . Lane Wells (now Dresser Atlas) ....._...______.....1 unit of radiation ______Scintillator _._...______...___2.16 PGAC (now Dresser Atlas) ______1 microroentgen/hour ______Scintillator _.______._____15.0 McCullough ._....____..______...... 1 microroentgen/hour ...______....__...___..___._...... 10.4 APPLICATION OF BOREHOLE GEOPHYSICS 67 B Counts per second is the unit used for less than twice the radius of investigation most laboratory radiation equipment, and will not be recorded at full amplitude because it is recommended for logger scales because the volume of influence is never completely of the ease with which the equipment can be filled by the bed. standardized and checked with pulse gener­ ators and nuclear counters. Counts per sec­ ond do not have any meaning with respect Extraneous effects to the intensity of a field of gamma radiation, The recorded natural-gamma intensity is except for a given measuring system. For decreased by any change in borehole condi­ this reason, counts per second should be tions that either increases the distance trav­ related to API gamma-ray units and also eled by gamma photons or increases the to a field standard in which the response of electron density of the material through the logging equipment is checked periodical­ which they must move. In general, however, ly. A small gamma source may be used as a these effects are much less than those caused field standard if it is always placed in the by similar changes in borehole conditions on same position relative to the detector, and if other types of nuclear logs. Shifts on gamma background radiation is constant or only a logs may be caused by changes in borehole small percentage of the total radiation. Two media (air, water, mud), casing, hole diam­ gamma field standards were built for the eter, gravel pack, grout behind the casing, Water Resources Division logging equipment or well development and are not necessarily which utilize two grades of uranium ore in related to changes in rock type. Increases in the annulus between telescoped aluminum natural-gamma intensity with time were pipes. The ore was sieved and mixed with found in wells that produce large quantities cement until it was completely homogeneous ; of salt water (Campbell, 1951). Anomalies after the mix had solidified, covers were of this type may be due to the transportation welded on to prevent the escape of radon, and precipitation of uranium or potassium. which would cause disequilibrium. Gamma probes are placed in the sleeves for field standardization. Standards of this type show no significant change with time, and they Gamma spectrometry reduce background effects to a minimum. Gamma spectrometry is a means of study­ ing the energy distribution of gamma pho­ Radius of investigation tons and provides information that is not available on the natural-gamma log. Both The radius of investigation of a gamma natural and artificial radioisotopes emit radi­ probe is a function of downhole instrumenta­ ation with energies characteristic of the iso­ tion, borehole fluid, borehole diameter, size tope. When dealing with the identification of casing, density of the rock, and photon and quantitative analysis of potassium, ura­ energy. Although the volume of the hole and nium, and thorium, one must recognize that the rock sampled by any radiation probe each of these constitutes a radioactive-decay may be visualized as roughly spherical, note series. Isotopes in these series decay to the that a unit volume of material close to the next nuclide by emission of alpha or beta detector will exert a much greater influence particles and a loss of mass. Some of these on the radiation intensity recorded than will nuclides or daughters also emit gamma radi­ the same unit of material farther away. De- ation. The following are simplified decay pending on the bulk density of the rock, 90 schemes for the common natural radioiso­ percent of the gamma photons detected prob­ topes with the source and energy of impor­ ably originate within 6-12 inches of the bore- tant gamma radiation indicated : hole wall. Figure 42 illustrates how a radioactive bed is detected before the detec­ Potassium-40 71.46 Mev --) Argon-40 (or tor enters the bed. Beds with a thickness of calcium-40). 68 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

Uranium-238 * thorium-234 + proctactin- correction of spectral-gamma logs due ium-234 + uranium-234 + thorium-230 to: The position of the detector in the bore- + radium-226 + radon-222 + polonium- hole, the diameter of the hole and the den­ 218 + lead-214 (or astatine-218) + bis- sity of the fluid, the thickness of the casing muth-214 (0.6, 1.12, 1.76, and 2.20 Mev) and cement, and the thickness of the for­ + polonium-214 (or to thallium-210) + mation. lead-210 + bismuth-210 + polonium-210 Figure 43 shows energy peaks for natu­ + lead-206 (stable) . rally occurring potassium and daughter prod­ Thorium-232 + radium-228 + actinium-228 ucts of uranium and thorium as they might (0.9 and 1.6 Mev) + thorium-228 + radi- appear on the oscilloscope of a multichannel- um-224 + radon-220 + polonium-216 + spectrum analyzer. The abscissa represents lead-212 + bismuth-212 + polonium-212 energy, increasing to the right, and the ordi­ (or thallium-208 2.62 Mev) --) lead-208 nate is the number of counts, or photons, (stable). detected. Counts are accumulated in channels on the abscissa that are subdivisions of the Each of these nuclides should be present energy range selected. Spectra of this type in an equilibrium amount which depends on must be accumulated over a period of time. half life. Changes in this quantity, which The length of time for accumulation depends may be caused by selective chemical removal on the gamma intensity. To obtain inhole from the system, produce disequilibrium. spectra, a linear-amplifier system for trans­ Gamma measurements of certain isotopes in mitting variable height pulses up the logging a decay series not present in equilibrium cable must be used. amounts will give incorrect quantitative To make continuous logs or stationary analyses for other isotopes in the series. The measurements that provide spectral data, a state of equilibrium in the radioactive-decay single-channel analyzer with a variable series is a potential source of information threshold or “window” adjustment is re­ on water chemistry and movement in a quired. This means that the energy range of ground-water system. Research underway pulses to be recorded per unit time on a log suggests that disequilibrium in uranium can can be selected. Figure 44 shows three se­ be detected by logging, but it is not known lected thresholds to distinguish between the whether detection is possible in rocks having a low uranium content. However, potassium, uranium, and thorium, as well as gamma- emitting radioisotopes in industrial wastes can be identified with logging equipment. Spectral equipment to make this possible can be added to most water-well loggers. The limitations and difficulties in utilizing .,,-Potosuium-40 inhole gamma spectrometry for quantitative analysis are numerous. Although the gamma photons from potassium-40 are essentially monoenergetic, approximately 80 gamma- energy peaks have been found in the uranium series and 60 were found in the thorium series (Belknap and others, 1959). Degrada­ tion of photon energy by Compton processes is one of the most serious limitations in gam­ ma spectrometry. (See p. 70.) For this Gamma Energy - reason, the low-energy spectrum probably will not be informative. Rhodes and Mott (1966) presented calculated departure curves Figure 43. - Gamma spectra - potassium, uranium, and thorium. a APPLICATION OF BOREHOLE GEOPHYSICS 69

uranium, thorium, and potassium peaks. Re- tive to uranium and thorium than the shales B cording all pulses in the energy “window” tested. In the Ogallala Formation of west between thresholds A and B should give a Texas, the zones containing more than 35- value that is related to the potassium-40 percent calcium carbonate were found to content, if the uranium and thorium con­ have a much higher radium-potassium ratio tributions are subtracted. The gamma activ­ than did the sands and clays having a lower ity between B and C should indicate the carbonate content (Keys and Brown, 1971). uranium content, if the thorium contribution Analyses of radioactive elements in ocean- is removed, and the count rate above C is bottom marine sediments indicated that con­ related to the thorium content. Figure 43 centrations of thorium and radium steadily shows how the variable height pulses, repre­ increase through silts to clay oozes, but the senting the various energies of detected gam­ uranium content remains constant and is ma photons, appear on the oscilloscope in apparently unrelated to grain size (Baranov relation to the thresholds selected. Continuous and Kristianova, 1966). These investigators gamma logs can be made from the number also found that red ocean clays contain the of pulses that occur in each of these “win­ highest concentrations of radioisotopes, and dows,” but, of course, the count rate will be that both thorium-230 and, less distinctly, relatively low unless very large crystals are thorium-232 increase from the ocean margins used. toward the open ocean. The possible relation The applications of inhole gamma spec­ of certain radioisotopes to grain-size distri­ trometry are several. The relative distribu­ bution and depositional environment would, tion of potassium, uranium, and thorium can of course, be extremely useful in identifying be determined by running successive logs, aquifers. The suggestion that zircon can be using different “window” settings. Diagnos­ used as a tracer in sedimentary petrology tic ratios of natural radioisotopes can be a was made as a result of a study of a Triassic significant aid to stratigraphic correlation sandstone in England that contained two ) and to the identification of lithology. Using types of zircons with widely different radio- ,inhole spectrometry, Brannon and Osaba activity (Rankama, 1963). (1956) found that a limestone in west Texas Gamma spectrometry has been found to had a lower concentration of potassium rela- be an accurate method of analyzing samples for potassium content (Bunker and Bush, 1967), based on the fact that potassium-40 makes up about 0.012 percent of all potas­ sium. It is not known whether accurate in- hole quantitative analysis of potassium can be made because of borehole effects and scat­ tering and attenuation within the rock ; how- ever, accurate uranium analysis is possible with gamma logs if the content is high enough. Clay minerals are selectively efficient in fixing potassium, and potassium is a major constituent of such clay minerals as illite and of feldspars and micas. The mineralogi­ cal source and isotopes causing radioactivity in sediments are not well known. Their dis­ tribution in igneous rocks has been more Tome - widely studied. Such accessory minerals as zircon, biotite, sphene, and apatite contain most of the uranium and thorium, and the content generally increases from basic to Figure 44. - Pulse-energy discrimination. granitic rocks. Gamma spectrometry in wells 70 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS has been successfully used to identify arti­ the gamma radiation absorbed is proportion­ ficial radioisotopes in liquid wastes. Gamma al to the electron density of the material spectrometry also has considerable applica­ penetrated, but it is affected by the chemical tion in neutron-activation logging. (See sec­ nature of the medium. Electron density is tion on “Neutron Logging.“) approximately proportional to the bulk den­ sity of most materials penetrated in logging ; Gamma-gamma logging however, for salt and gypsum, a Z/A correc­ tion should be made (Tittman and Wahl, Gamma-gamma logs are records of the in- 1965). A Z/A correction should be made for tensity of gamma radiation from a source in any material that does not have the same the probe after it is backscattered and atten­ ratio of atomic number to atomic mass as uated within the borehole and surrounding the calibration environment. After correc­ rocks. The main uses of gamma-gamma logs tion, the count rate recorded on a gamma- are for identification of lithology and the gamma log is inversely proportional to the measurement of the bulk density and po­ bulk density. The gamma-gamma log has also rosity of rocks. These logs may also be used been called the density log ; however, this for locating cavities and cement outside the implies a quantitative interpretation and cor­ casing of a well. rection for all interfering factors. Gamma-gamma density is widely used in Principles and applications the petroleum industry for the determination The gamma-gamma probe contains a of total porosity by application of the follow­ source of gamma photons, generally cobalt- ing .equation : 60 or cesium-137, shielded from a sodium Porosity = Grain density - Bulk density (from log) - , . iodide detector by Mallory-l,000 metal or -- lead spacers (fig. 45). Gamma photons from Grain density - Fluid density the source penetrate and are scattered and Grain density can be derived from labora­ c absorbed by the fluid, casing, and formation tory analyses of cores or cuttings, or, for surrounding the probe. Gamma radiation is quartz sandstone, a value of 2.65 g/cc is absorbed and (or) scattered by all material used. The fluid density in most water wells through which it travels. Degradation of may be assumed to be 1 g/cc, or, if the fluid photon energy takes place by three main is highly saline, laboratory measurement of processes : (1) The Compton scattering, in density may be necessary. If the same litho­ which a gamma photon loses part of its logic unit is present below and above the energy to an orbital electron, is proportional water table, or if gamma-gamma measure­ to the number of electrons (2) and occurs ments can be made after drawdown, it should with gamma photons from 0.1 to 1 Mev. (2) be possible to derive specific yield from gam­ The photoelectric effect, in which an ejected ma-gamma logs (Davis, 1967). Specific yield orbital electron completely absorbs the pho­ ton energy, is proportional to Z0 and occurs should be proportional to the difference be- with photons of 0.1 Mev or less. (3) Pair tween the bulk density of saturated and production, which occurs as the photon ap­ drained sediments, if the porosity and grain proaches the nucleus and completely converts density are not changed. itself into a pair of electrons, is proportional Bulk density may be read directly from to Z2 and requires gamma energy greater a calibrated and corrected log or derived than 1.02 Mev. Compton scattering is prob­ from a chart providing correction factors. ably the most significant process taking Errors in bulk density obtained by gamma- place in gamma-gamma logging. Some pho­ gamma methods are on the order of +0.03- toelectric absorption also takes place because 0.04 g/cc. Errors in porosity calculated from of degradation of photon energy by scattering, log bulk densities depend on the accuracy but the effect on a log may be reduced by of grain and fluid densities used. For certain energy discrimination. In the Compton range, source to detector spacings and over a limited APPLICATION OF BOREHOLE GEOPHYSICS 71

Lead or 1000 ’ Metal Spacers: Gamma-Ray Photons

After Correction for Bore Hole Effects Number of Pulses P & 0 0,=0,-&D@,) ’ I Source (g.4-0, 4 -4 -! COMPTON SCATTERING

Figure 45. -The equipment, principles, and interpretation of gamma-gamma logs. density range, a linear relationship is ob­ to make porosity increase to the left, as on tained when bulk density is plotted against long-spaced neutron logs. Confusion can be the logarithm of count rate. Most commercial eliminated by remembering that radiation gamma-gamma logs are reversed, so that recorded on a gamma-gamma log increases radiation increases to the left, rather than with increasing porosity. to the right as on natural-gamma and neu­ In addition to determining porosity, the tron logs. The purpose of this reversal was gamma-gamma log may be used to locate 72 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

casing or collars or the position of grout outside the casing. Figure 46 shows the use of the gamma-gamma log to locate the posi­ tion of cement or grout behind the casing. The gamma-gamma log made prior to ce­ menting indicated the sediment beds, between depths of 350 and 450 feet, which are also shown on the caliper and gamma logs. These beds were not detected by the postcement gamma-gamma log shown on the figure be- cause of shielding by the cement-filled an­ nulus. The gamma-gamma log can also be used to identify hole enlargements through casing as shown in figure 46. The water level and significant changes in fluid density will also be apparent on gamma-gamma logs.

Instrumentation Qualitative gamma-gamma logging can be done most economically with a gamma source, such as cesium-137 or cobalt-60, attached to 0,. RAOIAWJWINCREASESt -- - a conventional natural-gamma sonde by CALIPERLOG GAMMAGAMMALOG GAMMALOG heavy-metal spacers. The relative percentage PRECEMENT POSTCEMENT PRECEMENT of gamma photons absorbed and scattered Figure 46.-Gamma-gamma logs, used to interpret position of depends to a large degree upon the type and grout behind casing, and caliper logs, used tc select depth for size of the source, spacing between the source grouting and to estimate volume required. and detector, and the hole diameter. The radius of investigation depends on these same against the wall of the hole. Mallory-l,000 factors in addition to the bulk density (fig. metal is used as a shield around the source 45). In general, source strength and spacing and detector, so that most of the recorded must be increased to obtain proper logs in radiation is from the part of the borehole large-diameter holes, except when a decen­ wall in contact with the tool. The decentral­ tralized tool is used. Spacing that is too short ized tool has the advantage of being much may cause partially reversed log response less aff’ected by changes in drill-bit size, because of increased backscattered effect, and sonde position in the hole, or density of fluid will emphasize the hole-diameter changes. in the hole. It is, however, still affected by Cobalt-60 has a half life of 5.3 years, so that any bor’ehole rugosity between the source and radioactivity decreases about 1 percent per detector. Some commercial logging sources month, and correction must be made for the employ two detectors at different distances quantitative interpretation of logs. from the source in order to compensate for Both an axially symmetric and decentral­ mud-cake and borehole-diameter changes. ized side-collimated tool are used in the However, this system does not completely Water Resources Division research-logging eliminate the effect of borehole rugosity. program. With the axially symmetric tool illustrated in figure 45, 5-50 millicuries of Calibration and standardization cobalt-60 is used, with spacings of lo-35 Gamma-gamma logging equipment can be inches. These parameters depend on borehole calibrated in the API limestone pits, but conditions and rock density. This tool is not this does not provide correction for the sig­ positioned in the hole, nor is it directionally nificant borehole effects or Z/A effects. Until collimated. The decentralized sonde employs additional pits constructed of various earth two motor-driven arms to force the tool materials are available, it is highly desirable APPLICATION OF BOREHOLE GEOPHYSICS 73 l that core samples of the various lithologic Extraneous effects units penetrated be used for calibration of Because the gamma photon is affected to gamma-gamma logs that are to be interpret­ some degree by all media along its path from ed quantitatively. Accurate measurements of source to detector, changes in borehole pa­ grain density from samples are also needed rameters must be evaluated for quantitative- if porosity calculations are to be made. Field log interpretation. Factors which will cause standardization of logging equipment with a shift or anomaly on the gamma-gamma log blocks of various densities is desirable be- are as follows: Water level, change in fluid cause this method accounts for changes in density, mud cake, casing, collars, grout, and detector sensitivity, source decay, and spac­ most important of all, hole diameter. A de- ing ; however, such standards are heavy and, crease in measured radiation generally oc­ consequently, difficult to transport. The alter­ curs below the water table, and a smaller nate solution is to use a radioactive source, magnitude increase occurs in an open hole such as the natural-gamma field standards, below the casing. Diameter effects are great­ and to calculate radioactive decay and cor­ er in air-filled holes than in fluid-filled holes. rect for spacing. With proper standardiza­ If washouts, or changes of hole diameter are tion, gamma-gamma logs can be related to probable, a caliper log should be run. If the calibration runs in either pits or cored holes. hole is already cased, or if a caliper log can- not be run, zones of suspected enlargement Radius of investigation should be omitted from quantitative evalu­ ation. The decentralized sonde should nearly The radius of investigation of gamma- eliminate the effects of changes of bit size gamma logging devices is reported to be but not of borehole rugosity. Figure 60 shows about 6 inches, with 90 percent of the signal axially symmetric gamma-gamma logs and coming from the material within this volume. caliper logs in adjacent holes on the upper Note that the bulk density of the rock being Brazos River. The lithology is almost identi­ logged and fluid or casing between the sonde cal, so differences between the gamma-gam­ and rock will affect the radius of investiga­ tion. Furthermore, the spacing between the ma logs are nearly all due to hole-diameter source and detector will significantly alter effects. Hole C-l is a core hole where consid­ the volume of material measured. It was erable circulation was maintained to increase found experimentally that effects caused by core recovery ; therefore, the walls are badly casing collars or the position of a second washed. Rotary hole T-14 was drilled rapidly string of pipe on the outside can be empha­ to minimize washing effects. sized by short spacing and can be reduced Charts to correct for borehole conditions by longer spacing. In very high porosity are available for commercial gamma-gamma rocks, such as tuffs, a very long spacing is logs, and the newer devices are said to pro- necessary, and the radius of investigation vide a log that is electronically compensated probably exceeds 6 inches. In a recent Water for borehole parameters. Some of the more Resources Division experiment, radiation recent logs have a bulk-density scale, in from a cobalt-60 gamma source in a hole was grams per cubic centimeter, on the log. Prob­ measured in another hole through 4 feet of ably none of these techniques can accurately moist sand. A technique is now being em­ compensate for borehole rugosity or thin ployed to log the material between two holes washed-out zones between the source and by synchronously moving a source in one detector. Figure 47 shows examples of gam­ hole and a detector in the adjacent hole. This ma-gamma, neutron, and caliper logs made greatly increases the volume of material in­ in the same hole by a commercial service vestigated and reduces borehole effects. Ab­ company, A, and logs made with Water Re- solute values for bulk density cannot be sources Division equipment, B. The Water obtained unless the distance between the Resources Division gamma-gamma log was holes is accurately known. made with an axially symmetric tool, and B 74 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

DEPTH, FEET NEUTRON LOGS CALIPER LOGS REVERSED GAMMA-GAMMA LOGS 1300

20 40

60 80

1400 20

40

60 80

1500 20 40 60 80 1600 20

Figure 47. - Examples of logs made by IA) commerc!al oil-well lagging equipment and by (B) small water-well lagging equipment. the service company log was made, using a Principles and applications decentralized side-collimated tool with the output corrected for borehole effects. Deflec­ The various types of neutron logs are potentially the most useful techniques in tions caused by hole rugosity are apparent on both logs. Borehole corrections have re- borehole geophysics, as applied to ground- moved some of the rugosity effects, as at water :investigations, because most of the probe response is due to hydrogen and, there- 1,485 feet, but not others, as at 1,490 feet. fore, also to water. Neutron logs also have advantages peculiar to other radiation logs ; Neutron logging they can be used in liquid-filled or dry holes or in cased or open holes, and they have a A neutron source and a detector are ar­ relatively large volume of influence. The ranged in a probe so that the output is basic principles of deep well neutron-porosity primarily a function of the hydrogen content logging and the conventional neutron-mois­ of the borehole environment. Neutron logs ture logging are the same; however, the are used chiefly for the measurement of equipment and log interpretation differ. Most moisture content above the water table and older neutron-moisture meters provide only of total porosity below the water table. point measurements instead of logs, although APPLICATION OF BOREHOLE GEOPHYSICS 75 D continuous moisture loggers are now avail- tive mass of 1, and no electric charge ; for able. The moisture probes utilize a much this reason, the loss of energy when passing smaller source and a shorter source-to- through matter is caused by elastic collision. detector spacing that causes the count rate Materials which slow down neutrons are called to increase with a higher moisture content, moderators, and the effectiveness of natural­ rather than decrease as on a conventional ly occurring elements as moderators is the neutron-porosity log. For accurate quantita­ most important factor- in neutron logging. tive results, the moisture devices must be Neutrons from the source pass through the used in a small-diameter access tube that walls of the source and source sub, fluid fits the probe tightljr, in contrast to the con­ column, casing, and rock and are slowed ventional neutron-porosity logs, which can down by collisions with atomic nuclei. The be made in large-diameter open or cased most effective element in moderating neu­ holes. Table 5 summarizes the characteris­ trons is hydrogen because the nucleus of a tics of the two general types of neutron-log­ hydrogen atom has approximately the same ging systems. mass as a neutron. Neutrons must be slowed Neutrons for most logs are derived from to energies below 0.025 ev (thermalized) be- alpha particles impinging on beryllium. The fore they can be captured. The neutron may alpha emitter is intimately mixed with beryl­ be visualized as a ping-pong ball rolling across lium inside a sealed source. The first Water a billiard table. It loses but little energy in Resources Division research on neutron-po­ striking a billiard ball, but will lose most of rosity logging was done with a leased 2-curie its energy in a direct collision with another plutonium-239 beryllium source. A 3-curie ping-pong ball. It takes fewer collisions with americium-241 beryllium source, having an emission of 8.62~10~ neutrons per second, has been found to give superior logs under Table 5. - Comparison of neutron logging techniques most conditions and is now standard on many D Neutron “moistu+e Neutron “porosity” log oil- and water-well loggers. Americium-241 meter” widely used available on all oil- for point measure- well and some water­ has a half life of 458 years, so the decay- ments of moisture well logsers. correction factor is small. These sources emit content. fast neutrons (greater than lo5 ev or 0.1 Source-to-detector Usually less than Usually more than 15 Mev). Many of the older neutPon-logging spacing 5 inches. inches. devices and most of the thermal-neutron Source size Millicurie range. Multicurie range. moisture meters utilize radium-226 beryllium Type of detector Usually sensitive to Sensitive to thermal or thermal neutrons. epithermal neutrons sources. The latter type of source has the or gamma reys. disadvantage of strong gamma radiation that Usual application Moisture measurement. Saturated porosity and may produce gamma-gamma effects on log moisture measure­ ments. response under certain conditions. Most re­ Limitations Usually restricted to Useful in large- or cently, a californium-252 source was used for small-diameter holes smell-diameter holes experimental neutron logging and inhole above the water table above or below the or air-filled pipe water table. neutron-activation analysis. The 50-millicurie below the water table. californium source used emits 2.1~10~ neu­ Advantages Lower cost and Wider application : trons per second by spontaneous fission, reduced radiation reduced borehole exposure to effects : lower which is sufficient to activate several iso­ perSO”“d. statistical error. topes in boreholes ; however, its expected Graphic readoue Increases to right. Increases to right. high cost and short half life will probably neutron intensity limit the use of this source to specialized ap­ Moisture content Linear response - Logarithmic response- moisture increases moisture increases to plications. to right. left. In well logging, neutrons are artificially saturated Linear response - Logarithmic response- introduced into the rock-fluid system, and the porosity porosity increases to porosity increases to right; water in hole left. effect of the environment on the neutrons is causes significant measured (fig. 48). The neutron has a rela­ error. 76 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

Single-Conductor Cable to Logging Equipment Cash Land Surface

Epithrrmal Neutron Electronics and Power Suppl

NEUTRON MODERATION

Detector_

Spacers,

Neutrons

Source suf

When Beryllium is Bombarded with Alpha Fhrticlrs from Radium, Plutonium, Americium, etc.- Neutrons are rmifted PuBe Average Energy Loss Per Collision Am; Be Hydrogen 63% Source OxYPen 12% ._~ . NEUTRON CAPTURE

Figure 48. -The equipment, principles, and interpretation of neutron logs. hydrogen nuclei to thermalize a neutron than a neutron will collide with a hydrogen atom. with the nuclei of other elements. The aver- Cross section is the probability that a neu­ age energy lost by a neutron in a collision tron reaction will occur. For these reasons with the nucleus of a hydrogen atom is 63 the range of a neutron is largely a function percent, as compared to 12 percent for a col­ of the hydrogen content of the material lision with the nucleus of an oxygen atom. through which it passes. In addition, hydrogen also has a higher neu­ Various types of neutron logs are made tron-scattering cross section than common by counting the number of neutrons present elements found in sedimentary rocks. This at different energy levels or counting the means that there is a greater probability that gamma photons produced by neutron reac- APPLICATION OF BOREHOLE GEOPHYSICS 77 l tions. The type of radiation measured is a measured with a short-spaced moisture probe function of the type of source and detector, increases with increasing water content. If energy discrimination, shielding material, the short-spaced probe is used below the wa­ and spacing between the source and detector. ter table, the access pipe should be sealed to However, most neutron logs are hybrids, and exclude water. their names are derived from the type of In most rocks the hydrogen content is radiation that causes most of the measured directly proportional to the interstitial-water response. The term “neutron-gamma log” content ; however, hydrocarbons, chemically indicates that most of the radiation detected or physically bound water, and (or) any are gamma photons resulting from neu­ other hydrogenous materials can give anom­ tron reactions. The neutron-thermal-neutron alous values. For example, gypsum has a probe responds chiefly to thermal neutrons, large percentage of water of crystallization and the neutron-epithermal-neutron tool re­ which can be erroneously interpreted as ma­ sponds mostly to neutrons between 0.1 and terial having a high porosity. In the upper 100 ev. Brazos River basin, Tex., it was found that The gamma photons produced when neu­ this fact could be used to distinguish gypsum trons interact with matter are classified as and anhydrite and, thus, locate the depth of prompt, capture, or activation. Prompt gam­ hydration, which has a bearing on the hy­ mas are derived from the inelastic scattering drologic history of the area and the position of fast neutrons by nuclei; capture gammas of an interface between brine and fresh wa­ arise from decay or excited energy levels ter. Gypsum and anhydrite both emit very of a neutron ; and activation-gamma photons low intensity natural-gamma radiation, but result from the decay of an unstable nucleus anhydrite gives a high neutron count rate, caused by fast-neutron reactions or by the and gypsum gives a low neutron count rate. capture of thermal neutrons. Prompt and Figure 49 illustrates the combined use of capture gammas occur only during and im­ ) natural-gamma and neutron logs for strati- mediately after the neutron reaction that graphic correlation and for distinguishing produced them. Activation gammas may be gypsum from anhydrite. In figure 49 the produced for a long period of time, dependent anhydrite bed shown at an elevation of 1,700 upon the half life of the unstable isotope that feet in hole T-14 may have been partly al­ was created. tered to gypsum in well T-21. The anhydrite In a drill hole, the neutron source and bed registered a similarly low natural-gamma detector may be visualized as a roughly intensity in all four wells; however, the low- spherical cloud of neutrons that decrease in porosity deflection to the right on the neutron energy and number from the source outward. logs is somewhat reduced in well T-21. The detector is located in the region of lower Although a neutron log cannot be used energy neutrons at a distance dependent on for measuring porosity above the water table, the spacing that is selected. With long spac­ it is very useful for measuring changes in ing, in a medium with a high percentage of the moisture content. Figure 50 shows how hydrogen, most neutrons will be thermalized the perched source of contaminated water and captured prior to reaching the detector; (which was ultimately reaching the water whereas in very low porosity material, the table) was located behind an g-inch steel range of neutrons will be much greater‘ and casing in a well at the National Reactor Test­ more will be counted. In a tool designed for ing Station in Idaho. The neutron log shows moisture measurement, the detector, which a gradual decrease in moisture downward is located near the source, measures the in- through the clay “perching” bed, and the crease in thermalized neutrons that results clay is clearly indicated as a deflection, to from the increase in water content. Thus, the the right, on the natural-gamma log. number of neutrons measured with a long- Meyer (1963)) described how neutron-log­ spaced probe decreases with increasing ging devices can also be used to determine water content, and the number of neutrons the specific yield of unconfined aquifers. His 78 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

2000

1900

1800 d t / /’ E v) 1700 f 2 y1 NATURAL T-22 NEUTRON 9 1600 - GAMMA 4 L 8 NATURAL NEUTRON z 5 Z -- IL 1500 -

2 F G

1400 -

3 1300 - NATURAL NEUTRON GAMMA T-5 NATURAL GAMMA T-, I. NEUTRON

4000 FT

Figure 49. - Correlation with gamma and neutron-N logs. method utilizes a neutron probe to measure probe is 0.205. Thus, the two methods appear the difference between the moisture content to be equally reliable for determining the of saturated material and the moisture con- specific yield for unconfined aquifers. tent of the same material after it has been Epithermal neutrons are those in the ener­ drained. A conventional pumping test and gy range from 0.1 to 100 ev-that is, they are neutron methods were used simultaneously above the thermal energies at which neutron to determine the specific yield of alluvium capture and activation are possible. Epither­ near Garden City, Kans. Before the pumping mal-neutron measurements provide the high­ test was started, the percent-moisture by vol­ est percentage of response due to hydrogen ume was determined with a neutron probe and are least affected by the chemical compo­ for each foot of depth of saturated mate- sition of rocks and contained fluids. Boron rial. The amount of water drained by volume and chl!orine are two elements, having high- was then measured during the pumping test. neutron-capture cross sections, which might The Thiem equation (Wenzel, 1942) and a interfere with quantitative moisture or po­ modified form of the Theis nonequilibrium rosity measurements. For these reasons, equation (Cooper and Jacob, 1946) were used selective detection of epithermal neutrons to calculate specific yields of 0.19 and 0.21, should provide the most accurate neutron respectively, on the basis of drawdown data. log for measuring porosity in rocks saturat­ The average of the specific yields calculated ed with fresh water. for the three depths measured with a neutron A neutron-gamma log can be made with APPLICATION OF BOREHOLE GEOPHYSICS 79

also permits the measurement of half life as an aid to identifying isotopes. A recent feasibility study demonstrated that a 50-mc californium-252 source can be used for both stationary- and continuous- activation analysis in boreholes (Keys and Boulogne, 1969). The continuous-activation log was made using a 5.5-foot spacing be- tween the source and the detector. With this spacing, practically no radiation from the vicinity of the source reaches the detector, and activation logs can be made only with the source moving ahead of the detector down the hole. Aluminum-28 was

400 - identified as the chief radioisotope contribut­ ing to the log response. Use of this new technique may provide a log more closely

WATER TABLE related to clay content than the natural-gam­ ma log because the clay minerals are mainly

NEUTRON-EPITnEBMAL NEUTRON LOG NATURAL GAMMA LOG hydrous aluminum silicates, and there is a significant increase in the percentage of alu­ Figure 50. - Interpretation of a neutron log and a gamma log minum with decreasing grain size from fine indicates that water is perched on a clay bed. From Keys sand to clay. In stationary-activation experi­ (1967). ments made using a californium-252 source, , the same source, spacers, and electronic sec­ both sodium-24 and manganese-56 were tion used for neutron-epithermal neutron readily produced and identified in a borehole. B logging. However, the detector is a thallium- The technique provides a method for identi­ activated sodium iodide phosphor that is very fying water that is rich in sodium chloride sensitive to gamma photons and is also sensi­ in open holes or behind casing. Also, the tive to neutrons. A removable boron carbide quantity of oxygen, carbon, calcium, and shield around the crystal was used experi­ iron can be estimated from in-situ neutron mentally to reduce the sensitivity to neutrons. activation (Ferronsky and others, 1968). When thermal neutrons are captured, the Limitations of these techniques for use in a nucleus of the unstable atom created emits borehole include the difficulty of determining gamma photons that have energies character­ whether the nuclide is present in the rock istic of the irradiated element. Measurement matrix or in the fluid, and problems associat­ and identification of activation-gamma pho­ ed with maintaining the constant conditions tons in the laboratory is known as neutron- necessary for quantitative analysis. The gam­ activation analysis, and it is a most effective ma activity produced by neutron irradiation means of detecting and measuring minute is related to the neutron flux and to the quantities of various elements. Unfortunate­ nuclear characteristics of the parent and ly, in a drill hole many factors interfere with product nuclides, and this activity may be the use of neutron-activation analyses as a calculated (Lyon, 1964). Table 6 provides quantitative tool. Not the least of these is the data on some common nuclides easily activat­ degradation of gamma energies from the ed by thermal-neutron capture. point of emission to the detector. Another The neutron-gamma probe has been found serious problem is the interference of various experimentally to be very sensitive to chlo­ elements with similar energy peaks-par­ rine content ; hence, a comparison of neutron­ ticularly those elements in the hole and in the epithermal-neutron logs, which are relatively tool that might not be abundant in the rocks, insensitive to chemical composition of fluids, such as iron. Fortunately, activation analysis and of neutron-gamma logs, which are mostly 80 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

Table 6. - Data on ccmmn nuclides Mev for hydrogen. Also, hydrogen is most [After Sent&= and Hoyt (1966)) with additional data from effective in thermalizing the neutrons, so that Goldman ang Stehn (1961) 1 capture can take place. Most of the chlorine Counts oer detection in petroleum logging is done by

._ direct comparison of neutron-neutron logs irradiation’ with neutron-gamma logs. Increases of count “Al _._.__...... 100 =A1 2.7 x 10’ 2.3 min 1.18 =c1...... 24.6 ‘Fl 8.1 x 10’ 37.5 min 2.16. 1.63 rate on the neutron-gamma log are supposed “K . 6.88 “K 1.9 X 10’ 12.4 hr 1.63 to be due to chlorine and are therefore relat­ qig . 11.2 “Mg 3.1 X 102 9.6 min 0.84. 1.02 “Mn ...... 100 Tdn 1.2 X 10’ 2.58 hr 0.84, 1.81. ed to saline interstitial fluid. In our research, 2.13 =Na ______100 *‘Na 2.1 X lOa 15.0 hr 1.37, 2.16 we found that brine of, 200,000 parts per Wi ______3.09 *‘Si 5.9 2.62 hr 1.26 million, which is common in the upper Brazos ‘Based on 10 percent counting efficiency, a flux of lo8 n/cm’ River basin, Texas, produced a shift on both sec. and a normal abundance of nuelides. types of neutron logs, although of different a record of prompt-gamma photons, gives a magnitude (fig. 51). The neutron-epithermal­ theoretical basis for measuring the chlorine neutron and neutron-gamma logs shown in content in place and through casing. Of course, figure 51 coincide fairly well above the inter- most of the chlorine present will be due face, but not below. On the neutronepither­ to common salt, NaCl. Chlorine has a high- mal-neutron log, this difference may be due capture cross section for thermal neutrons, to the lower hydrogen content of brine. That and emits gamma rays of 7.4 and 7.77 Mev, is, sandstone with 25-percent porosity, sat­ compared with capture-gamma rays of 2.2 urated with a sodium chloride brine, is logged DEPTH FEE

23 >--

30

36( NEUTRON-EPITHERMAL NEUTRUN 3lNCiLt-I’CJIN I NEUTRON GAMMA RESISTANCE

Figure 51. - Location of the brine-fresh-water interface from geophysical logs, hole T-18, upper Braror River basin, Texas. APPLICATION OF BOREHOLE GEOPHYSICS 81

as rock having a lower porosity due to the D substitution of chlorine atoms for hydrogen atoms. There will be a greater increase in count rate on the neutron-gamma log than on the neutron-epithermal-neutron log be- cause the decrease in hydrogen and the increase in chlorine both raise the neutron- gamma count rate. This is balanced to an unknown degree by the smaller number of thermal neutrons available for capture, ow­ ing to the reduction in hydrogen content. At present, neutron-chlorine logging serves only as a qualitative technique for locating the interface between fresh water and brine through casing under the proper conditions. Inhole neutron activation of sodium may pro- vide a more positive means of doing this. A potential use for neutron, gamma and gamma-gamma logging is to locate the source of fine-grained material produced dur­ ing the development of a well by pumping. Presumably, most of the fines will be re- moved from that part of the aquifer where the fluid velocity is highest, if all other fac­ tors are equal. The difference between a neutron log made prior to the development D of a well and a second log made after well development should indicate zones of higher Figure 52.-Neutron logs, Yukon Senices well, Anchorage, porosity produced by the pumping and the Alaska. consequent removal of fines. Preliminary in­ rial developed from this zone (103-122 ft) vestigations in Alaska and Texas have indi­ consisted mainly of silt containing some cated that changes in porosity caused by both coarser particles. The increase in porosity well development and artificial recharge can could only have been caused by a leak in be detected by logging. the casing within this depth interval. Brine- Figure 52 illustrates the use of successive tracer logs, temperature logs, and neutron neutron logs to detect porosity changes due logs indicated that development and subse­ to development of a well. The well, which quent production from the deeper aquifers was drilled for oil exploration near Anchor- was small. (See fig. 62.) age, Alaska, was lined with 95/s-inch casing, plugged at 960 feet, and pressure-cemented Instrumentation back to the surface. It was decided to per­ The downhole and surface electronics for forate the casing at four deep zones selected making neutron logs can be exactly the same from geophysical logs, develop the well, and as that utilized for making natural-gamma then selectively test with a pump and packer. and gamma-gamma logs. This equipment Log A in figure 52 was made before per­ must be capable of handling a somewhat forating. The shallowest zone perforated was higher count rate than is typical for natural at 507 feet. Log B was made after several gamma, and pulse-energy discrimination is days of surging and bailing. It recorded a desirable. Considerable latitude in zero posi­ zone showing a significant increase in po­ tioning and sensitivity is necessary because rosity from 103 to 122 feet and a decrease of the high-amplitude deflections on the logs in porosity from 130 to 138 feet. The mate- and the shift in count rate at water table. A A 82 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

DEPTH, SINGLE-POINT FEET RESISTANCE LOG NEUTRON-EPITHERMAL NEUTRON LOGS

c WiSHOUiS ZT

*l-H -lOO- OHM COUNTSPER SECOND Ill/-INCH SPACING 11%/r-INCHSPACING 243/4-INCHSPACING 6-INCH DIAMETER I-INCH DIAMETER 4-INCH DIAMETER UNCASEDHOLE STEELCASING STEELCASING

Figure 53. - Neutron logs made with various spacings between source and detector and under different hole conditions. europiuqm-activated lithium iodide crystal en­ Calibration and standardization riched in lithium-6 may be used for neutron The API limestone pits in Houston, Tex., detection at temperatures below 150°F. Gas- are the only readily available models for filled tubes are commonly used at higher calibrating neutron-logging equipment at temperatures. Similarly, sodium iodide crys­ various porosities. Neutron-moisture-logging tals and Geiger-Mueller tubes can be used equipment can be calibrated with smaller for the detection of gamma photons. In models, available in many areas, or in boric the low-temperature epithermal-neutron tool, acid solutions of different concentrations. cadmium is used as a thermal-neutron shield, Core sa.mples provide the only other means and energy discrimination is used to reduce sensitivity to neutron-produced secondary- of calibration. The count rate can be related gamma photons to a low level. Character of either Lo API neutron units from the lime- the log is not affected by the spacing between stone pits or to percent porosity from core the source and detector, but spacing does af­ samples. Regardless of how the equipment is fect the quantitative relation between the calibrated, it should be standardized frequent­ log and the hydrogen content. Figure 53 illus­ ly-preferably after, or before and after log­ trates the manner in which sensitivity is af­ ging ea.ch hole, if the equipment has shown fected by casing and by spacing between the any tendency to drift. A neutron field stan­ source and the detector. Note the decrease dard can consist of an unvarying high- in detail and the reduced effect of washouts hydrogen environment. The standard can be vith the longer spacing. constructed with water, paraffin, or plastic, APPLICATION OF BOREHOLE GEOPHYSICS 83

and should be large enough that changes in ing a few pieces of core for laboratory anal­ moisture in the environment will have no ysis would have produced meaningless data effect on the probe output. Where possible, in a sedimentary section exhibiting such the use of two standards having different inhomogeneities. Laboratory analyses of the hydrogen densities is preferable, so that re- core were done by Core Laboratories, Inc. of corder span, as well as pen positioning, can Dallas, Tex., and the Hydrologic Laboratory be measured. of the U.S. Geological Survey in Denver. Calibration should be done in full-scale Some of the samples sent to the two labora­ models that have different porosities and that tories were duplicates, and some were split closely approximate borehole conditions. The for two analyses by one laboratory to deter- models, containing built-in neutron access mine the confidence that could be placed in tubes of various diameters, are filled with a core samples as a means of calibrating logs. water-saturated homogeneous sand. Models The analyses of some duplicated samples 20 inches in diameter were found to be less varied by more than 100 percent, whereas than infinite with respect to the radius of others tested exactly the same. This differ­ investigation of our neutron-porosity probes. ence does not necessarily indicate a problem An infinite model is defined as one that can in the laboratory, as the same piece of core be increased in size with no change in the could not be used twice; also, detailed sam­ radiation measured. To change fluids in a pling in some sections indicated a porosity sand model is difficult, and it is expensive to variation that was not apparent from exam­ construct the number of large models needed ination of the drill core. A recent article on to represent the various porosities, casing a gamma-gamma method for making a con­ sizes and types, and diameters of open hole. tinuous measurement of core porosity sug­ A heterogeneous model that permits changes gests that variations in porosity are much in interstitial fluid, porosity, and casing size greater than suspected, and that a small plug appears to be an alternate solution. Such cut from a drill core does not provide a rep­ B changes can be made with movable rods of resentative sample for most areas (Evans, limestone or novaculite (Allen and others, 1965; Harms and Choquette, 1965). Also, 1965). some differences apparently exist between The American Petroleum Institute has the analytical techniques used at the two constructed neutron-calibration pits of sat­ laboratories, as we noted a relatively consis­ urated limestone blocks with porosities of tent difference in the higher porosity range. 1.9, 19, and 26 percent (Belknap and others, A further difficulty in comparing core anal­ 1959). Because of possible instrument drift, yses with logs is caused by depth inaccura­ these pits should be used for calibration of cies on samples from zones with lost core. logging equipment only if a portable field standard is available for comparison. Cali­ Radius of investigation bration of neutron devices in these pits and The volume of influence, or radius of in­ the use of a field standard provide a basis vestigation, is an important factor in the for quantitative comparison of logs. Note, analysis of any neutron logs (fig. 42). The however, that calibration in a limestone radius of investigation is a function both of model is not directly applicable to a sand- the spacing between the source and the de­ stone environment because of matrix effects. tector and of the type of material within Figure 3 shows how the porosity of more the volume of influence. Figure 42 is some- than 200 core samples compares with a neu­ what idealized in that the true shape of the tron log. Note how much easier the log is to volume of material investigated is not known, interpret between depths of 200 and 300 feet and the radius of investigation decreases at deep than are the many core samples from higher saturated porosities, or greater per- this interval. The common practice of select­ cent-moisture, because of the reduced range a4 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

of neutrons prior to capture. Inasmuch as the sm;all vertical dimension and the consid­ the recorded response is an integration of erable horizontal extent of most washouts. the varying porosities present within a rath­ The best solution to this problem is not er large sample volume, it follows that a bed to attempt quantitative-log interpretations having lo-percent porosity but that does not where hole washouts are known or suspected. fill the volume of influence will not give the This is one reason why a caliper log is such same neutron value as that for a bed having an important tool in borehole geophysics. lo-percent porosity that more than fills the Figure 47 provides a comparison of a com­ sample volume. Thus, very thin beds are not mercial decentralized neutron log, on the left, accurately measured by neutron logging. and a log made with an axially symmetric The radius of investigation of neutron tool by the Water Resources Division re- devices is reported to be from 6 inches for search project, on the right. The gamma- high-porosity saturated rocks to 2 feet for gamma logs of the same hole are much more low-porosity or dry rocks. These estimates sensitive to the washouts shown on the cal­ may be conservative, for when we placed the iper logs. Figure 55 shows a neutron and a neutron sondes in 20-inch-diameter models gamma-gamma log with a resistance log. Be- that had a saturated-sand porosity of greater cause the hole was caving, the radiation logs than 30 percent, the sondes exhibited signifi­ were made through drill rods. Joints in the cant changes in count rate when the hydro­ drill stem apparently have very little effect gen density outside each model was changed. on the neutron log, but they have a marked An experiment run in two holes, which were effect on the gamma-gamma log. Note the drilled 4 feet apart in damp sand and gravel, similar character of the neutron and the indicated that significant numbers of neu­ single-point resistance logs. trons from a 3-curie americium source could Mud cake and invasion will also have a be detected at that distance. However, this minor effect on neutron logs and will de- is not directly comparable to logging in a crease with pumping and development of a single hole. well. Multiple-electrode electric-logging tech- 4 niques or dual-spaced neutron or gamma- Extraneous effects gamma. logs may provide data on the invaded Neutron logs are affected by changes in zone. This is a potentially important field borehole parameters, although to a lesser because of the close relation between the degree than most other geophysical logs that depth of invasion and the formation perme­ measure the properties of rocks. Figure 54 ability. YPIW. shows that little difference exists between WI- neutron logs made in an open hole and those made through two types of casing installed later in the same hole. The one major differ­ ence between these logs occurs at a depth of 80 feet, and is probably due to the caving of an anhydrite and shale unit, which oc­ curred when the plastic casing was being drilled out. The most marked extraneous effect on neu­ tron logs is that caused by changes in hole diameter. Such error can be reduced by maintaining a side-collimated tool against the wall of the hole. The effect of changes in bit size, or average hole diameter, would theoretically be eliminated by a decentralized tool. A decentralized tool probably would not Figure 54. -A comparison of neutron logs mode in the same reduce the effect of hole rugosity because of hole uncared and through PVC and steel casing. 4 APPLICATION OF BOREHOLE GEOPHYSICS 85 B SINGLE-POINT DEPl RESISTANCE LOG NEUTRON LOG GAMMA-GAMMA LOG FEE

700

800

900

OHM 200 300 600 500 COUNTS PER SECOND COUNTS PER SECOND

Figure 55. -Neutron and reversed gamma-gamma logs, made through drill rods, and a single-point resistance log. Compare the effect of joints on the radiation logs. The media in a drill hole has a consider- proximating the water level at the time the able effect on a neutron log. The marked shift log is made. Likewise, a water-brine inter- at the air-water interface is useful for ap- face or a water-mud interface in the hole is 86 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

apt to cause a shift that is not always readily that may be discharged to the environment, c recognizable. Fortunately, the fluid-conduc­ authorized disposal practices, and precau- tivity log will give exact locations of these tionary procedures and administration con­ fluid-column interfaces without showing in­ trols. All WRD [Water Resources Division], terference from hole effects. personnel authorized to handle radioactive materials, or in charge of activities requiring use of these materials, are required to be The use of radioisotopes in well logging familiar with the A.E.C. regulations and the All types of nuclear logging except nat­ contents of NBS [National Bureau of Stan­ ural gamma utilize radioactive materials or dards] Handbook No. 92, entitled “Safe sources. However, natural-gamma logging Handling of Radioactive Materials,” and equipment is calibrated, or standardized, IAEA [International Atomic Energy Agen­ with natural or artificial radioactive mate- cy], “Guide to the Safe Handling of Radio- rials. Most neutron-logging devices use either isotopes in Hydrology.” WRD memorandum radium-beryllium, plutonium-beryllium, or No. 68.98 also establishes the position of the americium-beryllium sources or electronic- Regional Radiation Protection Officer, who neutron generators. Gamma-gamma logging “will ensure that only qualified personnel, devices generally contain cobalt-60 or cesium- properly trained and licensed, oversee use of 137 sources. Downhole radioactive tracer or handle radioactive materials and equip­ techniques utilize a wide variety of gamma- ment * * * .” Because of the importance of emitting liquid or solid radioisotopes. (See nuclear-logging techniques, some of the pro­ section on “Fluid Movement Logging.“) cedures for the proper utilization of radio- Therefore, anyone planning a logging pro- active materials in wells will be briefly gram must be cognizant of the regulations described. Specific handling procedures will and procedures necessary to use radioiso­ depend on the size and nature of the source topes. The purpose of this brief description and the application, and should be developed is to minimize personnel exposure and the only by experienced personnel in compliance c risk of accident or loss of a source. with local regulations. Radium and small quantities of other nat­ Training and experience in radiation mon­ urally occurring radioactive materials, as itoring and the safe handling of radioiso­ well as microcurie sources as specified in title topes is a prerequisite to obtaining a permit 10, Code of Federal Regulations, Chapter 1, or license. Initial training should be designed Part 30.72, do not require a license from the to provide an understanding of the nature of U.S. Atomic Energy Commission or from radiation and its potential hazards; permis­ those States that have instituted formal reg­ sible doses, radiation levels and concentra­ ulation of radioactive materials. Higher tions, precautionary procedures, and waste levels of the specific radionuclides listed in disposal should be included. Courses in “Ra­ Part 30.72 of the code and all other radio- diological Monitoring for Instructors,” of­ active materials are regulated by the U.S. fered by the Office of Civil Defense, are Atomic Energy Commission or the States. adequate for initial training. On-the-job The following quotation is from the U.S. training must include the specific equipment Geological Survey, Water Resources Divi­ and procedures to be used in well logging. sion Memorandum No. 68.98, dated January Periodic refresher training is also necessary 16, 1968, which should be read by everyone to bring personnel up to date on changes in planning to utilize radioactive materials. regulations, equipment and procedures. “Federal regulations (U.S.A.E.C., Division Before radioisotopes are purchased or uti­ of Materials Licensing, 1966, Conditions and lized, the Regional Radiation Protection Of­ Limitations on the General License Provi­ ficer should be contacted for information on sions of 10 CFR 150.20) prescribe the limits licensing, film-badge service, wipe testing, governing exposure of personnel to radia­ and mo,nitoring equipment. Either an A.E.C. tion, concentrations of radioactive material license or a permit covered under one of the c APPLICATION OF BOREHOLE GEOPHYSICS 87 D existing broad licenses is necessary prior to tion-Radioactive Materials,” so they can be the purchase of regulated radioactive mate- identified if lost. Shields should be labeled rials. A monthly film-badge service for all as to type and amount of radioactive material personnel using the sources and portable contained. Monitoring equipment may be monitoring equipment must be obtained be- used to measure radiation levels during han­ fore the sources are purchased. Shields that dling ; in addition, all personnel utilizing or can be locked should be selected on the basis transporting sources must wear film badges. of maximum-radiation attenuation ; however, Care must be taken to assure that the general they must also be portable. Shields should public and visitors to a logging project do conform with Interstate Commerce Commis­ not receive exposure above acceptable levels. sion regulations so they can be shipped. Visitors in the immediate area of operations Despite high manufacturing standards, for more than a cursory examination of a sealed sources occasionally do leak; there- project should be required to wear film fore, when sources are received, they should badges furnished by the project. be wipe tested, and procedures should be Logging sondes containing radioactive ma­ set up for at least semiannual wipes. The terials should never be lowered in a well sources are wiped with filter paper, which until at least one other sonde has freely tra­ is first checked with portable monitoring versed the full depth interval to be logged. equipment and then sent to a laboratory If any caving is experienced or anticipated capable of detecting the presence of 0.005 or any obstruction is found, radioactive ma­ microcuries of removable contamination. terials should not be put in the hole. An Any suspected leaky sources should be report­ expensive effort is required to recover a lost ed immediately, and steps should be taken to source, and if it is not recovered, it may be prevent the spread of contamination. Sources necessary to cement the well. For these rea­ and other radioactive materials should al- sons, it is recommended that the radiation ways be stored in locked shields in a locked logs be run in the drill stem or temporary D room or vehicle. Approved radiation signs pipe if any difficulty is anticipated in the and labels are required on both the shield hole. If a radioactive source is lost, the and the outside of the room or vehicle. Radi­ Regional Radiation Protection Officer and ation levels outside transporting vehicles and the nearest A.E.C. Licensing Office must be storage roomsmust not exceed the prescribed notified immediately, and all attempted re­ limits. covery methods must be nondestructive. Var­ Source-handling procedures and equipment ious types of “fishing” tools are available should be designed on the basis of minimum commercially. personnel exposure necessary to do the job, Work with radioactive tracers requires ex­ not on the basis of maximum permissible tensive preparation and planning, as well exposure. Use the proper combination of as protective clothing and decontamination time, distance, and shielding to reduce expo- facilities. Procedures will depend on the type sure to a minimum. Sources should never be and amount of tracer used, the application, handled directly. Forceps or other remote- the ground-water environment, and the dis­ handling devices should be designed for rapid tance to nearest point of use. For these rea­ manipulation of the source, as well as in- sons, they will not be described here. The creasing the handler’s distance from it. Regional Radiation Protection Officer should Whenever possible, sources should be stored be consulted for this type of work, and the in a shield in the same sub used for logging. regulations and references cited herein must In this manner, the sub can be attached di­ be followed. Broad licenses to permit use of rectly to the logging sonde, which makes extra radioactive tracers in ground-water hydrol­ loading and unloading of the sub unneces­ ogy have not been issued ; instead, a separate sary. The sonde should be placed in the well license is required for each application of a as soon as the sub is securely attached. radioactive tracer. Considerable information Source subs should be metal, labeled “Cau­ is needed on the ground-water environment 88 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS before such licenses will be issued. This elastic wave through the media being logged. a requirement has tended to discourage the Figure 56 schematically illustrates a single- use of radioactive tracers in ground-water receiver probe and a two-receiver probe. In studies to date. The less strict regulations the single-receiver system, the sonic path is that have permitted a much wider use of T-A-B-R and includes the transit time in radioactive tracers in oil wells may also per­ the mud. In the two-receiver system, the mit application to deep waste-disposal inves­ transit time B1-Bz is recorded. Spacing T-R tigations in the future. is usually greater than 2 feet, and the spac­ ing between receivers is 1 foot or more. Velocity logs are recorded in microseconds Acoustic Logging per foot. Figure 5’7 shows an acoustic-veloc­ ity logging probe that is compensated or Generally, an acoustic log is a record of adjusted for borehole-diameter changes, os­ the transit time of an acoustic pulse between cilloscope presentation of the received sig­ transmitters and receivers in a probe. The nals, and a typical acoustic-velocity log. chief uses are for the measurement of po­ Because of the complex character of acous­ rosity and the identification of fractures. tic waves and the effect of various rock properties on these waves, a great potential exists for determining in-situ lithologic Principles and applications properties by use of acoustic techniques. Acoustic-velocity logs are widely used in Much useful data is available in the received- the petroleum industry for the measurement wave train. The types of acoustic waves re­ of porosity. Amplitude logs are often applied ceived after transmission through the rock to finding the location of fractures in a bore- are classified as compressional, or P waves ; hole and the determination of the character . of cement bonding between the casing and the formation. Acoustic logging, also called c sonic logging, utilizes many frequencies not audible to the human ear. Basically, all acous­ tic-logging devices contain one or two transmitters that convert electrical energy to acoustic energy, which is transmitted through the environment as an acoustic wave. One to four receivers reconvert the acoustic energy sensed to electrical energy for trans- mission up the cable. The tool is constructed so that the shortest path for the acoustic wave is that through the rock surrounding the well and then refracted along the bore- hole wall. The hole must be filled with water 6, or mud to permit transmission of the elastic waves from transmitter to formation, and from formation to receiver. Two general types of measurements may Bl be made in acoustic logging: 1 1. Interval transit time, which is the recip­ rocal of velocity. 2. Amplitude, which is the reciprocal of at­ tenuation. SINGLE-RECEIVER PROBE TWO-RECEIVER PROBE The continuous acoustic- or sonic-velocity log is a record of transit time (At) of an Figure 56. - Uncompensated acoustic-velocity prober. c APPLICATION OF BOREHOLE GEOPHYSICS 89

SURFACE DOWNHOLE along the wall of the borehole by particle EQUIPMENT EQUIPMENT movement both perpendicular and parallel OSCILLOSCOPE to the direction of wave movement, and their Pulse FromTrammutterNo2 velocity is less than that of the S wave. Therefore, the acoustic waves arrive in the , following sequence: P wave, S wave, and boundary wave. Ckntrctluer The frequency, or wave length, of trans­ mitted and received pulses is another impor­ 1~ronsmMer No 2 tant characteristic. Certain rock types exhibit frequency-selective acoustic characteristics (Chaney and others, 1966). For example, limestdne transmits the higher frequency \ Pulse Amvol , waves better than poorly consolidated sands

i Recwer No 2 and shales. As shown in the previous discus­ \ sion, the following acoustic-wave parameters GRAPHIC RECORDER and the ratios between them can be mea­ Recewer No. I sured : P- and S-wave velocity ; P- and S-wave amplitude, or attenuation ; and frequency. Research is presently underway in the pe­ troleum field on the relationship between permeability and the various acoustic pa­ rameters. Khalevin (1960) indicated that Transm~lter No I porous fractured limestone is more readily identified by wave-amplitude attenuation than by wave-velocity changes and suggested using continuous-amplitude logging for po­ rosity measurement. Taylor (1968) used UNCASED WELL sonic logs to estimate the vertical compress­ ibility of an artesian aquifer. He then used Figure 57. - Principles and equipment for making borehole­ the values of compressibility to plot the ef­ adjusted acoustic-velocity logs. fect of changes in net stress on the storage shear, or S waves ; and boundary, or surface coefficient of the aquifer. This use of acoustic waves (Guyod and Shane, 1969). The P logs in ground-water hydrology should be waves are propagated by the movement of investigated further. particles in the medium in the direction of At present, the continuous-acoustic-veloi­ wave propagation ; these waves arrive first ity log is used mostly for the measurement of and are characterized in consolidated rocks rock porosity in open holes (Pickett, 1960). by a lower amplitude than the S waves. The Recently, some acoustic-porosity logs were S waves have a velocity about one-half that also run in cased holes that have at least 40- of P waves and are characterized by particle to 50-percent cement bonding between the movement perpendicular to the direction of casing and the formation (Muir and Zoeller, wave propagation. S waves are not trans­ 1967). mitted through the fluid directly, but are The basis for porosity calculation from transmitted by means of an intermediate P acoustic logs is the time-average equation: wave. The amplitudes, or heights, of the P 1 # 1-e and S waves received are related to the v, =At=T+TI,, porosity, the degree of consolidation, the frac­ where turing of the rocks traversed, and the orien­ At = interval transit time, in seconds tation of the fractures (Morris and others, per foot ; 1964). Boundary waves are only transmitted @= porosity fraction ; 90 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

V, = velocity of signal in fluid, in feet porosity. Secondary porosity, due to vugs per second ; and fractures, will not be accurately mea­ V, = velocity of signal in matrix, in feet sured because the elastiawaves that give the per second; and first arrival apparently travel through the V, F velocity of signal in rock, in feet matrix around the openings. Furthermore, per second. correc.tion factors must be applied for un­ This equation is obtained by considering that consolidated or shaly sediments (Lynch, the path of an acoustic pulse through satu­ 1962). rated rock consists of two velocities, in series, The effect of secondary porosity on the V, and V,,. The length of the path in fluid various elastic-wave parameters can be used is equal to the porosity (Q) , and the length of to locate fracture zones near wells (Walker, the path in the rock matrix is equal to 1962). Fractures may also cause “cycle skip- (1-e). Because the equation is valid only ping” on sonic-velocity logs. This is due to for rocks with uniformly distributed inter- attenuation of the pulse to below the detec­ granular pore spaces, it can be considered tion level. A subsequent pulse not attenuated to provide values for effective porosity. The below that level is sensed as the first arrival; velocity of elastic waves in water or drilling thus, the recorded velocity is very low. Cycle mud is 5,000-5,400 feet per second (transit skipping is not uniquely caused by fractures ; time 200-185 +ecjft), and following are poor consolidation, gas, sands, and material velocity and transit-time ranges for the ma­ added to the mud to reduce circulation loss trix of some common rocks: can also cause it. P waves and S waves are attenuated to different degrees by fractures, Velocity Transit time Rock fft/sec) (rs=/ft) and the angle at which a fracture intersects Sandstones .._... 15,000-18,000 66.7-55.6 the drill hole affects the wave amplitude. Shales______6,000-16,000 167.0-62.5 Therefore, the total signal received must be Limestone ___.____._19,000-21,000+ 52.647.6 availa.ble for visual examination. The selec­ Dolomite ._.___.___...21,000-24,000 47.6-42.0 tive effect of fracture orientation on P- and 4_ The graph in figure 58 can be used to S-wave arrival and amplitude was discussed determine porosity from an acoustic-velocity in detail by Morris, Grine, and Arkfeld log when the acoustic velocity through the (1964). They were able to locate fractures rock matrix is known. If the matrix velocity and interpret their primary orientation from of a limy sandstone is 50 Psec/ft, and if At P- and S-wave amplitudes, but they were not from a log is 78 rsec/ft, the porosity is ap­ able to determine fracture density. proximately 20 percent. The acoustic-velocity Similar interpretation of a full acoustic log should only be used for porosity calcula­ signal provides the best means of determin­ tion in rocks having uniformly distributed ing the bonding of cement between the casing and the formation (Muir and Zoeller, 1967). At Log-At Matrix Porosity from Transtt Time 0 q Sonic velocity in steel casing is very high. At Llqutd-At Matrix When the casing is centralized in the hole 40 Dolomltes and not bonded to the formation, only a LImestones symmetrical casing signal will be recorded. Decentralized unbonded casing will be recog­ nized by a distorted casing signal, and bond­ Sandstones ing of cement to the casing but not to the formakion will produce a very low amplitude Shaly Sandstones casing signal but produces no formation sig­ nal. The amplitude of the casing signal is ‘“lb0 9’4 i8 8'2 7-6 7b 64 j8 52 66 4.0 decreased by good bonding between cement At Log M sec/ft and casing, and the amplitude of the forma­ tion signal is increased by good bonding be- Figure 58. -Graph based on the time-average equation for the determination of porosity from acoustic velocity. From Welex tween the cement and formation. Sufficient (1968). cement bond to permit signal transmission 4 APPLICATION OF BOREHOLE GEOPHYSICS 91 to and from the formation will allow the ter and the first receiver do not change the recording of a log of acoustic velocities of At. However, even with this system, anoma­ rock. A good casing-formation bond can be lous deflections occur on the log when the determined by comparing the velocity log edge of a cavity or washout in the borehole made after the hole is cased with an open- is located between the receivers. These de­ hole velocity log or a neutron log. flections are in the opposite direction at the Acoustic-transit time has also been related top and bottom of a cavity. Figure 57 illus­ to the engineering properties of rocks by trates the above explanation; when the edge Carroll (1966). Plots of transit time versus of a cavity is located between receivers, there static Young’s modulus and ultimate com­ is a difference in length of the travel path pressive strength show a correlation with from the formation to the receivers. The transit time. The in-situ elastic properties latest development in acoustic-logging de- of rock salt were calculated by Christensen vices incorporates two transmitters and (1964) from acoustic-log values of P- and either two or four receivers, and is termed S-wave velocities, and density was calculated “borehole compensated” or “corrected” (Ko­ from gamma-gamma logs. The properties kesh and others, 1965). calculated, assuming that the medium was Most transmitters and receivers used at isotropic, were the bulk-compression modu­ the present time consist of a roll of thin mag­ lus, Young’s modulus, the shear modulus, and netostrictive alloy wrapped with a coil of Poisson’s ratio. The values obtained through wire. Various types of piezoelectric crystals logging compared very favorably with aver- are also used. A pulse of current through the age laboratory measurements on rock salt. wire causes the metal to oscillate by means These characteristics are useful in studying of rapid contraction and expansion. This the influences of underground nuclear explo­ vibrational energy is transmitted through the sions and should also be useful in the in­ fluid in the hole to the rock and back through terpretation of surface seismic records. the fluid to the receivers. This type of trans­ Another presentation of acoustic-log data ducer emits a range of frequencies from is most useful in the identification of litho­ about 5,000 to over 40,000 hertz ; the center logic units causing seismic reflections, The frequency is usually about 20,000 hertz. integrated travel time is recorded along the So that the first arrival detected will be side of the standard acoustic-velocity log. The the formation signal, the body of an acoustic- time required for a seismic pulse to travel logging sonde must be constructed of rubber, through any depth interval may be obtained or of similar low-velocity high-attenuation by adding the time pulses that appear along materials, so that the amplitude of the elastic the side of the log for the interval of interest. wave propagated through the tool is less than the amplitude propagated through the formation. Acoustic-logging tools should be Instrumentation centered in the hole with bow springs or There are three general types of acoustic- rubber fingers so that the signal path length logging sondes. The earliest sondes developed is consistent. consisted of a single transmitter and a single The electronic equipment required to make receiver. Any change in the length of the acoustic logs is very complex. Power and wave path, such as that caused by a washout, timing circuits pulse the transmitters. Gating drastically affected the At that was measured. and amplifying circuits send the proper se­ The first step in improving this tool was to quence of pulses up the cable. Adjustable add a second receiver. With the two-receiver trigger circuits are used to select the proper sonds, which are still in use, At is measured point for first-arrival timing. For this pur­ from the time of arrival at the first receiver pose, an oscilloscope presentation in the (RI) until the time of arrival at the second logging truck is essential. Also, scope presen­ receiver (I&). With this arrangement, differ­ tation allows observing and photographing ences in acoustic path between the transmit­ of wave amplitude and frequency, as well as 92 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

arrival time. Circuits average the transit model having a given porosity is a relatively times and convert them to a DC voltage, simple matter, but to simulate the acoustic which drives the recorder pen. response of a certain rock type in an open Continuous or periodic pictures of the oscil­ hole would probably require a large, water- loscope trace of the wave form or full-wave saturated block of that type of rock with a recording (Muir and Fons, 1964) constitute hole drilled in it and pressure to simulate another type of data output that is used for inhole conditions. fracture-finding and cement-bond logging. Acoustic velocity, as well as porosity on Borehole-corrected sondes can be run on sin­ core samples, can be measured to determine gle-conductor cable, but the electronics is some of the reasons for errors in acoustic simplified and the cost is reduced when a measurement of porosity (Jenkins, R. E., multiconductor cable is used. 1960). Jenkins noted that, in quartz sand- stone, the best 30 percent of data points ob­ Calibration and standardization tained showed a deviation of only +l percent Interval transit time, in microseconds per porosity from the theoretical time-average foot-the standard unit for all acoustic-veloc­ line. The best 90 percent showed a deviation ity logs-can be read directly from a cali­ of +5 percent. In contrast to neutron logs, brated oscilloscope, which should be a part of acoustic-velocity Iogs are the most accurate every acoustic-logging system. Assuming that for logging consolidated rocks of a higher there are no electronic errors in transmission porosity but are inaccurate for logging rocks of the triggering, or transmitter, pulse and in whic’h porosity is less than 5 percent. the receiver signal, the transit times from Both acoustic-velocity and porosity mea­ the scope can be written on the graphic ana­ surements on core samples are recommended log record when the sonde is stationary in if the Ifogs are to be used for quantitative the hole. This can be done at the same time interpretation. Core samples used for this that the operator is selecting the first P-wave purpose should be undisturbed and, if pos­ arrival. The transit time measured at these sible, saturated with native fluids. Determin­ points in the hole is, of course, subject to the ing the relation between acoustic velocity and judgment of the operator in selecting ar­ porosity in the laboratory provides another rivals. Use of several built-in calibration means of learning the effects of such inhole signals that can be recorded on the log at conditions as confining pressure. any time is desirable, in order to check up- hole circuits and to provide standard points Radius of investigation on each log for determination of pen-zero The radius of acoustic investigation varies positioning and span, or sensitivity. Calibra­ with th’e wave length and frequency of the tion points should appear on every acoustic elastic wave, as well as with the sonic log, preferably before and after the run, as velocity.. The radius investigated is report­ an indication of uphole system drift. No con­ ed to be about 3 times the wave length, h venient field standard for checking the re­ (Pirson, 1963). sponse of the total acoustic-logging system is, as yet, available. x +, Theoretically, transit times from acoustic where logs can be converted to porosity values if V, = velocity in rock, and both the matrix and the fluid-velocity values n = frequency. are known. As with most other logs, environ­ At a frequency of 20,000 hertz, the radius mental calibration is desirable, to reduce the of investigation should be about 0.75 feet possibility of extraneous effects on logs. Be- for soft rocks having a velocity of 5,000 cause of the difficulty of building environ­ ft/sec, and should be about 3.65 feet for mentaI sonic modeIs, the relation of core hard rocks having a velocity of 25,000 ft/ samples to log deflection is probably the sim­ sec. Reducing the transmitter frequency will plest method of calibration. To construct a increase the radius of investigation. APPLICATION OF BOREHOLE GEOPHYSICS 93

Extraneous effects the caliper log. A borehole-adjusted acoustic B tool should greatly reduce this error. Also, if Cycle skipping on acoustic logs, an extra­ a logging sonde is not properly centered, neous effect which is fairly obvious (fig. 59)) it can cause signal attenuation due to phase occurs when the detecting circuits miss the shift and can result in missed arrivals. Re­ first arrival, so that time is measured to the ceiver span should always be considered next detectable arrival. It is caused either when interpreting acoustic logs: the shorter by excessive signal attenuation in the fluid the span, the greater the resolution of thin or the formation or by equipment malfunc­ beds, and the more detailed the log. tion. The cycle skipping on the acoustic log The greatest problem in the interpretation made with four receivers in figure 59 is of acoustic-velocity logs or acoustic-wave probably due to the rugosity of the wall of records is the variability in environmental the hole, as shown on the caliper log. Proper factors affecting the transmission and atten­ electronic gating in the equipment can great­ uation of elastic waves. Acoustic velocity in ly reduce this problem. The acoustic logs in porous media is dependent on such lithologic figure 59 also recorded deflections at depths factors as the type of matrix, the density, of 920 and 1,030 feet, which may be due to size, distribution, and type of grains and changes in the hole diameter, as shown on pore spaces, and the degree of cementation,

ACOUSTIC LOG ACOUSTIC LOG DEPTH 2 TRANSMITTERS, 2 TRANSMITTERS, FEET CALIPER GAMMA-GAMMA NEUTRON LOG 4 RECEIVERS 2 RECEIVERS LOG LOG 840 1

B 900

1000 “ifw.==z.jL

I 1lOll‘I 3x&? 6 7 -ii- 100 75 100 75 1600 1700 270 200 290300 HOLE DIAMETER - INCHES TRANSIT TIME - MICROSECONDS PER FOOT COUNTS PER SECOND

Figure 59. -Three porosity-related logs. Note the cycle skipping on the commercial acoustic log and the hole-diameter effect on the USGS gammagamma log. 94 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

and upon the elastic characteristics and prop­ measurement of the area of the hole, and 4 - erties of the interstitial fluids (Jenkins, R. independently recording arms are needed. E., 1960). The widely used time-average (Hilchie, 1968). equation (p. 89) has been criticized because Caliper logs are utilized for the identifi­ it does not account for most of these factors. cation of lithology and stratigraphic correla­ Most of these characteristics are unknown tion, for the location of fractures and other unless considerable sample analysis has been openings, as a guide to well construction, and done, and, therefore, calculation of correc­ to correct the interpretation of other logs for tion factors would be impossible even if the hole-diameter effects. Most changes in hole necessary equation were available. The time- size are caused by a combination of drilling average equation is an approximation; nev­ techniques and lithology. Drilling factors ertheless, it has been found to produce which can cause changes in hole diameter satisfactory porosity figures under most con­ include : Drilling technique ; weight and ditions. The time-average equation is recom­ straightness of the drill stem ; volume, pres­ mended to be used only after the results have sure, and type of fluid circulated ; and length been checked by means of adequate samples of time the drilling equipment is in the hole. in each geologic environment. Correct values Lithologic factors which will affect the hole for porosity are most likely to be ascertained diameter include: Type and degree of ce­ when several types of porosity logs are run, mentation or compaction ; porosity and per­ as shown in figure 59. meability ; bed thickness and vertical distance to adjacent hard beds; size, spacing, and orientation of fractures and vugs ; and the swelling or hydration of clay. Caliper Logging Figure 60 illustrates the differences in hole diameter in beds of the same lithology caused The caliper log is a record of the average by changes in drilling technique. The solid diameter of a drill hole. Its major use is line on the left is the caliper log of a core (1 to evaluate the environment in which other hole C-IL, in the upper Brazos River basin in logs are made in order to correct them for Texas. The hole was badly washed out during hole-diameter effects and to provide in.forma­ drilling operations, which were designed for tion on lithology. maximum core recovery. An adjacent offset hole was drilled rapidly with a tricone rotary bit. The reduction in hole-diameter changes Principles and applications can be ;seen on the dashed log of T-14. The Continuous logging of the average diame­ same lithologic units can be identified on both ter of drill holes is one of the most useful logs despite the difference in drilling tech­ and simplest techniques in borehole geo­ niques. Note the three anhydrite beds be- physics. Most caliper sondes consist of one tween depths of 100 and 150 feet, which are to four pads, or bow springs, or feelers which shown as hard layers where the hole diame­ follow the wall of the hole. The graphic rec­ ter is less than 5 inches. A major fracture ord, calibrated in inches, is the average hole system just above 200 feet was recorded as diameter. Averaging is generally done so a zone of thin washouts that attained a maxi- that an increase in hole radius in one direc­ mum diameter of 14 inches. The importance tion will not cause the recorded increase in of making caliper logs to aid in the interpre­ diameter to be as great as an equal radial tation of other logs is demonstrated by the increase that is symmetrical around the axis hole-dia.meter effects on the gamma-gamma of the hole. Some commercial calipers have logs in figures 4’7 and 60. only one feeler and, so, may not give a true The repeatable detail that can be detected picture of an asymmetrical hole. In an by a sensitive caliper sonde is shown on the elliptical hole, none of the usual feeler con- right side of figure 61. The two logs were figurations will necessarily give a correct made of core hole C-l on different dates by APPLICATION OF BOREHOLE GEOPHYSICS 95

DEP A B FEE 5 6 INYES8 9 lo RADIATIONINCREASES I I I I I L II -- z -Y w ------=--=--- -==-- 4 LL-- 1 ---_ _---- > -z------33 r-14\ 7 5- 3 I -- f \- x&‘@=---’ .-YZ

401 GAMMA-GAMMA

Figure 60. - Effect of drilling technique on hole diameter, and effect of hole diameter on gamma-gamma logs. different logging operators. Rotary hole T-5 the same stratigraphic position. The differ- is approximately 3 miles from C-l; yet, a ence in the drilling techniques is evident fracture zone was recorded in approximately from a comparison of the caliper logs. In 96 TECHNIQUES OF WATER-RESOURCEtS INVESTIGATIONS both holes, however, the very hard anhydrite packer tests. The caliper sonde detected most a beds in the upper part of the holes remained of the fracture zones found in coring and approximately bit size. The fracture zone was particularly effective in showing the also caused similar amplitude deflections on main water-bearing zone between 680 and the caliper logs, even though there were dif­ 800 feet deep. The fracture zones on the elec­ ferences in drilling technique. The fracture tric logs that were not detected by the caliper zone in T-5 was found to be the main source might have beenrecorded, if shorter arms and of brine and gas in the hole. Correlation, a higher sensitivity had been used. In the using caliper logs, of the lithology and frac­ Snake River Plain, Idaho, the sensitivity of tures is substantiated by the single-point re­ a three-feeler caliper was found to be suf­ sistance logs shown on the left in figure 61. ficient to detect the scoria and vesicles which Anhydrite beds are interpreted from logs by mark the top of basalt flows. Further, a cal­ their high resistance and small hole diame­ iper log can be run with enough sensitivity to ter, and fractures are interpreted by their pick joints in casing and to distinguish be- low resistance and large, irregular hole di­ tween smooth, new casing and rough, cor­ ameter. roded casing. The location of fractures in igneous and Caliper logs were one of the first success­ metamorphic rocks and of solution openings ful means of identifying and correlating in limestone is an important use for the aquifers in the Snake River Plain (Jones, caliper log. Figure 14 shows several logs 1961). Cinder beds and unconsolidated sedi­ made in a hole cored in granite in Clear ments in this area cave badly during cable- Creek County, Colo. Information on the fault tool drilling. Vibrations of the drill bit cause and fracture zones and the water-bearing caving of sediments that occur above layers zones (right of the spontaneous-potential of hard basalt. Beds of unconsolidated sedi­ log) was obtained from core, pump, and ments above the water table continue to cave

SINGLE-POINT RESISTANCE LOGS CALIPER LOGS HOLE= t-3 miles- HOLE T-5 C-l logged 2-77-65 w

-I- OHMS fi i ii i s i i s 44 lb 11 12 HOLE DIAMETER - INCHES

Figure 61.-Correlation of fracture zoner between rotary hole T-5 and cow hole C-l from caliper logs. Note the repeatability of caliper logs mode on different dates in hole C-l. APPLICATION OF BOREHOLE GEOPHYSICS 97

as a hole is deepened because of the water changes in the hole diameter. Mud squeezes poured in the hole during drilling. In the can be recognized on the caliper log by the Snake River Plain, caliper logs were also used areas that are significantly smaller than bit to determine the best place to seat the casing size, and, frequently, the mud squeezes cause for cementing and to estimate the volume of the caliper arms to close all the way. In addi­ the annulus to be filled (Keys,. 1963). The tion, mud squeezes can completely close a caliper logs in figure 46 were used to select drill hole and can even cause the loss of log­ a depth of 460 feet for seating the casing to ging sondes. Mud rings or mud cake build-up be cemented because only a few caved sedi­ can also cause hole diameters to be less than ment zones were found just above this depth. bit size, but do not necessarily indicate a The high permeability of some sediment caving hole. zones allows lateral movement of grout and Mud cake build-up on the wall of the bore- prevents fill-up of the annulus. hole may be related to the porosity and per­ An accurate caliper log can be used to meability of the rocks, and thick mud cake determine the size of casing a hole will ac­ can be detected with a caliper log. Most drill­ cept, and to what depth it can be set. Caliper ing muds are a suspension of clay particles logs are also extremely useful for calculat­ in water, and higher pressures in the hole ing the annulus volume for effective gravel cause the water to invade the rock and leave packing. Annulus fill-up with much less than a layer of clay or mud cake on the wall of the calculated volume of gravel would be the hole. Whether a caliper will cut through indicative of gaps not properly filled with or ride on top of a mud cake depends on the gravel. An accurate caliper log is also es­ pressure exerted on the springs or feelers. sential for selecting seats for straddle or Some pad-type devices are designed to ride isolation well packers. All packers have an ef­ on top of the mud cake. fective operational range of hole diameters. One of the most important applications of A packer either will not set, will blow out, or caliper logs is aiding in the correct interpre­ D will leak when inflated in a portion of the tation of other logs. Most logs that respond hole that is too large. Furthermore, a caliper to lithology are also affected by changes in log will locate fractures in hard rock that hole diameter. Figure 61 shows this effect may permit flow to bypass the packer or on gamma-gamma logs. Changes that are cause it to blow out. Packers should never consistent over a thick interval, such as those be set in rough, irregular parts of a hole. caused by a change in bit size, can be cor­ Caliper logs are essential for quantitative rected by use of empirically derived charts. interpretation of flowmeter logs or tracer In general, the changes in the log response work measuring vertical flow. All vertical in- caused by borehole rugosity cannot be cor­ hole flow measurements must be corrected rected, and very rough zones seen on caliper for changes in the hole or in the casing logs should be eliminated from quantitative- diameter. log interpretation. In sequences of detrital sediments that are poorly to fairly well consolidated, the mas­ Instrumentation sive sands and sandstones are generally closer to bit size and produce a smoother Two basic types of measuring systems are hole than do the clays and shales. Thin beds used in caliper tools-arms or feelers hinged are partly responsible for the sharp, closely at the upper end and maintained against the spaced caliper deflections that are common hole wall by springs, and bow springs fas­ in shale. Shale and compacted clay tend to tened at both ends. The arm-type device is become hydrated and to swell and slough much more sensitive than the bow-spring into a hole that is drilled with water-base device, which may span washed out or frac­ mud (Jones and Skibitzke, 1956). Processes ture zones. Most feelers also exert a higher of this type may continue long after a hole pressure than bow springs. The caliper logs has been completed and cause progressive in the center of figure 47 can be compared 98 TECHNIQUES OF WATER-RESOURCESINVESTIGATIONS

with a log on the left, made with a one-arm Caliper tools are available for small log­ pad-type caliper, and with a log made with gers that will measure hole diameters up to the three-arm probe used on most Geological 42 inc’hes ; however, arms that are longer Survey loggers. Devices employing small- than necessary reduce the sensitivity. In ad­ diameter feelers have a much greater vertical dition, with long arms, the measuring point resolution than either the pad or bow-spring or depth will differ significantly from the probes. If a hole is round, all types of calipers nearly closed position to the fully open posi­ should give the same log. In asymmetrical tion. Feeler-type calipers are generally pro­ holes, one- or two-arm calipers will generally vided with interchangeable arms of several measure a larger diameter than three- or different lengths. The arms should be selected four-arm calipers. If asymmetry is impor­ on the basis of the hole-diameter range ex­ tant, dual-type calipers that make indepen­ pected and the type of information desired, dent measurements should be used (Hilchie, and the basing and span adjustments should 1968). Both types of tools use the feelers be made to satisfy these same requirements. to change the resistance of a potentiometer, Spring pressure on the feelers is a factor which is monitored by voltage changes at the in caliper tools, inasmuch as inadequate pres­ surf ace. sure will not force the arms to a fully open Motor-driven feelers have proved to be the position in heavy mud. Spring pressure will most dependable means of opening the arms, also affect the degree of penetration of mud although heavy mud or sand in the mecha­ cake. IObviously, long arms will require a nism will occasionally prevent them from stronger spring pressure in the tool to achieve opening. A DC motor in the tool can be the same force at the ends of the feeler. operated at any position in the hole, and a light on the caliper module indicates when the arm-drive mechanism has reached the Clalibration and standardization end of its travel. By reversing the DC po­ Calibration of caliper tools should be done larity, the arms can be closed at any time, in cylinders of different sizes that contact thus permitting the tool to travel downhole all the feelers. Because it generally is not and to relog without returning to the surface. feasible to carry many cylinders in a logging An early device that opened the feelers when truck, a board-type standard inay be used, the sonde wasbounced on the bottom of the instead. The board consists of a large hole, hole was not satisfactory because the bottoms to fit the body of the sonde, and small holes, of drill holes are generally filled with soft drilled to accept a single feeler, in order to mud and loose drill cuttings. Another type of simuhte the various hole sizes. Rings may caliper tool uses an electric blasting cap to also b’e used, but if they are made of rod break an arm-retaining wire, thus permitting stock, they are difficult to use. The correction the arms to open. This system does not al­ factor between the field standards and the ways work, and any relogging necessitates use of a one-feeler calibration board and returning the tool to the surface for time- calibration cylinders must be established if consuming rewiring. A system that does very accurate measurements are required. not permit closing the feelers in the hole Before logging a hole, the basing and sen­ means that the open caliper must be dragged sitivity adjustments and field standard should through the entire cased part of the hole, be used to set up or to verify the logging span causing excessive arm wear. Also, it is not desired. One inch of chart width per inch of aIways possible to pull an open caIiper into hole diameter was found to be sufficiently a small-diameter pipe or drill stem with the sensitive for fracture location and is an easy arms open. In addition to the controls for scale to use, Standard measurements should opening and closing the feelers, preselected be plotted directly on the log, and enough scales, or a variable scale having independent points, should be plotted to establish the lin­ zero positioning and span, are provided. earity of system response. If any system drift APPLICATION OF BOREHOLE GEOPHYSICS 99

is suspected, several points should also be log if the fluid in the well is in thermal equi­ B plotted on the caliper log after the tool is librium with the adjacent rocks and if there removed from the hole. It is also desirable to is no vertical circulation of fluids in, or log at least 20 feet of casing before closing adjacent to, the well bore. Such static con­ the tool as a further check of the calibration ditions are unusual, but where they occur, and as an indication of the electronic noise the temperature gradient is largely a func­ on the log. tion of the thermal conductivity of the rocks. Thermal conductivities (calories per second­ centimeter-C) vary from as high as 10 for Radius of investigation and extraneous some types of sandstone, and range from 4.8 effects to 8.1 for limestone, and from 2.2 to 3.9 for clay. In most wells the geothermal gra­ The radius of investigation of a caliper dient is considerably modified by fluid move­ is the point at which the feeler touches the ment in the borehole and adjacent rocks. A borehole wall. The caliper has a more nearly geothermal gradient measured under com­ unique response than most other logging pletely static conditions is one extreme, and devices. Instrumental malfunctions are more no gradient produced by a very high flow likely to cause spurious log response than are rate through the hole is the other extreme. the borehole parameters. Heavy mud can All logs fall between these extremes. In gen­ prevent the tool from opening, and mud eral, the geothermal gradient is steeper in cake can prevent the feelers from reaching rocks with low permeability than in rocks the wall of the hole. However, temperature with high permeability, possibly because of drift and cable leakage are much more fre­ ground-water flow. This relation offers a quent sources of anomalous log response. potential interpretation of thermal data to determine lithology or the relative magni­ tude of permeability. Typical geothermal gra­ Temperature Logging dients are between 1” and 1.3”F per 100 feet of depth. The depth-temperature relation is Temperature logs are the continuous rec­ also reported as reciprocal gradient, or the ords of the temperature of the environment depth interval per degree. immediately surrounding a sensor in a bore- Seasonal recharge to the ground-water hole. They can provide information on the system may be reflected in cyclic-tempera­ source and movement of water and the ther­ ture fluctuations, and the vertical movement mal conductivity of rocks. of water in the unsaturated zone can be investigated with very sensitive temperature sensors (Stallman, 1965). Temperature logs Principles and applications may also be used to identify aquifers or perforated sections contributing water to a Most temperature logs are continuous rec­ pumped well. Water in different aquifers is ords of the thermal gradient of the fluid in seldom the same temperature. Where two a borehole, and are made with a single sensor aquifers contribute water of different tem­ moving down the hole. Most commercial logs peratures to a well, it may be possible to are recorded in degrees Fahrenheit. Recent­ estimate the relative contribution of each ly, some of the WRD logging equipment has zone from the temperature of the well dis­ been calibrated in degrees Celsius. The tem­ charge. perature recorded is only that of the fluid Figure 62 shows a series of temperature surrounding the sensor, which may or may logs of an unsuccessful oil well that was not be representative of the temperature in drilled and then plugged near Anchorage, the surrounding rocks. The geothermal gra­ Alaska. It was decided to jet-perforate the dient may be derived from a temperature casing at intervals, selected from geophysical 100 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

DEP”’ OEPTH FEE 1 FEET

-I 00

C D

20

300

6OY

6( 600

Figure 62.-Temperature logs, Yukon Services well, Anchorage, Alaska. APPLICATION OF BOREHOLE GEOPHYSICS 101 logs, for selective aquifer testing with a a temperature of 62°F attains the normal pump and packer. Log A was made after the ground-water temperature of 52°F within 9%-inch casing had been filled with water 1.5 miles downgradient. Temperature logs and allowed to stabilize for several months. clearly show the horizontal and vertical dis­ Above 150 feet, the temperature gradient persion of warmer water from the discharge was small, apparently because of significant well. Temperature logs can be used to locate ground-water circulation. The gradient be- an unknown source of thermal pollution. low this interval was typical of that logged Further, the temperature distribution in oil wells in the Cook Inlet field. Log B was around a disposal pond at the NRTS is ap­ made after the casing had been perforated parently related to the velocity of the ground- and after the well had been surged and bailed water movement. for several days. The perforated intervals Figure 63 shows a series of temperature between 500 and 600 feet are clearly distin­ logs made during artificial recharge of the guished; however, temperature log B did not Ogallala Formation near Lubbock, Tex. indicate two deeper intervals that were also The recharge water was obtained from perforated. Nonetheless, logs B through D a nearby playa lake and fed to a well suggested that some warmer water from the by gravity at 100 gpm for 24 hours. The deeper aquifers did move up the well during recharge water contained about 700 ppm development and pumping. Log C shows that suspended sediment and had an initial most of the water moving up the casing at temperature of 22°C compared with 17°C 14 gpm was coming from the 575foot-deep for the ground water. The casing in the re- aquifer. The flow rate was determined from charge well was slotted for its entire length, brine tracejector logs. (See section on “Fluid- and the well was gravel packed. Auger holes Movement Logging.“) Brine-tracer logs and were drilled at various distances from the neutron logs showed a casing leak at about well and cased with 2-inch-diameter pipe 115 feet that was contributing most of the equipped with well points. The logs shown water pumped. (See fig. 52.) Temperature in figure 63 were made in auger hole H-3, log D shows that, after pumping for about 24 which was 6.5 feet from the injection well. hours, the aquifer at 515 feet had developed Recharge was started at 1600 hours, and and was contributing some water. temperature logs A and B show that the Temperature logs can be extremely use­ water reached auger hole H-3 between 1615 ful in identifying recharge water or liquid and 1628 hours. The zone of maximum ve­ wastes discharged to the ground. Olmsted locity at 105 feet coincides with the interval (1962) mapped the temperature distribution of lowest clay content indicated by the low of ground water from wells logged at the natural-gamma intensity. The recharge wa­ NRTS (National Reactor Testing Station) ter gradually spread vertically through the in Idaho. He attributed the zones of warm sand until it reached the top of the underly­ water, some of which are coextensive with ing clay (logs C and D) . The well lost ap­ chemical changes, to hot-spring sources, low­ proximately one-half of its specific capacity er permeability rocks, and (or) recharge during the test, and gamma-transmittance from excess warm irrigation water. Zones logs made before and after recharge suggest­ of cooler ground water in the Snake River ed significant plugging by sediment at a Plain aquifer may be related to recharge depth of 105 feet. from cold surface water. Temperature logs Because of the effect of temperature on permit the identification of warm waste wa­ electrical resistance, temperature logs are ter, discharged through a well at the NRTS, necessary for the correct interpretation of as it moves downgradient in the aquifer resistivity logs. Alger (1966) showed the (Jones, 1961). Water injected into a well at magnitude of this correlation (fig. 2). Spe- 102 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

DEPTH FEET

WATER LEVEL BEFORERECHARGE - 100 I

1628 HOURS is ( 110 2215 HOURS U-J

,,n I- \ LA RECHARGESTARTEDAll600 HOURS

\ Y -+ . --- FOLLOWING DAY, 1030 HOURS ----

C- \ _--a_-­ NATURAL GA~~~~-_-, c - -me-- -1,150 c Figure 63.-Temperature logs during artificial recharge, auger hoIce H-3, Hufstedlerwell, nearLubbock,Tex. cific conductance may be calculated from becomes more difficult. The temperature fluid resistivity and temperature logs. The anomaly should be greater where there is density and viscosity of water are very im­ more cement behind the casing. A neat port- portant characteristics affecting its move­ land-cement grout should have a temperature ment, and they are related to temperature of 160°F after 4 to 8 hours and should have (figs. 64,65 ; Olmsted, 1962). The importance a temperature of 100°F after 8 to 12 hours of viscosity is emphasized by Jones and Ski­ at shallow depths, where the normal borehole bitzke (1956) who pointed out that an aqui­ temperatures are considerably below 100°F. fer having an effective permeability of 2,000 The differential-temperature log utilizes a meinzers at 60°F has an effective permeabil­ different measuring principle than the tem­ ity of about 3,200 meinzers at 100°F. perature log and offers a potentially greater Temperature logs are also used to deter- sensitivity. The differential system detects mine the location of grout outside the casing the difference in temperature over a fixed after cementing a well (Keys, 1963). For interval of depth. Differential-logging sys­ this purpose, the well should be filled with tems are of two types. One system uses two water to a level considerably above the ex­ thermal sensors that are spaced from 1 to pected top of the grout, and a temperature several feet apart along the axis of the sonde log should be run within 24 hours of cement­ (Basham and Macune, 1952). This type of ing. With a temperature tool, it is possible tool is shown on the right in figure 66. The to detect grout for several days after cement­ differential-temperature log illustrated is a ing. Anomalous temperatures may prevail record of the temperature difference between for longer periods, but log interpretation the two sensors. Another type of differential c APPLICATION OF BOREHOLE GEOPHYSICS 103

perature log run near Roswell, N. Mex. The sharp change in gradient at a depth of about 340 feet seems to be related to the increase in porosity shown just below this point on the neutron log. A temperature log should always be run simultaneously with a differential-tempera­ ture log. This not only provides the actual temperature values, but often the first log made down the hole is the only one that is correct because passage of the tool disturbs the fluid column. Preselecting the tempera­ ture range for logging is difficult because the temperature is generally not known; yet, a sensitive scale is desirable. Figure 67 shows Figure 64. - Relation of viscosity of water to temperature. From repeat temperature logs and dual-sensor dif­ Olmsted (1962). ferential-temperature logs in a deep well in Colorado. The left trace shows the tempera­ ture log on a sensitive scale. The numerous changes in zero positioning, or basing, neces­ sary to prevent the pen from going off-scale, make the log very difficult to decipher. The log was reconstructed in the office to pro­ duce the smooth trace shown (second curve from left). The depth interval from 1,780 feet to 2,100 feet is shown as the third enlarged curve from the left. Both the temper­ ature gradient and the differential-tempera­ ture logs repeated very well from the first to the fourth logging traverse through the interval. All the temperature logs had to be

Figure 65.-Relation of specific gravity of water to temperature. reconstructed to be interpreted, but the From Olmsted (1962). differential-temperature logs are understand- able as is. The gradual changes on the tem­ system uses one sensor and an electronic perature log are sharply accentuated on the memory, so that the temperature at one time differential-temperature log, and the repeat- can be compared with the temperature at a ability of minor changes is good. Good log selected previous time (Johns, 1966). With repeatability suggests that disturbances by either tool the recorder response may be set at the logging sonde had a negligible effect on zero in a reference-temperature gradient in temperature distribution. In many holes the the borehole. In this way the log will demon­ thermal logs do not repeat very well, and strate a passive response to the reference- the fluid must be allowed to return to equi­ temperature gradient and an active response librium before logging is repeated. Gas enter­ to changes in gradient. Any changes from ing a well generally produces a significant reference gradient should be recorded as thermal anomaly, and, although this is not sharp deflections from a baseline, as shown a common occurrence in water wells, gas- on the hypothetical curve in figure 66. The produced thermal anomalies occasionally oc­ differential log can be considered as the first cur in deeper test holes (Van Orstrand, derivative of the temperature. Note the dif­ 1918). This may be the explanation for the ference in sensitivity between the tempera­ low-temperature anomaly shown at a depth ture log and single-sensor differential-tem­ of 1,750-2,100 feet in figure 67. 104 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

DEPTH, FEE MASK WELL, ROSWELL, NEW MEXICO THEORY AND EQUIPMENT

201 TEMPERATURE INCREASES-+ POROSITY DECREASES- I

AT T NEUTRON-NEUTRON LOG DIFFERENTIAL-(AT) T HYPOTHETICAL CURVES PROBE TEMPERATURE LOGS c

Figure 66. - Principles, equipment, and examples of differential-temperature logging.

The greater sensitivity and the sharp de­ Instrumentation flections provided by the differential tech­ nique suggest the possibility of using water Basically, temperature-logging sondes con­ as an inhole tracer. Water may be injected tain a thermistor whose internal electrical in a well and a series of time-lapse differen­ resistance changes in response to tempera­ tial-temperature logs used to locate the zones ture changes. A thermistor is a glass-in­ accepting the water. Basham and Macune sulated semiconductor that has a negative (1952) reported that the difference in tem­ temperature coefficient of resistance. A small perature between the cooler, injected water current must be fed through the thermistor, and the water of normal temperature below in order to measure changes in resistance, an aquifer is proportional to the total flow and if this current is too high, self heating accepted by the aquifer. Their tests also will occur, and, consequently, an error will showed that the differential system was sen­ be introduced. Any material used to enclose sitive to a flow of a fraction of a gallon per the thermistor or other sensor should have a minute leaving the borehole. Temperature high thermal conductivity and a small mass logs made shortly after a well is drilled may in order to reduce the thermal-response time. indicate the location of deeply invaded zones Furthermore, thermal conductivity between by means of temperature anomalies. Simi­ the sensor and body of the probe must be larly, a series of time-lapse logs before, dur­ minimized. The sensor is generally mounted ing, and after a pumping test will at least inside a cage or tube to protect it and to indicate the base of the lowest producing channel the fluid past the sensor. These tubes aquifer. (See fig. 62.) are usually constructed such that they are c APPLICATION OF BOREHOLE GEOPHYSICS 105

DEPTH, TEMPERATURE.DEGREES CENTIGRADE TEMPERATURE,DEGREES CENTIGRADE FEET -18- 20 2i 24 26 28 30 26 27 28 29 30 5DDr I I I

600 - 1800

700 - 800 -

900 - 1000 - 1100- 1900

1200 -

1300 -

1400 -

21oou 4 x 2100 ‘FIELD RECONSTRUCTED RECONSTRUCTED FIELDDIFFERENTIALLOGS TEMPERATURELOG TEMPERATURELOG TEMPERATURELOG FIRSTLOGDOWN FIRSTLOGDOWN SECONDLOGUP SECONDLOGUP FIRSTLOGUP

Figure 67. - Field- and reconstructed-temperature logs (left). The interval from 1,780 to 2,100 feet is enlarged (right) to show the detail and repeatability of differential-temperature logs.

more effective in channeling the fluid past measuring both of these fluid parameters be- the sensor when logging down a hole. Elec­ fore they are disturbed by subsequent passes tronics should be designed so that the changes of the sonde. in resistance elsewhere in the circuit or log­ ging cable have a negligible effect on the readout. Important controls are zero position­ Calibration and extraneous effects ing and sensitivity or span, which may either Many thermistors and other types of tem­ be continuously adjustable or be included in perature sensors are supplied with calibra­ a scale-selector switch. tion data in terms of resistance as a function Each temperature sensor and probe has an of temperature. An accurate thermometer is inherent response lag, or time constant, so used in a stabilized fluid bath to determine that logging speed must be constant and slow whether these values are correct. When the enough that temperatures are accurately re­ relation between temperature and resistance flected at true depths on the log. A single is properly determined, a single standardiza­ tool that makes simultaneous logs of fluid tion technique will suffice for accurate logs. temperature and resistivity is now available A resistance-decade box can be substituted commercially and offers the advantage of for the thermistor to check the response of 106 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS the entire logging system and to place stan­ reported the results of field and laboratory dardized values on each log. Field standard­ tests .and concluded that convection may ization is particularly important because of cause temperatures in the upper zone of the possibility of drift of the electronics in deep wells to depart widely from ambient the hole or at the surface in response to temperatures in the formations penetrated. changes in environmental temperature or This’ relationship is not only important in AC voltage, or frequency. temperature logging, but can be responsible In addition to instrumental effects, such as for introducing an error in tracer measure- thermal lag, selfheating, and drift of electron­ ments of very slow flow in a hole. ics, borehole conditions can cause significant Differential-temperature logs can display errors in the interpretation of tempera­ two unique types of errors. The log from a ture logs. Drilling, cementing, and testing two-sensor probe can be inaccurate because disturbs the thermal environment of a bore- the first sensor disturbs the thermal environ­ hole, and considerable time (up to several ment of the fluid through which it passes. years) may be necessary for equilibrium to The memory type of differential device is be attained. Furthermore, the borehole fluid based on the principle that time between the is disturbed during logging. Consequently, stored and the actual temperature is a func­ fluid-temperature and fluid-conductivity logs tion of the depth interval traversed. Error is should always be run down the hole before introduced if the logging speed is not con­ any other logs are run. If more than one stant. log is run, the first log down the hole should be considered the most nearly representative of equilibrium conditions. Convection in a well can cause significant Fluid-Conductivity Logging disturbance of the thermal gradient, particu­ larly in large-diameter wells. Krige (1939) Fluid-conductivity logs provide a measure­ indicated that the critical temperature gra­ ment of the conductivity of the inhole liquid dient above which convection occurs in the between electrodes in the probe. After ap­ absence of viscosity is propri,ate corrections, the logs provide data gaT on the chemical quality of fluid in a borehole. cp’ where Principles and applications g = acceleration due to gravity; T = absolute temperature ; Fluid-conductivity logs are a continuous (Y= coefficient of thermal expansion; record of the conductivity or resistivity of and the fluid in the borehole, which may or may cp = specific heat at constant pressure. not be related to the conductivity of fluids When viscosity is considered, the critical in the adjacent rocks. The most common gradient becomes sonde measures the AC-voltage drop across -Cvk +-, gaT two closely spaced electrodes, which is a g aa4 CP function of the resistivity of the fluid be- where tween the electrodes. Fluid resistivity is gen­ v = kinematic viscosity ; erally :measured in ohm-meters, the same unit k = thermal conductivity ; used for rock-resistivity logs. Use of the a = radius of the borehole ; and same units simplifies calculations involving C = a constant, which is 216 for a hole the two logs. Logs may also be recorded as whose length is great compared the reciprocal of resistivity-that is, conduc­ to its diameter. tivity. Conductivity is measured in micro­ Sammel (1968) plotted critical thermal gra­ mhos per centimeter, which is equal to 10,000 dients as functions of temperature, dissolved- divided by the resistivity, in ohm-meters. solids content, and well diameter. He also Logs of the borehole fluid are called fluid APPLICATION OF BOREHOLE GEOPHYSICS 107

conductivity in this manual to avoid confu­ concentration of 375 mg/l at 100°F and 760 sion with resistivity logs, which measure the mg/l at 50°F. A list of factors to convert rocks and their interstitial fluids. A tempera­ other salts to a NaCl equivalent is given on ture log made simultaneously with the fluid- page 8. conductivity log allows the most accurate Fluid-conductivity logging was first used conversion to specific conductance because it in ground-water investigations by Livingston eliminates the possibility of measuring a and Lynch (1937) of the U.S. Geological fluid column disturbed by the first trip down Survey in 1930. They developed and used the hole. Conductivity can be converted to conductivity equipment to locate salt water specific conductance at the standard temper­ leaking into artesian wells in Texas. Fluid- ature of 25°C or 77”F, using figure 68 (from resistivity or conductivity logs are widely Olmsted, 1962). For example, a conductivity used to determine water quality in wells and of 700 pmhos/cm at 50°F would equal a spe­ to aid in the interpretation of standard elec­ cific conductance of about 1,000 pmhos/cm tric logs. Spontaneous-potential logs and for a NaCl solution. Jones and Buford most types of resistivity logs are affected by (1951) presented a similar graph to convert differences in the conductivity of fluid in the resistivity to standard temperature. borehole. Electric logs are generally made The relationship of resistivity and temper­ shortly after the drilling of a well has been ature for a pure NaCl solution is shown in completed. Rotary-drilled holes are filled with figure 2 (Alger, 1966). The NaCl content, in a mud, which may be segregated by gravity ; milligrams per liter (mg/l) , can be read therefore, the fluid column will exhibit a directly from the chart if both the fluid resis­ gradual change in resistivity with time and tivity and the temperature are known. A depth. The resistivity of samples of circu­ resistivity of 10 ohm-meters indicates a NaCl lated mud does not give information on this condition, which can seriously affect the

‘Cl quantitative interpretation of electric logs. Olmsted (1962) gave some excellent exam­ ‘.05I”““” / ples of the relationship between the chemical 1.00 character of ground water and the sources

0.95 and movement of that water. There is no known way to directly calculate the concen­ 0.90 tration of specific dissolved ions in water from conductivity. However, if water sam­ 0.85 z ples analyzed in the laboratory show a con­ z 0.80 sistent relation between dissolved solids and 5 specific conductance for a given aquifer, the 4 0.75 correlation may be useful in interpreting ,o regional water-quality patterns from logs 5 0.70 (Jones and Buford, 1951). Conductivity logs 5 also provide a rational basis for selecting = 0.65 depths for the collection of water samples. Information from fluid-movement and (or) 0.60 multielectrode-resistivity logs is necessary to determine whether the water measured by fluid-conductivity probes is representative of 0.505 A the water in the surrounding rocks. Logs also 35 40 45 50 55 60 65 70 75 80 TEMPERATURE, IN DEGREES FAHRENHEIT provide a convenient and inexpensive way of vertically and horizontally extrapolating wa­ ter-sample data from a well, and they indi­ Figure 68. - Relation of electrical conductivity to temperature in dilute solutions of potassium chloride and sodium chloride. cate temporal changes in quality. From Olmsted (1962). Fluid-conductivity logs may be used in 108 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS the same general way as fluid-temperature Instrumentation logs to locate the source of water in produc­ ing wells, or to locate the most permeable The usual fluid-conductivity probe utilizes zones under injection conditions. The source two or four ring-type gold or silver elec­ of water can be determined only if detectable trodes, spaced several inches apart, inside differences exist between the conductivities a tube through which the water flows. Gold of water in several aquifers. Injecting saline electrodes are preferable, as they reduce the water in a well and following it with a fluid- changes in contact resistance caused by conductivity probe is an inexpensive means chemical reaction at the electrodes. Probes of determining the relative magnitude of for multiconductor cable utilize a four-elec­ permeabilities, if corrections are made for trode arrangement, as in formation-resistivi­ head. This technique is described in the sec­ ty logging, and probes for single-conductor tion on “Fluid-Movement Logging.” cable use two electrodes. Inviolable rules for the interpretation of Alternating current instead of direct cur- fluid-conductivity logs cannot be given; the rent across these electrodes tends to reduce logs must be analyzed on the basis of all electrode polarization. The voltage drop available data. For example, a fresh-water- across these electrodes is related to the resis­ saline-water interface in a well might be at tance or conductance of the fluid. An em­ the same depth as a similar interface in the pirically determined constant can then be aquifer system, or it might be a residual used to determine the resistivity or conduc­ condition caused by drilling, cementing, or tivity of a unit volume of fluid. Like the pumping operations. Due to cross flow be- temperature sonde, the circuit should be de- tween aquifers in a multiaquifer well, the signed so that changes in temperature of the fluid-column interface may be related to an electronics or changes in resistance of the interface in the rock at a different depth. logging cable do not cause inaccurate logs. Setting of screens at the wrong depth can Pulses can be used to transmit the data from cause the measurement of fluid conductivi­ the probe to a frequency-to-voltage converter (. ties that are not representative of fluid in the in the module at the surface, so that cable aquifer. Figure 69 shows a very sharp inter- length will have no effect on the recorded face between the overlying brackish water signal. and the brine. This interface is not shown Fluid-conductivity probes should be run on the temperature log but can be identified at slow logging speeds to assure the proper on the neutron log, and it is probably located flow of fluid through the tool. at the same depth in the fluid column as in the adjacent rocks. Calibration and extraneous effects Calibration of the fluid-conductivity sonde should always be done in fluids of known conductance and corrected to standard tem­ perature. After the relationship of logger response to conductivity or resistivity is de­ termined, a resistance-decade box can be J--w- cul­ used for standardization of the logging-sys­ tem response in the field. Standardization r before and after logging is particularly im­ portant until the stability of both surface and downhole electronics is known. Recali­ bration with fluids of known conductivity is necessary only if a change occurs in the contact resistance of the electrodes.

Figure 69. -A brine-brackish-water interface, shown by fluid- The most common extraneous effect on resistivity and neutron logs. fluid conductivities recorded on logs is caused c APPLICATION OF BOREHOLE GEOPHYSICS 109

by a disturbance of the fluid in the borehole. with impeller, thermal, or tracer probes. Fluid-density differences and thermal con­ Techniques for measuring the horizontal vection can cause movement of fluid in a movement of fluids through a single borehole completely sealed casing. (See section on are new and little used to date, but have “Temperature Logging.“) Water in a well considerable pote&ial for application to may require months to attainchemical equi­ ground-water hydrology. librium with the surrounding rocks after One of the most common applications of drilling or cementing operations are com­ the measurement of vertical-fluid movement pleted. Unlike thermal equilibrium, the is in wells that are open to a multiaquifer changes in water chemistry and, thus, in artesian system (Bennett and Patten, 1960). conductivity require the actual movement of If there are differences in head between aqui­ water or ions. It may take longer to reestab­ fers, flow will occur within the well. Such lish chemical equilibrium than thermal equi­ flow lowers the head in the contributing librium after some disturbance. If there is aquifer and may result in either thermal or much internal movement of water in the chemical pollution of the aquifer having the well, equilibrium may never be established. lower head. Vertical flow in a well may invali­ date pump-test data if not detected. Vertical flow will also complicate sampling for water quality and the interpretation of fluid-con­ Fluid-Movement logging ductivity or temperature logs. An example of the measurement of natural Fluid-movement logging includes all tech­ flow in a well with an impeller-type flow- niques for measuring natural and (or) arti­ meter is shown in figure 70. This same well ficially induced flow within a single borehole. was logged with a radioactive-tracer sonde, Data on inhole flow is related to well con­ and the results were similar. The left side struction, differences in head, and the relative of figure 70 shows continuous integrated­ B magnitude of permeability of the water-bear­ flowmeter logs made to locate the zones of ing units open to the well. flow. In this mode of operation, the logs were run with the sonde moving down and up the hole at a relatively constant logging speed of Principles and applications 40 feet per minute. The individual pulses on Devices used to measure the vertical and these logs are caused by rotation of the im­ horizontal components of flow in a single peller. The pulses are integrated, so that a well include impeller flowmeters, thermal deflection of the trace to the right indicates flowmeters, and various systems for injecting an increase in revolutions per unit time. If and detecting radioactive and chemical trac­ it were possible to maintain the same con­ ers. The impeller flowmeter transmits pulses, stant logging speed up and down a hole, and which indicate the number of revolutions of if the position of a symmetrical sonde in the the impeller per unit time, whereas the ther­ hole were consistent, logs made up and down mal flowmeter detects water heated by an in a stagnant column of water would be the element in the tool. Various tracers are in­ same. Therefore, the zone of maximum curve jected in the column of water, and their separation in figure 70 represents a zone of movement to a detector is timed. (Techniques upward flow. The impeller revolutions in- for measuring flow rate between wells are creased when the sonde was moving against not considered to be in the field of borehole the upward flow and decreased when the geophysics and are not discussed here.) sonde was moving in the same direction as Techniques for measuring the natural or in­ the flow. The deflections on one log not duced vertical movement of fluids in bore- matched by deflections in the opposite direc­ holes have been widely applied in both oil tion on the other log were probably caused and water wells. The interpretation of all by changes in logging speed or by the probe types of vertical-fluid-movement logs is simi­ bouncing off the casing. When the logs were lar, regardless of whether they are made repeated, only the curve separation between 110 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

DEPTH, FEET CONTINUOUS INTEGRATED LOGS !STATIONARY TIME-DRIVE LOG 240 5 259 lo detectable flaw

260 250 261

262 260 InteAal of 263 stationary upward flow c measurments 15 seconds 32F shown at right -J 270 I 265

266 280 - Flowmeter 267

2 682tionory position 290 of flowmeter, 2 bL in feet

270 300 271

hlo det;;;ble flow l 310 INCREASING FLOW­ NUMBER 0 F PULSES I‘ER UNIT TIMEN FLOW LOGGINGSPEED:40 FEETPER MINUTE MEASUREMENTSMADE WITH FLOWMETER STATIONARYAT l-FOOT INTERVALS

Figure 70. -Continuous flowmeter logs, used to locate the zone of flow (left) and to determine stationary time-drive measurements across that interval (right).

259 and 272 feet were duplicated. By moving through the same zone. In this mode, the re- the flowmeter, flow can be detected that is corder on the logger is set to operate on time too slow to detect with a stationary flow- drive--4 inches per minute on this log-and meter. When the velocity of flow is equal to individual pulses caused by rotation of the the logging speed, the impeller ceases to impeller are recorded. Note on this log that rotate. only minor changes in velocity occurred The right-hand log in figure 70 shows across the zone of interest. The average ve­ stationary-flowmeter measurements made locity, indicated by the movement of radio- APPLICATION OF BOREHOLE GEOPHYSICS 111

active tracers, in the zone from 272 to 259 B feet was 18 feet per minute, or 29 gallons per minute. Apparently, all the water was entering and leaving the screen in two very 500 - thin zones separated by relatively imperme­ 700 - able material. A second broad category of application for inhole-flow measurement is to determine the relative magnitude of permeabilities under an imposed hydraulic stress. In the petroleum industry, this is termed “injectivity profil­ ing” and is done during injection or recharge of water into a well. An injectivity profile is OtPTH. made by plotting the water lost within each FEET

depth interval of hole where vertical-velocity 1250 measurements are made. Several studies of I.500 this type have shown a poor correlation be- tween injectivity and horizontal permeability measured on core samples (Kaveler and 1400 Hunter, 1952 ; Piety and Wiley, 1952). This lack of correlation is suggested to be due 1500 chiefly to two factors : (1) the sands in these studies showed a difference of as much as 1600 300 percent in the horizontal permeability

of adjacent samples, and (2) the vertical GALLOYSfER DAYLOSS GALLON5PERDA1 permeability showed wide variations and PER1001 INJECTlVllY PROFILES equallecl as much as one-half of the horizontal permeability; hence, it could contribute sig­ Figure 71. - The movement of slugs of radioactive tracer under nificantly to the actual flow pattern, as could injection conditions (upper half) and injectivity profiles and fractures not present in the small core sam­ their relation to o caliper log (lower half). ples. Lack of correlation between sample as a few feet per day. The curves in the data and injectivity profiles may also be due upper half of this illustration show the move­ to failure to correct for head differences, to ment of four injections of an aqueous solu­ lower permeability caused by drilling mud, tion of iodine-131, from February 25 to or to plugging of the formation from organic March 9. These slugs of tracer were located matter in the recharge water. Nonetheless, by periodic gamma logs, and are indicated inhole-flow data can show the relative by dots in figure 71. Because no system was amounts of water moving into an aquifer available for maintaining a constant injec­ system under injection conditions designed tion head, it was necessary to place a refer­ to simulate artificial recharge. Similarly, a ence slug of tracer in the 61/b-inch I.D. (in- flow-measuring device can be placed in a well side-diameter) casing. The dashed curve beneath a pump during a test, and the rela­ shows the velocity of this reference slug tive contributions of various aquifers can be corrected for a 5$‘&inch hole. Two types of determined at various discharge rates. injectivity, or water-loss profiles, were cal­ When permeability and flow rates are ex­ culated ; “gallons per day” and “gallons per tremely low, tracer techniques are the best day per foot.” This was clone by comparing means of measuring water movement in a the volume of water moved through the cas­ hole. Figure 71 illustrates a technique used ing with the volume moved through a given to analyze the vertical distribution of perme­ depth interval of open hole for the same ability in fractured crystalline rocks at the period of time. AEC Savannah River Plant in South Caro­ At the National Reactor Testing Station lina, where vertical flow in wells was as low in Idaho, a radioactive tracejector and other 112 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

WELL DOWNWARD FLOW OF WATER, IN GALLONS PER MINUTE CONSTRUCTION D 100 200 300 400 500 600

I “- Traceiector Log E. II August 13, 1964 4 /lD-Inch Casing 1 d-L.1 Perforations: * 200 (0) Slot I--I (b) Lower Gun + N 8inch Casing (c) Upper Gun 1f I I -tt

-512- I

Upper Gun Perforations -697-

-930- Lower Gun Perforations -1070- c 6-Inch Casing 1200- --1183- lot Perforations --1268-

1400 - -.-I--’ Figure 72. - Radioactive-tracejecor logs, National Reactor Testing Station, Idaho. From Morris and others (1965). nuclear logs were used to determine the rea­ but the well was bridged at 1,005 feet, and son for inadequate capacity and to guide the tracejector log C showed part of the water recompletion of a waste-disposal well (Mor­ leaving through the bottom of the hole. On ris and others, 1965). The waste-disposal the basis of caliper, gamma-gamma, and well was originally drilled to a depth of 1,275 natural-gamma logs, it was decided to gun feet, with slot perforations from 1,182 to perforate the casing interval from 512 to 1,267 feet (fig. 72). The specific capacity of 697 feet. Tracejector log E showed that this the well was not adequate for the disposal zone took practically all the water injected anticipated, and it decreased with time. at 550 gpm and that the head rise was insig­ Tracejector log B showed that about 30 per- nificant. Tracejector log D was run during cent of the water injected at 100 gpm left a period of no injection, and a natural-down- the well through the annulus between the 6- ward flow of about 25 gpm was noted. and g-inch casing. The casing was then gun In addition to measuring vertical veloci­ perforated between 935 and 1,070 feet. An ties, there are several techniques, which have improvement in specific capacity was noted, received only limited use in ground-water APPLICATION OF BOREHOLE GEOPHYSICS 113

hydrology to date, for measuring horizontal determine the direction of exit of the injected movement in a single well. However, these radioisotope (Payne and others, 1965). An- techniques may only provide data on local other possibility is the use of a number of velocities and direction of movement, and it radially spaced detectors, which may be re- may be necessary to repeat the procedure corded individually, with the whole device in several wells to establish an area1 pattern. oriented by compass. A device of this type One method for measuring velocity is to in­ has been used for measuring the horizontal ject a liquid radioisotope into a well and component of flow in lakes. Selecki and Fili­ make in-sit,u measurements of the dilution, pek (1966) proposed a method that relied on as untagged water moves through the well the neutron moderation in a saturated solu­ (Raymond and Bierschenk, 1957). The equa­ tion, containing lithium, boron, and cad­ tion for analysis of this data is based on hori­ mium, which is introduced into the well. The zontal flow, so dilution must be measured in direction of maximum movement of the a section of the well that is isolated by pack­ tracer was measured by means of a rotating ers. The velocities calculated are greatly af­ collimated-neutron detector. fected by well loss due to the screen, gravel Gamma-emitting radioactive tracers that pack, or mud cake ; therefore, they are closely are water soluble have the following advan­ related to the yield of that interval of the tages over chemical tracers : They are detect- well. Lewis, Kriz, and Burgy (1966) utilized able at very low concentrations so that nonradioactive tracers and a dilution-sam­ density effects are minimized, they are easily pling technique to determine the hydraulic detected in a well bore and outside the casing, conductivity of fractured rock. Hydraulic and they are obtainable in numerous chemi­ conductivities of 0.02 to 0.5 feet per day cal and physical forms suited to the specific determined in small wells agreed favorably test requirements. Relative cost and diffi­ with similar conductivity values obtained culties in obtaining a Government license for from pumping tests. use in ground-water environments are the B The single-well pulse technique utilizes a chief drawbacks to the use of radioactive radioisotope dissolved in the stream of water tracers. (See page 86, section on “The Use injected into a well (Borowczyk and others, of Radioisotopes in Well Logging.“) 1967). Water injection continues until the A brief discussion of radioactive tracers tracer moves some distance out into the aqui­ that can be used is germane to the selection of fer. The well is then pumped, and the time a flow-measuring system. A radioactive tracer of recovery for 50 percent of the injected that has a short half life and is detectable tracer indicates the velocity of movement in at low concentrations should be selected so the aquifer under injection conditions. The that the concentration at the nearest point of technique is most successful in a single, ho­ reuse does not exceed limits (recommended mogeneous aquifer. A gamma probe can be by the U.S. Public Health Service) for drink­ used to measure concentration and to deter- ing water. Iodine-131 is commonly used be- mine the zone that is taking the water and cause it has an 8-day half life and a high tracer. The hydraulic conductivity, calculated specific activity of energetic gamma photons. from pumping tests in the area, averaged However, other tracers can be used, such as 0.4 cm/set, and the transit time between two bromine-82 with a 36-hour half life. The observations wells, as determined by tracer possible sorption or delay of tracers must study, was 0.47 cm/set. Hydraulic conduc­ always be considered when measuring flow tivity calculated from the single-well pulse through aquifers, but this is not an important technique was 0.45 cin/sec. factor in determining velocities in a fluid Another single-well technique utilizes di­ column. An attractive possibility is the use rectional detectors to determine the azimuth of neutron-activated tracers. Radioisotopes of movement of a liquid-radioactive tracer. formed in this way generally have a very One method employs a shielded detector that short half life and are not subject to strin­ may be rotated from the surface in order to gent regulation. In general, however, an in- 114 TECHNIQUES OF WATER-RESOURCE,S INVESTIGATIONS

tense neutron flux is required to produce a watch, were empirically related to flow. The radioisotope that has a long enough half life, oil-well-type flowmeter has some improve­ a high specific activity, and a high enough ments, but still exhibits the shortcomings gamma energy to penetrate the fluid and described under “Calibration” and “Extra­ sonde. The half life needed is a function of neous IEffects.” I?riction on the shaft is re­ the velocity to be measured. A californium- duced by filling the switch chamber with oil 262 source recently used for well logging was or by allowing it to fill with water. A very found to have sufficient neutron flux to pro­ sensitive magnetic switch and a rotating duce activated radiotracers in a relatively magnet eliminate friction caused by a me­ short time (Keys and Boulogne, 1969). chanical switch, but they introduce some Another type of inhole-tracer technique magnetic bias. Pulses can be coded so that utilizes fine particulate or insoluble radioiso­ the direction of rotation can be ascertained topes, such as gold-198 or cobalt-60. They without moving the flowmeter. Recording of are introduced into the well under injection data h.as been considerably improved over conditions and are filtered out on the face the stopwatch and notebook method. Pulses of aquifers in proportion to the relative from the flowmeter can be integrated in a permeability of the units. Gamma logs made standard gamma-logging module, and the after injection correspond closely to the per­ time constant can be adjusted so that a meability profiles measured by other meth­ smooth integrated trace, or an integrated ods. Soluble tracers may also be introduced trace with pulses superimposed, can be re- into the water during its injection into a corded (fig. 70). Loggers with time drive well. Periodic gamma logs made during sub- on the recorders also permit the graphic sequent pumping of the well locate the zones recording of pulses per unit time. In this where the greatest amount of tracer has manner, all data from either continuous or entered. stationary flowmeters is automatically re- corded. The oil-field flowmeter is also avail- able; it has a number of interchangeable c Instrumentation baskets, and impellers of different diameters. The chief advantages of an impeller flow- The various methods of flow measurement meter are its simplicity and its relatively low in wells, with the exception of radioactive cost. Disadvantages include nonlinear re­ tracers, are discussed in detail by Patten sponse, lack of sensitivity at low velocity, and Bennett (1962). Their discussion and and po’or accuracy at high velocity ; also, new developments are summarized here. The none are available for very small diameter most widely used device for the measure­ holes. ment of flow in water wells utilizes an im­ A significant improvement can be made in peller, a rotor, or vanes housed inside a the performance of any type of flowmeter protective cage or basket. The Price current by the addition of a device to force all or meter was first used by Meinzer (1928) to most of the flow through the cage or basket. locate zones of leakage in wells in Hawaii. Commercial service companies have inflat­ Fiedler (1928) described the use of the Au able-packer flowmeters that operate on log­ deep-well current meter in wells in the Ros­ gers. T:he packer may be inflated at any point well artesian basin. Recent work by the in the ‘hole that is within the size range of research project in borehole geophysics has the packer to provide a reading with all the been done with flowmeters designed for work fluid chianneled through the flowmeter. Such in oil wells. This type of flowmeter was used equipment is not available for purchase at to make the logs shown in figure 70. The present (1971) ; however, the performance earlier devices used a mechanical switch, of most flowmeters can be improved with a attached to the rotor, which completed an simple centralizing flange. A thick rubber electrical circuit through a battery. An audi­ flange arndan annular bristle-type brush have ble click was detected on earphones, and the both been used successfully. The rubber clicks per unit time, measured with a stop- flange can be cut to fit the hole diameter, as c APPLICATION OF BOREHOLE GEOPHYSICS 115 B determined by a caliper log. This method at the station. This probe may be used to has been used to increase the sensitivity of measure velocity with the tool stationary, as a flowmeter sufficiently to detect the move­ shown, and then the slug of tracer can be ment of air out of a well in response to fall­ followed up or down the hole to the point of ing barometric pressure. exit. If the velocity is extremely low, the hole The thermal flowmeter, developed by H. E. may be relogged periodically to determine Skibitzke (1955)) has promise of offering movement. Moving the tool through a slug improved flow measurement, but it has not of tracer does tend to spread it out to some yet received wide use. A resistance heating extent. element is located between two thermistors A common type of tracejector used in in a small-diameter tube. The amount of both oil wells and water wells consists of a water heating that is achieved is inversely positive-displacement piston-type pump that related to the fluid velocity through the tube. will eject from a fraction of a milliliter to The upstream thermistor is at ambient tem­ 20 milliliters of a water-soluble tracer, such perature, and the downstream thermistor as iodine-131. The ejector is operated by measures the higher temperature due to direct current from the surface. The module heated water reaching it. The imbalance for operating a motor-driven caliper may caused in a Wheatstone bridge circuit is be used. The radiation probes used for nat­ measured on a meter. Because of nonlinear ural-gamma logs may be attached to the response, thermal convection, and internal ejector in any one of a number of combina­ and external flow variables, this device must tions. When direction of flow is not known, be calibrated empirically. Patten and Bennett they may be placed above and below the (1962) reported a functional velocity range ejector. When direction is known, they may _. of 2 to 75 fpm, with errors likely to be less both be attached above or below the ejector than 0.5 fpm at low velocities and less than to increase the accuracy of measurement, as 1 fpm at high velocities. The thermal flow- shown in figure 73. The recorder is operated B meter is readily adaptable to small-diameter on time drive, and an event marker is auto­ tools and can probably be improved to mea­ matically triggered when the switch is de- sure velocities less than 2 fpm. Variations pressed to eject a drop of iodine. The upper of the system developed by Skibitzke include left part of figure 73 shows the repeatability the measurement of changes in voltage re­ of single-detector logs. Note the sharp ar­ quired to heat a single thermistor, which rival front of the slug of iodine at about 22 should be related to the velocity of the fluid seconds, and the gradual decrease in concen­ removing the heat. Similarly, the changes tration. The lower left part of figure 73 in resistance of a hot-wire or hot-film ane­ shows an actual double-detector log. In this mometer have also been used to measure mode, traveltime is measured between the fluid flow. arrival of a slug of tracer at the first and the Radioactive-tracer techniques for inhole second detectors. This measurement is con­ flow surveys are widely used in the petroleum sidered to be more accurate because the slug industry and in ground-water investigations of tracer is well formed by the time it reaches in Europe, but have only had limited use in the first detector, and errors caused by dis­ this country. Basically, the downhole equip­ persion and actual time of ejection are elimi­ ment consists of a device for injecting a nated. Note in figure 73 that a higher velocity gamma-emitting radioisotope in the well and is recorded between detectors than from the one or several gamma detectors (Edwards injector to detector No. 1. With this system, and Holter, 1962). The tracejector detec­ natural vertical flows as high as 30 feet per tor described here was developed for injec­ minute have been measured in wells at the tivity profiling of oil wells and was first National Reactor Testing Station in Idaho. used in ground-water investigations by the Horizontal flpw in some of these aquifers senior author at the National Reactor Test­ removes all of the tracer from the well within ing Station in 1961. Figure ‘73 shows the several seconds. In contrast, vertical veloci­ double-detector probe and some logs made ties of a few feet per day were measured in 116 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

Gamma Radlatlon Incroorr -

REPEAT LOGS NO. 2 DETECTOR

- InJection .Sml. of 270 Foot ‘. c

- 6 l/4*­ 0 Inlwtlon .!l ml al 270 Feel NM-=-+- INJECTION - DETECTOR Concentration of Iodine Solution 0.2 Millicurie per Milliliter :H,O PROBE

Figure 73. - Doubledetector radioactivetracejector (right) and logs of iodine-131 injections (left). fractured crystalline rocks at the A.E.C. directionally sensitive ; it will detect flovV Savannah River Plant in South Carolina. The behind casing; and all data are automaticall: V advantages of this method are that it oper­ plotted. ates over a wide velocity range with very A brine ejector-detector system, utilizing 3 small quantities of tracer; it should not be the rasdioactive-tracejector and one or sev c APPLICATION OF BOREHOLE GEOPHYSICS 117

RESlSTlVlTY DECREASES I

- BEGIN INJECTON- 4’10 CASING Na Cl NHd.3 NH41 SATURATED SOLUTIONS

BRINE TRACER LOGS SlLVE.4 ELECTRODES MADE IN 4” ID CASING AVERAGE DOWNWARD FLOW 117 (pm

EJECTOR - DETECTOR I FROBE

Figure 74. - Brine ejector-detector probe (right) and logs of three different salts used or tracers (left). era1 fluid-conductivity tools, has recently made automatically. After the slug of tracer been developed (fig. 74). The system has the has passed the detector, the tool may be advantage of utilizing a less expensive, more moved to follow it, but this causes some readily available tracer that does not require mixing and disturbance in the flow system. licensing. However, chemical tracers have A new tool uses two sets of electrodes for disadvantages in that results are more affect­ tracer detection above or below the ejector. ed by the specific gravity of the solution; Patten and Bennett (1962) described a brine- they have a lower detection sensitivity ; and tracer system utilizing a plastic hose for they are not detectable through casing. The tracer ejection and a fluid-conductivity probe tool shown on the right in figure 74 is set and stopwatch for velocity measurement. up to measure downward flow. On the left They pointed out that the method is inher.ent­ are actual time-drive logs of three different ly accurate and usable at low velocities, but salt tracers in the same flow system. Note that certain improvements in instrumenta­ how the NHJ solution indicates a higher tion are needed as follows : An automatic and downward velocity because of the greater repeatable brine-release system incorporated specific gravity of the solution. Recording is in the same tool with two detectors, an auto­ done exactly the same as for the radioactive matic time-recording system, and the capa­ tracer, and the logs look nearly the same. An bility of making stationary measurements. event marker records the instant of ejection, The brine ejector-detector system now avail- and a plot of fluid conductivity versus time is able meets all these requirements and is 118 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS probably the most useful flowmeter presently Flow-measuring devices should be cali­ available for a wide variety of borehole con­ brated in the size of casing or hole in which ditions. they are to be used. Open-hole flow surveys should always be checked against a caliper log. The research project facility for testing Calibration and extraneous effects and calibration of flow sondes consists of All the flow-measuring devices described plexiglass cylinders of different diameters are subject to extraneous effects; hence, they that can be connected to a recirculating pump must all be calibrated by empirical methods. in a tank. A system of valves and a calibrated Aside from the disadvantages mentioned for Sparling meter allow the flow to be measured each tool, a problem common to all is the and the tlow direction to be reversed. Cali­ apparent change of fluid velocity caused by bration should be done for both flow direc­ varying the probe position radially within tions, if possible. Field standardization, or the borehole. The velocity of a fluid moving calibration, for one hole size can be done through a smooth cylinder is lowest near the either when a well is being pumped or when wall, and is a fairly constant maximum water is being injected into the well at a across most of the central part of the con­ known rate. With careful calibration and duit. Thus, a flow-measuring device that mea­ field standardization, errors at low-flow ve­ sures the velocity of only a small part of the locities can be reduced to 10 percent, and total cross-sectional area can give low results errors ,can be reduced to as low as 1 percent if it is located near the wall. For this reason, at higher velocities. use of centralizing devices, channeling flanges, or packers improves the accuracy of flow measurement. Transparent plexiglass Casing Logs cylinders, used to calibrate flow sondes for the research project on borehole geophysics, per­ The casing-collar locator is a useful and c mit observation of this effect. Dye was mixed inexpensive device that can be added to any with other tracers in order to study their geophysical logger. The continuous-collar log behavior. The greatest error was achieved can a1s.obe interpreted to accurately locate with the ejector port located against the side perforations and screens in a well. Accurate of the cylinder. Lateral dispersion of the information on a well’s construction is very tracer into the central zone of maximum ve­ important for pumping tests and as a guide locity took place along with vertical move­ to the correct interpretation of other geo­ ment. The directional effect on the output physical logs. The simplest type consists of of a flow-measuring sonde is significant if a permanent magnet wrapped with a coil of the upper and lower parts of the sonde are wire. Changes in the magnetic properties of not mirror images. Generally, a symmetrical material moving through the lines of flux tool is not feasible to construct because of from the magnet cause a small direct current the cable connector. Directional errors due to flow in the coil of wire, and the resulting to specific-gravity differences between trac­ voltage can be used to drive the recorder ers and water are only significant at low pen. In collar and perforation logging, DC- velocities and can be checked in a stationary voltage fluctuations are caused by changes column. If spacing between sensors is too in the -massof metal cutting the lines of fux great, a depth interval with varying fluid and by changes in the velocity of the tool. velocity may be averaged; however, constant Commercially, the typical collar log is run water flow across the interval being mea­ simultaneously with radiation logs as a means sured must be assumed for quantitative in­ of providing depth reference in the hole. The terpretation. Tools that have a large diameter CCL log is generally recorded as “event in relation to the diameter of the well will marks” on the left side of the log (fig. 75). cause the greatest vertical dispersion when This is accomplished by adjusting an event they pass through a slug of tracer. marker so that it trips only when the DC APPLICATION OF BOREHOLE GEOPHYSICS 119

EPTH, - FEET

- - - - 200

- STEEL CASING

YCOLLAF - -

- - 300 -

-

c - - - T - --L - F t TOP OF 400 - TORCH -CUT PERFORATION! -

-

- - - 500

:cL 200 MV 50MV

Figure 75.-A typical CCL log, made with an event marker (on the left of the log), and continuous logs, at two different potential scales, which were used to locate perforations in the well casing. voltage exceeds a certain predetermined tial module at sensitivities of 50 and 200 level. The continuous-collar logs shown in millivolts. Extraneous effects that may cause figure ‘75 were run with the same tool. The misinterpretation of CCL logs include signal was fed through a spontaneous-poten- changes in response due to logging speed, di- TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

Baker, P. E., 1957, Neutron capture gamma-ray rection, and position of the sonde in the hole. a In some holes, it may be helpful to decen­ spectra of earth formations: Am. Inst. Mining Metall. Petroleum Engineers Trans., v. 210, p. tralize the sonde. Where interpretation is 97-101. questionable, repeat logs should be run in Basham, R. B., and Macune, C. W., 1952, The Delta- order to distinguish real from extraneous log, a differential temperature surveying meth­ deflections. od: Am. Inst. Mining Metall. Engineers (Petro­ The effect of casing on other geophysical leum) Trans., v. 195, p. 123-128. logs is described in the sections on “Borehole Baranov, V. I., and Kristianova, L. A., 1966, Radio- activity of ocean deposits in chemistry of the Effects” and “Extraneous Effects.” Gamma- Earth’s crust, v. 1: Jerusalem, Israel Program gamma logs have also been used to locate Sci. Trans., Weizmann Science Press of Israel, joints and one string of casing outside an- p. 425-432. other. (See fig. 53). The caliper log is par­ Belknap, W. B., Dewan, J. T., Kirkpatrick, C. V., ticularly useful for measuring casing size Mott, W. E., Pearson, A. J., and Robson, W. R., and for locating joints and screens; it can 195B, API Calibration Facility for Nuclear Logs, Drilling, and Production Practice: Am. even be used to identify badly corroded pipe. Petroleum Inst., p. 289316. The spontaneous-potential sonde has also Bennett,, G. D., and Patten, E. P., Jr., 1960, Bore- been used in casing to locate differences in hole geophysical methods for analyzing specific anodic activity in wells where there is active capacity of multiaquifer wells: U.S. Geol. Sur­ corrosion (Kendall, 1965). This method can vey Water-Supply Paper 1536-A, p. l-25. only be used to locate areas of active corro­ Birch, F., Schauer, J. F., and Spicer, H. C., 1942, Handbook of physical constants: Geol. Sot. Am. sion and cannot determine the extent of Spec. Paper 36, p. 243-266. corrosion. Of course, both the SP and single- Borowcayk, M., Mairhofer, J., and Zuber, A., 1967, point resistance log will generally provide Single well pulse technique, in Isotopes in hy­ an accurate depth to the bottom of the casing, drology: Vienna, Austria, Internat. Atomic En­ and sometimes they can also be used to locate ergy Agency, Trans., 740 p. .. screens. Brannon, H. R., Jr., and Osaba, J. S., 1956, Spectral gamma-ray logging: Am. Inst. Mining Metall. a At least one commercial logging service Petroleum Engineers Trans., v. 207, p. 3035. is available for detecting corroded pipe. Bredehoeft, J. D., 1964, Variation of permeability in An electromagnetic casing-inspection log the Tensleep Sandstone in the Bighorn Basin, measures the changes in metal mass between Wyoming, as interpreted from core analyses and two coils (Edwards and Stroud, 1964). The geophysical logs: U.S. Geol. Survey Prof. Paper 501-D, p. D166-D1’70. log indicates possible total metal loss and, Bunker, C. M., and Bradley, W. A., 1961, Measure­ therefore, does not indicate whether the cor­ ment of bulk density of drill core by gamma-ray rosion has occurred on the inside or the out- absorption, in Short papers in the geologic and side of the pipe. hydrologic sciences: U.S. Geol. Survey Prof. Paper 424-B, p. B310-B313. Bunker, C. M., and Bush, C. A., 1967, A comparison of potassium analyses by gamma-ray spectrom­ etry and other techniques: U.S. Geol. Survey Selected References Prof. Paper 575-B, p. B164-B169. Caldwell, R. L., 1962, Gamma-ray spectroscopy in Alger, R. P., 1966, Interpretation of electric logs in well logging: Cracow, Poland, Nuclear Geo­ fresh-water wells in unconsolidated formations: physicists Conf., 1962, v. 1, p. 165-199. Sot. Prof. Well Log Analysts, 7th Ann. Logging Campbell, J. L. P., 1951, Radioactivity well logging Symposium, Tulsa, Okla., May 1966, Trans., p. anomalies: Petroleum Engineer, v. 23, p. B7- Ccl-CC25. BlE!. Allen, L. S., Caldwell, R. L., and Mills, W. R., 1965, Canadian Well Logging Society, 1967, Review of Borehole models for nuclear logging: Sot. Petro­ logging activity in Canada: Sot. Prof. Well Log leum Engineers Jour., v. 5, no. 2, p. 109-112. Analysts, v. 8, no. 3, p. 37-38. Archie, G. E., 1942, The electrical resistivity log as Carroll, R. D., 1966, Rock properties interpreted an aid in determining some reservoir character­ from sonic velocity logs: Am. Sot. Civil Engi­ istics : Am. Inst. Mining Metall. Engineers neers Proc., Jour. Soil Mechanics and Founda­ Trans., v. 146, p. 54-62. tions Div., v. 92, no. SM2, Paper 4715, p. 43-51. APPLICATION OF BOREHOLE GEOPHYSICS 121

Chaney, P. E., Jr., Zimmerman, C. W., and Ander­ Technology, v. 14, February, p. 121-124. son, W. L., 1966, Some effects of frequency upon Edwards, J. M., and Stroud, S. G., 1964, Field re­ the character of acoustic logs: Jour. Petroleum sults of the electromagnetic casing inspection Technology, April, p. 407-411. log: Jour. Petroleum Technology, v. 16, April, Christensen, D. M., 1964, A theoretical analysis of p. 377-382. wave propagation in fluid-filled holes for the Electra-Technical Laboratories, 1969, Operators interpretation of three-dimens’ional velocity logs : Manual, Widco Logger: Houston, Tex., 78 p. Midland, Tex., So& Prof. Well Log Analysts, Evans, H. B., 1965, GRAPE-A device for continu­ 5th Ann. Logging Symposium, May 1964, Trans., ous determination of material density and poros­ p. Kl-K30. ity: Sot. Prof. Well Log Analyst, 6th Ann. Crew, M. E., and Berkoff, E. W., 1970, Twopit, a Logging Symposium, Dallas, Tex., May 1966, different approach to calibration of gamma-ray Trans., v. 2, p. Bl-B26. logging equipment: Log Analyst, v. 11, no. 6, Ferronsky, V. I., Danilin, A. I., Dubinchuk, V. T., p. 26-32. Goncharov, V, S., Polyakov, V. A., Seletskiy, Croft, M. G., 1971, A method of calculating perme­ Yu. B., and Flekser, N. Ya., 1968, Radioisotope ability from electric logs, in Geological Survey investigative methods in Engineering Geology research, 1971: U.S. Geol. Survey Prof. Paper and Hydrogeology (Radioizopnye methody is­ 750-B, p. B265-B269. sledovanija v inzhenernoy geologii i gidro­ Cooper, H. H., Jr., and Jacob, C. E., 1946, A geologii) : Moscow, U.S.S.R., Atomizdat. graphical method of evaluating formation con­ Fiedler, A. G., 1928, The Au deep-well current meter stants and summarizing well-field history: Am. and its use in the Roswell artesian basin, New Geophys. Union Trans., v. 27, no. 4, p. 626-634. Mexico: U.S. Geol. Survey Water-Supply Paper Davis, R. W., 1967, A geophysical investigation of 696-A, p. 24-32. hydrologic boundaries in the Tucson Basin, Gaur, R. S., and Singh, Inderjit, 1966, Relation be- Pima County, Arizona: Arizona Univ. Ph. D. tween permeability and gamma-ray intensity thesis, p. 42-43. for the Oligocene sand of an Indian field, India Desai, K. P., and Moore, E. J., 1969, Equivalent (Republic) : Oil and Natural Gas Comm. Bull. NaCl determination from ionic concentrations : 2, p. 74-77. Log Analyst, v. 10, no. 3, p. 12-21. Goldman, D. T., and Stehn, J. R., 1961, Chart of the Dewan, J. T., Stone, 0. L., and Morris, R. L., 1961, nuclides : Schenectady, N.Y., General Electric Chlorine logging in cased holes: Jour. Petroleum Co., Chart APH-66-E. Technology, June, p. 531-537. Gondouin, M., and Scala, C., 1958, Streaming po­ Desbrandes, R., 1968, Theorie et interpretation des tential and the S.P. log: Jour. Petroleum diagraphies [in French]-Publication of the Technology, v. 10, August, p. 170-179. Petroleum Institute of France: Sci. Petroleum Guyod, Hubert, 1952, Electrical well logging fun­ Techniques, no. 13, p. 127. damentals : Houston, Tex., Well Instrument Dodd, P. H., and Droullard, R. F., 1966, A logging Developing Co., 164 p. system and computer programme to determine -1966, Interpretation of electric and gamma rock density of uranium deposits, in Radio- ray logs in water wells: Log Analyst, v. 6, no. isotope instruments in industry and geophysics, 6, p. 29-44. v. 2, Symposium: Warsaw, Poland, Internat. Guyod, Hubert, and Shane, L. E., 1969, Introduction Atomic Energy Agency, 1965, Proc., p. 206-225. to geophysical well logging-Acoustical logging, Doll, H. G., 1948, The S.P. log-Theoretical analysis v. 1 of Geophysical well logging: Houston, Tex., and principles of interpretation: Am. Inst. Hubert Guyod, 256 p. Mining Metall. Engineers Tech. Pub. 2463, 40 p. Hallock, W. B., 1897, Subterranean temperatures at -1949, Introduction to induction logging and Wheeling, W. Va., and Pittsburgh, Pa.: Colum­ application to logging of wells drilled with oil- bia School Mines Quart., v. 18, January, p. base mud: Jour. Petroleum Technology, v. 1, 148-154. June, p. 148-162. Harms, J. C., and Choquette, P. W., 1966, Geologic -1951, The laterolog, a new resistivity logging evaluation of a gamma-ray porosity device: Sot. method with electrodes using an automatic Prof. Well Log Analyst, 6th Ann. Logging focusing system : Am. Inst. Mining Metall. Symposium, Dallas, Texas, May 1966, Trans., Engineers Trans., v. 192, p. 306-316. v. 2, p. Cl-c37. -1955, Filtrate invasion in highly permeable Hilchie, D. W., 1968, Caliper logging-Theory and sands: Petroleum Engineer, v. 27, no. 1, p. Practice: Log Analyst, v. 4, no. 1, p. 3-12. B53-B66. Jeffries, F. S., 1966, Computer correlation of wire- Edwards, J. M., and Holter, E. L., 1962, Applications line log parameters and core analysis param­ of a subsurface solid-state isotope injector to eters: Log Analyst, v. 7, no. 3, p. 6-14. nuclear-tracer survey methods : Jour. Petroleum Jenkins, R. E,. 1960, The continuous velocity log and 122 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

acoustic core measurements: Core Lab. Tech­ Krige, *J. J., 1939, Borehole temperatures in the nical Memo. 16, p. l-9. Transvaal and Orange Free State: Royal Sot. Jenkins, R. J., 1966, Accuracy of porosity determina­ [London] Proc. Series A, v. 173, p. 450-474. tions: Log Analyst, v. 7, no. 2, p. 29-34. Larionov, V. V., 1961, Evaluation of the porosity and Johns, E. S., Jr., 1966, Tracing fluid movements with petroleum saturation of clay-sand collectors by a new temperature technique: Fort Worth, Tex., the neutron gamma method from the chlorine Gearhart-Owen, Inc., Bull. EJ416, 23 p. content, in All-Union Scientific Tech. Conf. on Johnson, A. I., 1967, Specific yield - Compilation of Use of Radioactive and Stable Isotopes and specific yields for various materials: U.S. Geol. Radiations in the Natural Economy and Science, Survey Water-Supply Paper 1662-D, 74 p. Trans. : U.S. Atomic Energy Comm. Pub., Jones, P. H., 1961, Hydrology of waste disposal, AE C-tr-4475, p. 134-140. National Reactor Testing Station, Idaho: Oak Lewis, 13. C., Kriz, G. J., and Burgy, R. H., 1966, Ridge, Tenn., Issued by U.S. Atomic Energy Tracer dilution sampling technique to determine Comm. Tech. Inf. Service, U.S. Geol. Survey hydraulic conductivity of fractured rock: Water IDO-22042. Resources Research, v. 2, no. 3, p. 533-542. Jones, P. H., and Buford, T. B., 1951, Electric log­ Livingston, Penn, and Lynch, Walter, 1937, Methods ging applied to ground-water exploration: Geo­ of locating salt-water leaks in water wells: U.S. physics, v. 16, no. 1, p. 115-139. Geol. Survey Water-Supply Paper 796-A, p. Jones, P. H., and Skibitzke, H. E., 1956, Subsurface l-26. geophysical methods in ground-water hydrology, Lyon, W. S., Jr., 1964, Guide to activation analysis: in Advances in Geophysics, v. 3: New York, Princeton, New Jersey, D. Van Nostrand Co., Academic Press, Inc., p. 241-300. 186 p. Kaveler, H. H., and Hunter, Z. Z., 1952, Observations Lynch, E. J., 1962, Formation evaluation: New York, from profile logs of water injection wells: Am. Harper and Row, Publishers, 422 p. Inst. Mining Metall. Engineers (Petroleum) Meinzer, 0. E., 1923, Outline of ground-water hydrol­ Trans., v. 195, p. 129-134. ogy, with definitions: U.S. Geol. Survey Water- Keller, G. V., and Frischknecht, F. C., 1966, Elec­ Supply Paper 494, 71 p. trical methods in geophysical prospecting, v. 10 -1928, Methods of exploring and repairing of International Series of Monographs in Elec­ leaky artesian wells: U.S. Geol. Survey Water- tromagnetic Waves : New York, Pergamon Press, Supply paper 596-A, p. l-3. 519 p. Meyer, ‘W. R., 1963, Use of a neutron moisture probe Kendall, H. A., 1965, Application of SP curves to to determine the storage coefficient of an un­ corrosion detection : Jour. Petroleum Technology, confined aquifer, in Short papers in geology, September, p. 1029-1032. hydrology, and topography: U.S. Geol. Survey Keys, W. S., 1963, Pressure cementing of water wells Prof. Paper 450-E, p. El74-E176. on the National Reactor Testing Station, Idaho: Morris, D. A., Barraclough, J. T., Chase, G. H., Teas- U.S. Atomic Energy Comm. IDO-12022, 19 p. dale, W. E., and Jensen, R. G., 1965, Hydrology -1967, The application of radiation logs to of subsurface waste disposal, National Reactor ground-water hydrology: Vienna, Austria, In­ Testing Station, Idaho - Annual progress re- ternat. Atomic Energy Agency Symposium on port: U.S. Atomic Energy Comm. IDO-22047. Use of Isotopes in Hydrology, Nov. 14-18, 1966, Morris, R. L., Grine, D. R., and Arkfeld, T. E., 1964, Proc., p. 477488. The use of compressional and shear acoustic Keys, W. S., and Boulogne, A. R., 1969, Well logging amplitudes for the location of fractures: Jour. with californium 252: Sot. Prof. Well Log Ana­ Petroleum Technology, v. 16, June, p. 623-632. lysts, 10th Ann. Logging Symposium, Houston, Morrison, B., 1969, A side-wall sampler: Canadian Texas, May 1969, Trans., p. Pl-P25. Geotech. Jour., v. 6, no. 4, p. 431439. Keys, W. S., and Brown, R. F., 1971, The use of well Muench, N. L., 1957, Identification of earth materials logging in recharge studies of the Ogallala by induced gamma-ray spectral analysis: Jour. Formation in west Texas, in Geological Survey Petroleum Technology, v. 9, no. 3, p. 89-92. research, 1971: U.S. Geol. Survey Prof. Paper Muir, D. M., and Fons, L. H., 1964, Acoustic param­ 750-B, p. B270-B277. Khalevin, N. I., 1960, Measurement of rock porosity eter log: Drilling International, Exposition in by sonic well logging: Kazvedochnaya i Promy­ Print Issue, May 15, 1964, p. 7. slovaya Geofizika 30, 3-9, Associated Technical Muir, D. M., and Zoeller, W. A., 1967, New acoustic Services, Inc., Translation RJ-2875. tool logs cased holes: Oil and Gas Jour., Oct. 23, Kokesh, F. P., Schwartz, R. J., Wall, W. B., and p. 306-112. Morris, R. L., 1965, A new approach to sonic Olmsted, F. H., 1962, Chemical and physical charac­ logging and other acoustic measurements: Jour. ter of ground water in the National Reactor Petroleum Technology, v. 17, March, p. 282- Testing Station, Idaho: U.S. Atomic Energy 286. Comm. IDO-22043. APPLICATION OF BOREHOLE GEOPHYSICS 123

Patten, E. P., and Bennett, G. D., 1962, Methods of Schlumberger, Conrad, Schlumberger, Marcel, and flow measurement in well bores: U.S. Geol. Leonardon, E. G., 1934, A new contribution to Survey Water-Supply Paper 1544-C, 28 p. subsurface studies by means of electrical mea­ -1963, Application of electrical and radioactive surements in drill holes: Am. Inst. Mining well logging to ground-water hydrology: U.S. Metall. Engineers Trans., v. 110, p. 273. Geol. Survey Water-Supply Paper 1544-D, 60 p. Schlumberger Well Surveying Corp., 1958, Introduc­ Payne, B. R., Cameron, J. F., Peckham, A. E., and tion to Schlumberger well logging (Schlum­ Thatcher, L. L., 1965, The role of radioisotope berger Doe. No. 8) : Houston, Tex., 176 p. techniques in hydrology, in United Nations -1966, Log interpretation charts : Houston, International Conference on Peaceful Uses of Tex. Atomic Energy, 3d, Geneva, Switzerland, 1964, Scott, J. H., 1963, Computer analysis of gamma-ray Proceedings, v. 15, Special Aspects of nuclear logs: Geophysics, v. 28, no. 3, p. 457465. energy and isotope applications: New York, Scott, J. H., Dodd, P. H., Droullard, R. F., and United Nations, p. 226-238. Mudra, P. J., 1961, Quantitative interpretation Pickett, G. R., 1960, The use of acoustic logs in the of gamma-ray logs: Geophysics, v. 26, no. 2, evaluation of sandstone reservoirs : Geophysics, p. 182-191. v. 25, no. 1, p. 250-274. Selecki, A., and Filipek, S., 1966, The single-well Piety, R. G., and Wiley, B. F., 1952, Flowmeter for method for determining the direction of flow of water injectivity profiling: World Oil, May, p. ground-water using a neutron source, in Inter- 176-188. national Journal of Applied Radiation and Iso­ Pirson, S. J., 1963, Handbook of well log analysis: topes: Northern Ireland, Pergamon Press, Ltd., Englewood Cliffs, N.J., Prentice-Hall, Inc. v. 17, p. 351352. Rabe, C. L., 1957, A relation between gamma radia­ Senftle, F. E., and Hoyte, A. F., 1966, Mineral ex­ tion and permeability, Denver-Julesburg basin : ploration and soil analysis using in-situ neutron Am. Inst. Mining Metall. Petroleum Engineers activation : Amsterdam, Netherlands, North- Trans., v. 210, p, 358360. Holland Publishing Co., Nuclear Instruments Rabson, W. R., 1959, Chlorine detection by the spec­ and Methods, v. 42, p. 93-103. tral log: Petroleum Engineer, v. 31, March, Skibitzke, H. E., 1955, Electronic flowmeter: U.S. p. B102-B106. Patent No. 2,728,225. Rankama, K., 1963, Progress in isotope geology: New Stallman, R. W., 1965, Steady one-dimensional fluid York, John Wiley & Sons, 705 p. flow in semi-infinite porous medium with sinusoi­ Rapolova, V. A., 1961, Experience of the use of dal surface temperature: Jour. Geophys. Re- nuclear methods in the investigation of cross search, v. 70, no. 12, p. 2821-2827. section of boreholes being drilled for water, h -1967, An outline of criteria for pumping-test All-Union Scientific Tech. Conf. on Use of design, observation, and data analysis: U.S. Radioactive and Stable Isotopes and Radiations Geol. Survey Techniques of Water-Resources in the Natural Economy and Science, Trans.: Inv., Book 3, Ch. B-l, 35 p. U.S. Atomic Energy Comm. Pub., AEC-tr-4475, Taylor, J. O., 1968, Ground-water resources of the p. 67-74. northern Powder River valley, southeastern Mon­ Raymer, L. L., and Biggs, W. P., 1963, Matrix char­ tana: Montana Bur. Mines and Geology Bull. acteristics defined by porosity computations : 66, p. 12-20. Sot. Prof. Well Log Analysts, 4th Ann., Logging Tittman, J., Sherman, H., Nagel, W. A., and Alger, Symposium, Oklahoma City, Okla., May 1963, Trans., p. X1-X21. R. P., 1965, The sidewall epithermal neutron Raymond, J. R., and Bierschenk, W. H., 1957, porosity log labs.] : Jour. Petroleum Technology, Hydraulic investigations at Hanford: Am. Geo­ v. 17, no. 9, p. 1060-1061. phys. Union Trans., v. 38, no. 5, p. 724-729. Tittman, J., and Wahl, J. S., 1965, The physical Rhodes, D. F., and Mott, W.E., 1966, Quantitative foundations of formation density logging interpretation of gamma-ray spectral logs : Geo­ (gamma-gamma) : Geophysics, v. 30, no. 2, p. physics, v. 31, no. 2, p. 410-418. 284294. Russell, W. L., and Steinhoff, R. O., 1961, Radio- Tixier, M. P., Alger, R. P., Biggs, W. P., and Car­ activity of volcanic sediments in Brazos County, penter, B. N., 1963, Dual induction-laterolog- Texas: Geophysics, v. 26, no. 5, p. 618-625. A new tool for resistivity analysis: Sot. Petro­ Sammel, E. A., 1968, Convective flow and its effect leum Engineers, Am. Inst. Mining Metall. Pe­ on temperature logging in small-diameter wells: troleum Engineers Paper SPE-713, p. 1-18 Geophysics, v. 33, no. 6, p. 1004-1012. (Preprint). Schlumberger, Conrad, and Schlumberger, Marcel, Turcan, A. N., Jr., 1966, Calculation of water quality 1929, Electrical logs and correlations in drill from electrical logs - Theory and practice: holes: Mining and Metallurgy, v. 10, no. 275, p. Louisiana Geol. Survey, Water Resources Pamph. 515-518. 19, 23 p. 124 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS

Van Orstrand, C. E., 1918, Apparatus for the mea­ mea.bility of water-bearing materials with spe­ surement of temperature in deep wells, and cial reference to discharging well methods: U.S. temperature determinations in some deep wells Geol. Survey Water-Supply Paper 887,192 p. in Pennsylvania and West Virginia, in Reger, Wood, IE. D., 1966, The gamma ray-neutron log D. B., and Teets, D. D., Jr., West Virginia (Dresser Atlas Gamma Ray Neutron Technical Geological Survey County Reports of Barbour Bull.) : Houston, Tex., Dresser Industries, Inc., and Upshur Counties: Wheeling, W. Va., West 18 p. Virginia Geological Survey, p. LXVI-CIII. Winsauer, W. O., Shearin, H. M., Masson, P. H., and Vonhoff, J. A., 1966, Water quality determinations Williams, M., 1962, Resistivity of brine-satu­ , from spontaneous-potential electric log curves: rated sands in relation to pore geometry: Am. Amsterdam, Netherlands, North-Holland Pub­ Assoc. Petroleum Geologists Bull., v. 36, no. 2, ” lishing Co., Jour. Hydrology, v. 4, p. 34134’7. p. 253. Walker, T., 1962, Fractive zones vary acoustic signal Wyllie, M. R. J., 1963, The fundamentals of well log amplitudes, World Oil, v. 164, no. 6. interpretation : New York, Academic Press, Welex, A Division of Halliburton Services, 1968, 238 p. Charts for the interpretation of well logs (Welex Youmans, A. H., and Hopkinson, E. C., 1964, The Bull. A 133) : Houston, Tex., Welex, A Division neutron lifetime log: Sot. Prof. Well Log Ana­ of Halliburton Services, 76 p. lyst,, 6th Ann. Symposium, Midland, Tex., May, Wenzel, L. K., 1942, Methods for determining per­ Trans., p. Ml-M26.

c

c INDEX

A Page Page Divalent ions. effect on SP .._ __...... ______... ______..,__.____.__...... 27 Activation analySi.9...... 79. 80 Dilution. point _.______...... ___...... ______..._.______...... 113 Acoustic Drift. instrumental ...... _.______.______.._..__.______...... 13, 23 amplitude...... 88. 89, 90 Drillhe. effect On logs ______...._.______19. 20, 73, 84, 93, 94, 96 logging...... 88-94 VelocltY...... 88, 90 E Alpha particles ...... 69 American Petroleum Institute...... 12, 18, 24, 66, 67, 72, 82, 88 Elastic constants ..______. ______...... _...._. 91 Anhydrite...... 77 Electric logging ...... ______.._...... __...... ‘28 Archie equation...... 6, 17, 41, 44 table of applications . ..______...... ______._..___...... 24 Attenuation. acoustic. See Acoustic amplitude. Electron volt ...... ______.______.___.______.. 69 Electra-chemical potential .._.___...... ______.____....._.__.__...... 24 B Energy of radiation .._. __...... _..______.. ._...._.______69 Equipment. logging __.______. ______.._...... ______21 Beats, effects on point resistance ...... 86 See also Specific logs. Bed thickneaa effect...... 18 nuclear logs ...... 68 F resistivity logs ...... 47, 61, 68 Fluid density ______. _... ______.______73. 102 Beta rays or particles...... ~...... 69 Fluid movement. ..______...... _..._. _._.._.. ._.______10, 109 Borehole effecta ...... 19. 82. 68 Flushed zone . ______..... ______.__..______64 Brine tracer ...... 116. 117 Flowmeter. impeller ______.______.______.______.______109 Bulk density ...... 70 Focused-current logs or devices ,_...__._ ____.______...... 68 Fluid parameters... ______..______. . 8. 99. 106 C Formation-resistivity factor. ______..._.. . 6. 41. 43. 44 Fractures. detection ..______._...... 89, 90, 94, 96. 111 Calibration. See Specific logs. effect on point resistance... ,___ .. .. . 32. 84 Caliper ...... 19, 72, 94 detected with caliper ,__.______..__....___ 94. 96 Casing logs...... 118 Frequency, acoustic ______..______.______.__..___.._._ 89, 91 effect...... - ...... 19 . . Fluid conductwrty ..__...______.._....______.._..._._..__8, 106 Cement...... 19. 97 Fluid movement ______.___... .____.. .______..__..__..____.__...... IO. 109 / bond ...... ______.______...... 90 detection...... 70. 72, 102 Chlorine logging, neutron ...... 79 G Clay...... 40 E Galvanometer ______.__. . .._.__...... 46 effect on gamma log...... 16, 66 Gamma rays .______._..__..______69, 64 effect on resistivity log...... 40. 41 Geometric factor. See Specific logs. Coil spacing of induction device ...... 68 Geothermal gradient ______.____...... ____.______...... 99 Composite log interpretation ...... 16 Ground-current loop ______.._. ___....______66 Compressional wave...... 88 Grain size distribution ______. ..______6. 8. 66 Compton scattering...... 70 Guard log ______.._._..______63. 64 Computer interpretation of loge...... 17 Gypsum 77 Conductivity, electrical...... 8, 9. 66. 106, 107 Conductivity, thermal...... 99 H Constant current...... 30, 31, 46, 47 Contact resistance...... 47 Half life ..______.._...... ____....__...... 69, 113 Cost of logging...... 3 Hole-diameter effect. See Drilling, effect on logs, and specific Core, use for log calibration ...... 3, 12. 18, 83 logs. Convection. See Thermal convection. Hydraulic conductivity. See Permeability. Conventional single point ...... 81. 82 Conversion factor I natural-gamma log ...... 66 fluid conductivity or reslstwity...... 8 Injectivity profiling. .._..______,.___,....___...... 111 Cratering on normal curves ...... 18, 61 Interface. saltwater-fresh-water.. ______..._...... 79. 108 Curie ...... ______...... 61 Induction device ______.___..._...... ____...... 66 Current electrodes...... 31, 37, 49 Interpretation of logs Cycle skipping, acoustic...... 90. 93 qualitative. ______.______. _...._. .._ ...... 10 quantitative __.______.___ .._ __._... ._...... 11 D See also Specific logs. Invasion, mud filtrate _. __ ,.____.___. _._._ .._.______.____..._.39 63 Dead time ...... Isotope .._...... _. ,.._.. .._____...... 69 61 Dead zone on lateral curve ...... Ion exchange __ _. _.__.__ _. _. ._ __.______.___._ ___.______41 Density. See Bulk density. Density of water ...... 102 L Departure graphs...... 39. 61 Depth errors...... 18 Lateral log. ______.... 49 Detectors, radiation ...... 61 Laterolog... ______64 Development. well...... 20, 81 comparison with single point __ . ..______..__...... 86 Differential single point ...... 81, 32 Limestone ____..______,__.______. __. ______.____...____...... 17, 41 Differential temperature ...... 102. 103 Liquid junction .______.______.. ._.____.___.._.__ 26 Digitized logs ...... 8 Lithology ______4, 10. 16, 24, 31, 61, 64, 70, 74, 88. 94

126 Page h3s Radius of investigation ______.______...... IS. 40, 68 acuustic ...... 88 See also Specific logs. calfper ...... 94 Recharge ...... ~...... 101 eas,ng ...... 118 Resistivity. defined ...... 6 fluid-conductivity...... _...... 106 fluid ...... 8, 106 fluid-movement ...... 109 rocks...... 87 focused...... ~...... 68 Resistance; deAned ...... 80 gamma-gamma ...... 70 Roentgen...... 61 gamma-spectrometry ...... 67 gamma. natural ...... 64 S induction ...... 66 lateral...... 49 Sampling ...... ~...... 8. 12, ia. 83 neutron...... 74 Sandstone...... 16. 66 normals ...... ~...... 88 Sand-shale ratios ...... 26 resistance...... 80 Saturation ...... 6, 10, 97 resiativity ...... 86 Scintillation detector ...... 61, 69 spontaneous-potential ...... 28 Shadow peak on lateral curve ...... 61 temperature ...... 99 Shale cont.ent. See Clay. wall-resistivity ...... 61 Shale line. SP log ...... 26 Lag interpretation ...... 10 Shear wave, acoustic ...... 89 See a&o Specific logs. Single-point or single-electrode system ...... 80 Logging equipment ...... 21 Sonic logging. See Acoustic logging. See ah Specifle logs. Source to detector. spacing ...... 10. 68. 72. 82 Spacing. electrode ...... 38 M Specific gravity of water ...... 102 Specific yield ...... Membrane junction ...... 26 7. 77 Mho. unit of conductivity ...... 67, 106 Speed of logging ...... 60. 61 Microfocused device ...... 64 Spontaneous potential ...... 23 Standardi,sation of loga Microlateral device ...... 61 ...... 18 Micronormal device ...... 61 See duo Specific logs. Statistical fluctuation.. Millivolts ...... ~...... 24, SO. 37, 47 ...... 69 Moisture content ...... 10. 74 Mud, drilling ...... 20 T Mud cake ...... 20, 61, 97 Temperature ...... 1, 10. 99 Multiple electrode ...... 88 Thermal convection in wells ...... 106 Thermal flowmeter ...... 116 \ N Thorium ...... 64. 68 Neutron ...... 69, 74 Three-electrode system ...... 49 capture cross section ...... 76 Time-average equation ...... 89 Noise. effect on point reaietance ...... 86 Time constant ...... 69. 60 effect on SP ...... 29 Tortuosity...... 6. 4S Nonlinear point-resistance scale ...... 34 Tracers ...... 109 Normal curves ...... 88 radiorxtive ...... 87. 111. 113, 116 with single-conductor loggers ...... _...... 46 chemical...... 117 Nuclear logging ...... 68 Transit time ...... 88 Two-electrode system ...... 88 0 Turcan’s method ...... 41 True resistivity ...... 38, 40 Ohm’s law ______.______.______...... ______. _.._...... ______..SO. 47 Ohm-meter, unit of reaistivity ..._...______..__..._.____...___..._....__..__...___6, 37 U

P Uranium __ _.______._.___._ _.______..__...... 64, 68

Pair production ...... 70 V Perched water ...... 77 Permeability ...... 6, 44, 86. 111 Velocity, acoustic... ______..______....__...... 88, 90 Photoelectric effect ...... 70 Viscosity

Radioactive sources and tracers _..______.__.______72. 76, 79. 86, 111. 118 Radioactivity of rocks ______..._._...... ______._...... _...... 66 Zero error of induction device ______._.______...... 68