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Borehole Geophysical Logging of -Supply in the Piedmont, Blue Ridge, and Valley and Ridge, Georgia

Crystalline and carbonate-rock in northern Georgia provide water to an ever- increasing number of private and public wells in the region. Understanding the depth and Atlanta yield of water-bearing zones in such wells is crucial for the development and long-term sustainability of ground-water resources and for keeping wells in good operating condition. GEORGIA Portable geophysical logging units are now widely available and have greatly increased the ability of geoscientists to obtain subsurface information from water wells.

What Is Geophysical Logging? ered below a water-bearing fracture and water cascades into the open portion of the and mixes with air (sketch B, below). Many different types of are used to collect subsurface data This situation can be avoided by maintaining the pumping level in wells (Haeni and others, 2001; Keys, 1990, 1997). These above any water-bearing zone (A) or using a casing or liner above include submersible cameras and highly sophisticated instru- the fracture (sketch C, below). Although the depths of some ments that measure water quality, detect lithologies, and measure water-bearing zones are known from the driller’s log, not all formation properties. Geophysical logging tools are lowered zones are detected. Borehole geophysical logs help locate all of into a well by a long cable and continuously record the physi- the water-bearing zones in a well. With this information, a well cal properties of the rock and fluid. Resulting data are presented can be constructed or refurbished to prevent water from entering in graphs called borehole geophysical logs. A caliper log, for the borehole from undesirable locations. Logs also are used example, is a continuous record of the well diameter that can be to evaluate casing condition; well construction; grouting seal used to detect fracture openings or changes in well construction. of new, old, or refurbished wells; lithologic or geologic units; A. B. Caliper, in inches relations among water-producing zones and lithologic units; and 5 6 7 8 0 water-quality problems as described on the following pages.

10 6-inch A. Correctly B. Over- Casing C. Well casing pumped well pumped well with liner 20 to 28 feet without liner without liner

Well casing Well casing Well casing 30

40 Pumping Fracture Fracture water level Upper Liner water- 50 Upper Cascading bearing water- water and zone bearing fouling sealed 60 zone off by (A) As the caliper crosses a fracture, the arms open slightly, liner creating the inflection on the caliper log (B). This example log Pumping water shows a fracture at about 42 feet below land surface. level Why Log a Well?

Geophysical logs are extremely important to gain a better under- Pump Pump standing of the system and fractures supplying a well. Drilled wells in northern Georgia are almost always constructed with an “open hole,” which means the borehole is unlined and Lower Lower Lower open to the below the surface casing (sketch A, right). water- water- water- bearing bearing bearing Water enters the well through fractures and other openings in zone zone zone the otherwise solid rock. In such open-hole wells, it is impor- Geophysical logs can be used to detect and prevent water-bearing tant to ensure that water-bearing fractures are not exposed to air zones from being exposed in the open portion of the borehole. (A) shows due to overpumping or declining water levels. Iron fouling and a correct pumping water level, (B) shows an over-pumped well without water-quality degradation may result if the water level is low- a liner, and (C) shows a liner that seals off shallow-water zones.

U.S. Department of the Interior Fact Sheet 2007–3048 U.S. Geological Survey June 2007 Caliper logging continu- CALIPER, IN INCHES Natural gamma logging GAMMA, IN APIU 5.5 6.5 7.5 8.5 9.5 10.5 0 100 200 300 400 500 0 ously records well diam- 8.3-inch casing measures gamma radia- eter and can be used to to 25 feet tion emitted by subsurface detect fracture openings rock layers and is recorded or changes in borehole in American Baseline gamma diameter. A log is collected Institute units (APIU). response of by lowering the tool to the 100 This tool can be run in amphibolite

bottom of a well, opening either open or cased holes Amphibolite the arms, and then pulling and is used to determine it up the borehole, allowing lithology. Typically, slight the -loaded arms to changes in the APIU base- 8-inch borehole open or close as they pass 200 line may be observed for Gamma borehole enlargements, or igneous and metamorphic response

restrictions (Keys, 1990). rocks. The tool also may of button- Schist schist Changes in the borehole be used to detect the pres- Open diameter may be related to ence of radioactive zones, Gamma water- Fracture fractures, changes in lithol- 300 which may degrade water response and bearing Fracture fractures fractures ogy, borehole construction, DEPTH BELOW LAND SURFACE, IN FEET quality in a well. related to or hole integrity. A caliper lithologic Fracture contacts in log mainly is used to locate the biotite- Fracture gneiss fracture openings in the Biotite Gneiss bedrock as shown in this 400 caliper log obtained from a 6-inch crystalline-bedrock well. borehole

Long- and short-normal LATERAL RESISTIVITY, Fluid logging methods FLUID TEMPERATURE, IN OHM METERS measure properties of the IN DEGREES FAHRENHEIT and lateral resistivity log- 0 100 200 300 400 65 66 67 68 69 70 0 ging measures the apparent water column in a borehole 0 resistivity in ohm meters. and typically include the The tool applies a constant simultaneous measure- current across two elec- ment of fluid resistivity trodes while measuring and temperature. Changes the potential between two 100 or inflections in the fluid 100 other electrodes. All three Response temperature or resistivity of resistive, resistivity logs are collected unfractured are an indication of pos- crystalline sible water-producing and simultaneously and can rock be compared. Long- and water-receiving zones in short-normal logs mea- 200 a well. Fluid logs also can 200 sure the resistivity over a be collected during pump- larger area surrounding a ing to identify water-bear- Potential borehole. Lateral resistivity Water- ing zones. Borehole grab water- bearing producing measures resistivity close to zones samples may be used to zone DEPTH BELOW LAND SURFACE, IN FEET DEPTH BELOW LAND SURFACE, IN FEET a borehole. In hard, resistive 300 collect water samples at 300 discrete depths. crystalline bedrock, water- Inflection bearing zones typically are (potential water- indicated by low resistivity. producing This example lateral resis- zone) tivity log helps distinguish 400 400 Note: This is from the water-bearing zones in the same well as the caliper log above. a crystalline-bedrock well. Flowmeter logging measures the direction and Flowmeter log magnitude of vertical fluid flow within a borehole, Tool positioned Tool positioned UPWARD FLOW, IN making it an ideal tool to identify water-produc- for measurement B for measurement E GALLONS PER MINUTE ing or water-losing fractures in a well. Flowmeter 0 2.5 5 40 measurements are collected at discrete locations, G usually above and below fractures identified using logs, or as a continuous log in a trolling mode (Paillet, 1998, 2000). 80 How flowmeter logging works. The example to the right shows measurements taken at discrete F locations. The measurements taken above and 120 below the bottom fracture (at locations A and B) Measure- are both near zero, indicating no measurable flow ment E from that zone. Measurements taken straddling E the next fracture (C and D) show an increase 160 in vertical flow from 0 to about 2.5 gallons per Inflow D minute (gal/min); therefore, that fracture produces C Measure-

2.5 gal/min. The next pair of measurements show DEPTH BELOW LAND SURFACE, IN FEET ment B 200 an increase of about 1.5 gal/min indicating another B water-bearing zone (between D and E). Flow measurements above E indicate no additional A contribution to flow in the well (F and G). Fracture 240

Flowmeter types: The electromagnetic flow- The electromagnetic flowmeter log shown below was collected meter with a flow diverter under both nonpumping and pumping conditions. Under non- can measure flows between pumping conditions, water enters the borehole through multiple 0.1 and 15 gal/min. Greater fractures between 100 and 250 feet and exits the borehole into flows (100 gal/min or more) shallow fractures near the base of the casing (30 feet). Under can be measured with proper pumping conditions, the flow contribution from various zones calibration of the flowmeter was determined; however, pumping did not change the flow while using an underfit flow relations in the well and water continued to exit the borehole diverter that allows some of the near the base of the casing. A deeper casing was added to vertical flow to bypass the tool rectify this problem. (Paillet, 2000). Electromagnetic flowmeter The heat-pulse flowmeter CALIPER, with a flow diverter can UPWARD FLOW, IN GALLONS PER MINUTE IN INCHES 100 20 30 40 50 60 70 80 5 7 9 11 13 measure flows as small as 0 0.01 +/– 0.005 gal/min and as Casing to 30 feet, well not flowing great as 1.5 gal/min. The heat- 50 pulse flowmeter uses a thermal trace to measure the direction 100 and rate of vertical flow in a 150 borehole (Paillet, 2000). The spinner flowmeter mea- 200 sures vertical flow by recording the rotation rate of an impeller. 250 The lowest velocity a typical 300 spinner flowmeter can measure Non-pumpingPumping flow conditions at 50 gallons per minute Inflow Measured flow is about 5 feet per minute, DEPTH BELOW LAND SURFACE, IN FEET 350 Outflow which limits its use to higher Interpolated flow rates. 400 Camera logging records Downhole view Optical televiewer (OTV) logging produces high- both downhole and side-look- resolution, oriented color digital images of a borehole wall.

ing views of a borehole and Fracture This method of imaging is relatively time-consuming; how- permits direct inspection of ever, it is preferred over camera logging in open bedrock the borehole wall and details wells because of the ability to map lithologic units and of the well construction. fractures using a single log. Because it is an optical tool, The images can be used to the log can be only run 2-D PROJECTION CORE VIEW identify changes in rock type NESWN in relatively clear water. and small-scale geologic 309 Acoustic televiewers structures, locate and describe Side-looking view (not shown) can be run fractures, and identify prob- in both water and mud- lems with borehole integrity filled holes. Low-angle and possible signs of contam- Fracture 311 foliation ination (Johnson and Dun- This example OTV log Steep stan, 1998). This method of shows lower angle foliation joints or layering by steeper borehole imaging is relatively joints in the borehole. The cost-effective and logs can be 313 location and orientation of collected quickly. However, fractures in relation to rock detailed interpretation of Borehole camera images fabric can be particularly

showing views of water- DEPTH, IN FEET BELOW LAND SURFACE useful in water-resource video logs can be time-con- producing fracture. evaluations. suming. 315

Geophysical Log Database Keys, W.S., 1990, Borehole geophysics applied to ground-water inves- tigations: U.S. Geological Survey Techniques of Water-Resources Geophysical logs provide a basis for larger-scale evaluations Investigations, book 2, chap. E-2, 150 p. of the occurrence and availability of ground-water resources Keys, W.S., 1997, A practical guide to borehole geophysics in throughout Georgia. Paper copies and digital records of well- environmental investigations: USA, CRC Press, Inc., 176 p. log data for wells in Georgia are stored at the U.S. Geological Paillet, F.L., 1998, Flow modeling and permeability estimation using Survey (USGS) office in Atlanta. New logs are being collected borehole flow logs in heterogeneous fractured formations: Water in cooperation with State and local water agencies and commu- Resources Research, v. 34, no. 5, p. 997–1010. nities through the USGS Cooperative Water-Resources Program Paillet, F.L., 2000, Flow logging in difficult —Making the (Clarke, 2006). best of a bad deal, in Proceedings of the 7th International Sym- posium on Borehole Geophysics for Minerals, Geotechnical, and Summary Applications, Denver, Colo., 2000: The Minerals and Geotechnical Logging Society, A Chapter at Large of the Society of When planning for the long-term use of a ground-water source, Professional Well Log Analysts, Houston, Tex., p. 125 –135. borehole geophysical logs are extremely important to gain a better understanding of the aquifer system and water-bearing Shapiro, A.M., Hsieh, P.A., and Haeni, F.P., 1999, Integrating multi- disciplinary investigations in the characterization of fractured rock, fractures supplying a well. Logs are used to in Proceedings of the Technical Meeting of the U.S. Geological • locate and describe fractures and rock units; Survey Toxic Substances Program, Morganwalp, D.W., • evaluate relations among rock types and yield; and Buxton, H.T., eds., Charleston, South Carolina, held March 8 –12, 1999: U.S. Geological Survey Water-Resources • measure yield and water quality of producing zones; Investigations Report 99-4018C, v. 3, p. 669 – 680. • identify water-quality problems; and Williams, J.H., and Johnson, C.D., 2000, Borehole-wall imaging • check casing condition, well construction, and with acoustic and optical televiewers for fractured-bedrock aquifer grouting seal of new, old, or refurbished wells. investigations, in Proceedings of the 7th Minerals and Geotechnical Logging Symposium, Golden, Colo., October 24 –26, 2000: Miner- als and Geotechnical Logging Society, p. 43–53, CD–ROM. Selected References Clarke, J.S., 2006, Helping Solve Georgia’s Water Problems—The For more information on the USGS Geophysical Logging Program USGS Cooperative Water Program: U.S. Geological Survey Fact Contact: Lester J. Williams or Michael F. Peck Sheet 2006-3032, 4 pages, Web-only publication available at http://pubs.usgs.gov/fs/2006/3032/. U.S. Geological Survey, Georgia Water Science Center 3039 Amwiler , Suite 130 Haeni, F.P., Lane, J.W., Jr., Williams, J.W., and Johnson, C.D., 2001, Atlanta, Georgia 30360-2824 Use of a geophysical toolbox to characterize ground-water flow in Phone: 404.906.5761 or 770.903.9100 fractured rock, in Proceedings of the Fractured Rock Conference, E-mail: [email protected] or [email protected] March 26 –28, 2001, Toronto, Ontario, CD –ROM. Johnson, C.D., and Dunstan, A.H., 1998, Lithology and fracture characterization from drilling investigations in the Lake area, U.S. DEPARTMENT OF THE INTERIOR Design and layout by Caryl J. Wipperfurth DIRK KEMPTHORNE, Secretary Editing by Patricia L. Nobles Grafton County, New Hampshire: U.S. Geological Survey Water- U.S. GEOLOGICAL SURVEY Resources Investigations Report 98-4183, 211 p. Mark D. Myers, Director