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Electrical Methods by A Techniques of Water-Resources Investigations of the United States Geological Survey CHAPTER Dl APPLICATION OF SURFACE GEOPHYSICS TO GROUND-WATER INVESTIGATIONS By A. A. R. Zohdy, G. P. Eaton, and D. R. Mabey BOOK 2 COLLECTION OF ENVIRONMENTAL DATA Electrical Methods By A. A. R. Zohdy The electrical properties of most rocks in crust in the form of large sheets, and that the upper part of the Earth’s crust are de- constantly change in intensity and in direc­ pendent primarily upon the amount of water tion. Their presence is detected easily by in the rock, the salinity of the water, and placing two electrodes in the ground sepa­ the distribution of the water in the rock. rated by a distance of about 300 meters Saturated rocks have lower resistivities than (984 feet) or more and measuring the po­ unsaturated and dry rocks. The higher the tential difference between them. The origin porosity of the saturated rock, the lower its of these telluric currents is believed to be resistivity, and the higher the salinity of in the ionosphere and is related to ionospheric the saturating fluids, the lower the resistiv­ tidal effects and to the continuous flow of ity. The presence of clays and conductive charged particles from the Sun which be- minerals also reduces the resistivity of the come trapped by the lines of force of the rock. Earth’s magnetic field. Two properties are of primary concern in If the ground in a given area is horizontal­ the application of electrical methods : (1) the ly stratified and the surface of the base­ ability of rocks to conduct an electric cur- ment rocks is also horizontal, then, at any rent, and (2) the polarization which occurs given moment, the density of the telluric cur- when an electrical current is passed through rent is uniform over the entire area. In the them (induced polarization). The electrical presence of geologic structures, however, conductivity of Earth materials can be stud­ such as anticlines, synclines, and faults, the ied by measuring the electrioal potential dis­ distribution of current density is not uni­ tribution produced at the Earth’s surface by form over the area. Furthermore, current an electric curren.t that is passed through the density is a vector quantity, and the vector Earth or by detecting the electromagnetic is larger when the telluric current flows at field produced by an alternating electric cur- right angles to the axis of an anticline than rent that is introduced into the Earth. The when the current flows parallel to the axis measurement of natural electric potentials (fig. 1). By plotting these vectors we obtain (spontaneous polarization, telluric currents, ellipses over anticlines and synclines and and streaming potentials) has also found ap­ circles where the basement rocks are hori­ plication in geologic investigations. The prin­ zontal: The longer axis of the ellipse is ori­ cipal methods using natural energy sources ented at right angles to the axis of the are (1) telluric current, (2) magneto­ geologic structure. telluric, (3) spontaneous polarization, and The measurement of telluric field intensity (4) streaming potential. is relatively simple. Four electrodes, M, N, M/, and N’ are placti on the surface of the ground at the ends of two intersecting per­ Telluric Current Method pendicular lines (fig. 2), and the potential differences are recorded on a potentiometric Telluric currents (Cagniard, 1956 ; Ber­ chart recorder or on an z-g plotter (Yungul, dichevskii, 1960; Kunetz; 1957) are natural 1968). From these measurements two corn­ electric currents that flow in the Earths ponents E,, and Ey of the telluric field can 5 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS 8 c3 t + Figure I.--Flow of telluric current over on onticline. Ellipse and circles indicate telluric field intensity as a function of direction with respect to axis of anticline. M’ very much the same manner as a gravity map or magnetic map. However., a telluric M N map (fig. 3) delineates rock structure baaed -I-- on differences in electrical resistivity rather I. I N’ than on differences in density o:r magnetic M’M N susceptibility. Figure 2.- Examples of electrode arrays for measuring x and y components of telluric field. M, M’, N, and N are potential electrodes. Magneto-Telluric Method be computed, and the total field obtained by The magneto-telluric method (Berdichev­ adding E, and Ey vectorially. skii, 1960; Cagniard, 1953) of measuring re­ The intensity and direction of the telluric sistivity is similar to the telluric current current field vary with time; therefore method but has the advantage of providing measurements must be recorded simultane­ an estimate of the true resistivity of the ously at two different stations to take into layers. Measurements of amplitude variations account this variation, One station is kept in the telluric field E, and the associated statronary (base station), and the other is magnetic field H, determine earth resistivity. moved to a new location in the field (field Magnet&&uric measurementi at several station) after each set of measurements. frequencies provide information on the varia­ The ratio of the area of the ellipse at the tion of resistivity with depth because the field station to the area of a unit circle depth of penetration of electromagnetic (Keller and Frischknecht, 1966) at the base waves is a function of frequency.. A limita­ station is calculated mathematically. When tion of the method is the instrumental dif­ a contour map of equal elliptical areas is ficulty of measuring rapid fluctuations of the prepared (Migaux, 1946, 1948 ; Migaux and magnetic field. Interpretation techniques others, 1962 ; Migaux and Kunetz, 1955 ; Sch­ usually involve comparisons of observed data lumberger, 1939) it reflects the major geo­ with theoretical curves. The method is useful logic structures of the basement rocks in in exploration to depths greater tlhan can be APPLICATION OF SURFACE GEOPHYSICS 8 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS 0 reached effectively by methods using artifi­ neering studies which lead to an understand­ cially induced currents. ing of the merits of utilizing electrical re­ To the author’s knowledge the telluric and sistivity methods for exploring the subsur­ magneto-t&uric methods have not been used face (Compagnie GBnerale de Gbphysique, extensively in the Western Hemisphere ; 1963). According to Breusse (1963)) the real however, the methods have been used exten­ progress in applying electrical methods to sively in the Eastern Hemisphere by French ground-water exploration began during and Russian geophysicists in petroleum ex­ World War II. French, Russian,and German ploration. The use of the methods in ground- geophysicists are mainly responsible for the water exploration is recommended at present development of the theory and practice of di­ only for reconnaissance of large basins. re&current electrical prospecting methods. Spontaneous Polarization and Definition and Units of Resistivity Streaming Potentials It is well known that the resistance R, in Spontaneous polarization or self-potential ohms, of a wire is directly proportional to its methods involve measurement of electric po­ length L and is inversely proportional to its tentials developed locally in the Earth by cross-sectional area A. That is: electro-chemical activity, electrofiltration ac­ tivity, or both. The most common use of self- R = L/-A, potential surveys has been in the search for or R=,,-, (1) ore bodies in contact with solutions of dif­ A ferent compositions. The result of this con- where p, the constant of proportionality, is tact is a potential difference and current flow known as the electrical resiativit,y or elec­ which may be detected at the ground surface. trical specific resistance, a characteristic of Of more interest to ground-water investiga­ the material which is independent of its tions are the potentials generated by water shape or size. According to Ohm’s law, the re­ moving through a porous medium (stream­ sistance is given by ing potentiala). Measurements of these po­ tentials have been used to locate leaks in R = AV/I, (2) reservoirs and canals (Ogilvy and others, where AV is the potential difference across 1969). the resistance and Z is the electric current Spontaneous potentials generally are no through the resistance. larger than a few tens of millivolts but in Substituting equation 1 in equation 2 and some placee may reach a few hundred milli­ rearranging we get volts. Relatively simple equipment can be used to measure the potentials, but spurious AAV (3) sources of potentials often obscure these P=t7 natural potentials. Interpretation is usually qualitative although some quantitative in­ Equation 3 may be used to determine the terpretations have been attempted. resistivity p of homogeneous and isotropic materials in the form of regular geometric shapes, such as cylinders, parallelepipeds, Direct Current-Resistivity and cubes. In a semi-infinite material the re­ sistivity at every point m,uat be dlefined. If Method the cross-sectional area and length of an element within the semi-infinite material are In the period from 1912 to 1914 (Dobrin, shrunk to infinitesimal size then the resis­ 1960) Conrad Schlumberger began his pie- tivity p may be defined as APPLICATION OF SURFACE GEOPHYSICS 9 ern United States, whereas in certain areas 20 (AV/L) in California the resistivity of fresh-water 0= r bearing sands generally ranges from 100 to ;To U/A) 250 ohm-m. In parts of Maryland resistivi­ or ties have been found to range Ibetween about EL 300 and 600 ohm-m, which is about the same p=­ range as that for ,basaltic aquifers in south- J ern 1,daho. These figures indicate that the where EL is the electric field and J is the cur- geophysicists should be familiar with the rent densi,ty.
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