Chap. 10] ELECTRICAL METHODS 625 



(c) In resistivity methods current is supplied to the ground at two points 

 and the potential is measured between two additional points whose spacing 

 or distance from the primary electrodes is varied. The ratio of voltage 

 and current, multiplied by a spacing factor, gives what is known as ap- 

 parent resistivity as a function of spacing and, hence, as a function of 

 depth penetration. This application makes possible a determination of 

 depth to bedrock, to sulfide ore bodies, to water level, and to beds of 

 stratigraphic significance. If the spacing (and therefore the depth pene- 

 tration) is kept constant and the arrangement as a whole is moved, hori- 

 zontal variations in character or in depth of a given formation may be 

 determined. An adaptation of this procedure is the process of electrical 

 logging discussed in further detail in Chapter 1 1 . 



(d) The potential-drop-ratio method involves a comparison of voltage 

 differences with reference to magnitude and phase in successive ground 

 intervals. This method is also applied to a determination of depth of 

 horizontal and vertical formation boundaries. Although the potential- 

 drop-ratio method has greater resolving power in determination of strati- 

 fied formations than the resistivity method has, it is best adapted to an 

 investigation of vertical formation boundaries, that is, to the location of 

 ore bodies, quartz veins, and the like. 



(e) Eltran (transient) methods derive their name from the fact that, not 

 quasistationary fields, but transients are studied. The so-called electro- 

 chemical method measures the time that elapses between the application 

 of a current impulse and the peak of the polarization current released by 

 the primary impulse. This time interval is said to be dependent on the 

 electrolytic properties of the formations affected. In the "Eltran" methods 

 proper, a current impulse is impressed on the ground, and the time decay 

 of the corresponding potential impulse is determined. The time constant 

 is primarily dependent on the resistance characteristics of the ground circuit. 



2. Electromagnetic methods may be classified according to the manner in 

 which the currents, whose electromagnetic field is measured, are caused 

 to flow in the ground. In electromagnetic galvanic methods, current is 

 supplied by grounded electrodes. In electromagnetic inductive processes, 

 currents are induced to flow in subsurface conductors by insulated loops 

 or cables. A sharp line cannot be drawn between these methods because 

 there are some that employ either power supply. It is difficult to classify 

 the various electromagnetic methods except to enumerate them by name: 



(a) The Lundherg-Sundherg methods involve the measurement of hori- 

 zontal and vertical field components with compensator devices giving 

 their in-phase and quadrature constituents in reference to the primary 

 field supply. (6) Ambronn^s method employs a similar arrangement, ex- 

 cept that the compensator gives the phase and magnitude of three field 



