ELECTRICAL METHODS 



551 



E/I. A paper by Karcher and McDermottf gave the results of experi- 

 mental work in determining time constants and resistivities using a fixed 

 current electrode spread and outlined the general limitations of the work, 

 A subsequent paper by Statham^ described a modified configuration utiliz- 

 ing two current circuits so positioned that their elTects nullified each other. 

 Investigations using alternating current of rectangular or other non- 

 sinusoidal wave form were described by West.§ In this work, the two 

 detecting electrodes were placed in line with, but outside of, the current 

 electrodes. The changes in wave form caused by anomalous subsurface 

 structures were determined by a null method in which the surface poten- 



-(^ 



///. 



A 



■'My/////////3/ 



Fig. 338. — Electrode and apparatus for transient pros- 

 pecting studies. A shows the electrode configuration with 

 the detecting electrodes at the left and the current electrodes 

 at the right. R is usually 1000 feet. B shows the detecting 

 and analyzing circuit, which is represented in diagram A 

 as a simple voltmeter. Arrows indicate direction of control 

 or current propagation. (West, Geophysics.) 



tial transient was balanced against an electromotive force produced by 

 passing the output of an oscillator through an adjustable network. The 

 electrical apparatus used by West is shown schematically in Figure 338. 

 Numerous other applications of non-sinusoidal wave form currents have 

 been made in recent years.* 



Studies were conducted by Hawley** to determine the characteristics 

 of the transients associated with the flow of current when a source of 

 constant voltage is suddenly applied between two electrodes imbedded 

 in the ground. In particular, detailed studies were made of: (a) the 



t J. C. Karcher and E. McDermott, "Deep Electrical Prospecting," A.A.P.G. Bull., vol. 19, 

 Jan. 1935. pp. 65-77. 



t L. Statham, "Electric Earth Transients in Geophysical Prospecting," Geophysics, vol. 1, No. 

 2, Feb. 1936, pp. 271-277. 



§ S. S. West, "Electrical Prospecting with Non-Sinusoidal Alternating Currents," Geophysics, 

 vol. 3, No. 4, October 1938, pp. 306-314. 



* S. S. West, "Method of Subsurface Prospecting," U. S. Patent No. 2,237,643, April 8, 1941. 

 P. W. Klipsch and S. Bilinsky, "Electrical Prospecting," U. S. Patent No. 2,243,428, May 27, 

 1941. 



K. H. Evjen, "Method and Apparatus for Making Geological Explorations," U. S. Patent No. 

 2,294,395, Sept. 1, 1942. 



J. P. Minton, "Electrical Prospecting," U. S. Patent No. 2,304,739, Dec. 8, 1942. 

 W. B. Lewis, "Frequency Converter System for Geophysical Prospecting," U. S. Patent No. 

 2,342,676, Feb. 29, 1944. 



** Paul F. Hawley, "Transients in Electrical Prospecting," Geophysics, vol. 3, No. 3, July 

 1938, pp. 247-257. 



