550 EXPLORATION GEOPHYSICS 



the measurements in surveys by the transient method.) It may be shown 

 that if either of these functions is known the voltage due to any other 

 driving current, however arbitrary, may be computed. That is, a single 

 measurement of A{t) or A' {t) is sufficient to compute the response that 

 would be observed for any other applied current surge, provided only 

 that the surge is sufficiently short.* 



In an early application of this method t it was desired to record the 

 transient characteristics obtained when a direct current was established 

 through a path, a portion of which included the area under investigation. 

 For a single layer, a direct current transient will have a time constant t 



rp_Ah 



R 



where A is a constant, h the depth of the layer, and R the resistance of 

 the layer. 



Blau postulated that if more than one layer were present the build-up 

 time for the first layer would be relatively unaffected, and each subsequent 

 layer would contribute a transient whose duration would be proportional 

 to its thickness. The various transient effects would therefore be super- 

 posed on one another and would produce a composite transient composed 

 of "ripples." The various ripples would thus be indicative of particular 

 subterranean strata. It was futher postulated that a thin layer would create 

 a short ripple on the transient curve while a thick layer would create 

 a long ripple.** Lateral changes in the thickness of any stratum would 

 be indicated by corresponding changes in the time interval of the ripples 

 observed at dift"erent locations. 



Because the transient methodsff utilize a surge of current into the 

 ground, studies may be made of both the time constant and the ratio of 



* It may be shown also that the ordinary direct-current resistivity is equal to the 

 area of the A(t) oscillogram, measured by a planimeter or other means, divided by 

 the total electric charge Q of the surge. This is true irrespective of the slowness or 

 rapidity of the surge. Hence, by employing a surge generator to supply power and a 

 ballistic type of meter to integrate the amplified voltage transient, readings cap be 

 taken rapidly and accurately without using calibrated potentiometers, non-polarizing 

 electrodes, or photographic recordings. 



t L. W. Blau, "Method and Apparatus for Geophysical Exploration," U. S. Patent 1,911,137, 

 issued May 23, 1933. 



L. W. Blau and L. Statham, "Apparatus for Recording Earth Current Transients," U. S. 

 Patent 2,079,103, issued May 4, 1937. 



t Franz Ollendorff, "Elektromagnetische Ausgleichsvorgange in Geschichtetem Erdreich," 

 Archiv Fur Elektro-Technik, Vol. 23, No. 3, pp. 261-278, 1930. 



** The data would be plotted with the time duration of the ripple as ordinate and 

 the distance on the surface from the point of reference as the abscissa, just as in 

 seismic work where the times of arrival are plotted against distance from the point of 

 reference. 



tt G. E. White, "Electrical Transient Prospecting," U. S. Patent No. 2,251,537, Aug. 5, 1941; 

 "Geophysical Prospecting." U. S. Patent No. 2,291,596, July 28, 1942. 



P. W. Klipsch, "Mixing Circuit for Electrical Prospecting," U. S. Patent No. 2,251,549, Aug. 

 5, 1941. 



T. Zuschlag, "Apparatus for Geophysical Prospecting," U. S. Patent No. 2,278,506, April 7, 

 1942. 



S. J. G. Pirson, "Method and Apparatus for Geoelectrical Exploration," U. S. Patent No. 

 2,319,764, May 18, 1943. 



