Chap. 10] ELECTRICAL METHODS 817 



It is difficult to say just what type of electrical field or current distribu- 

 tion causes the observed type of anomaly. It probably results from dis- 

 tortions of equipotential lines of the electrical field induced in the near- 

 surface strata. These distortions occur near the boundaries of good and 

 poor conductors. Similar anomalies, observed near subsurface salt water 

 contacts in oil fields and above salt domes and igneous intrusions, are be- 

 lieved to reflect near-surface resistivity variations sometimes associated 

 with such geologic features."^ Otherwise (with the exception of fissures, 

 and the like, acting as carriers) it appears impossible to reconcile the ap- 

 parent depth penetration of this method with the limited range postulated 

 by the theory. 



Petrowsky^^^ observed that radio waves in salt mines had about the 

 same range as in air, and that waves of from 50-250 m in length could be 

 transmitted through quartz porphyrj^ rocks, tuffs, and conglomerates in a 

 pyrite mine. In one case it was definitely possible to locate a pyrite stock 

 by its "shadow." 



2. In the interference methods, variations in signal strength due to in- 

 terference of waves which travel by different routes are utilized to de- 

 termine the difference in path and, if possible, the path itself. Interfer- 

 ences may be measured (o) by leaving the wave length constant and 

 vaYying the position of the receiver, and (6) by leaving the receiver con- 

 stant and varying the wave length of the transmitter.'^" If the path dif- 

 ference between two waves is Ar, a minimum in reception will result when 

 Ar = X/2 or Ar = (2n -1- 1) X/2. When the wave length is held constant 

 and the receiver is moved at the surface, the distance between successive 

 minima in reception will be X, or twice the path difference. If, on the other 

 hand, the receiver is left constant and if two wave lengths, Xi and X2 , 

 produce successive reception minima, the path difference is 



Ar = ^^^ . (10-70a) 



X2 — Xi 



Since iff = 4r^ -f- a^ and Ar = 2r — a, the distance of a reflecting surface 

 follows from the path difference Ar and is given by 



d = ^ V(Ar)2 + 2a(Ar). (10-706) 



"* The apparent reaction of the Eltran methods to deep structures and fields, at 

 first unexplainable, is now believed to result from near-surface resistivity variations 

 accompanying such features. See E. E. Rosaire, Geophj'sics, 3(2), 96-121 ^Mar., 

 1938), and E. E. Blondeau, Geophysics, 4(4), 271-278 (Oct., 1939). 



"8 Loc. cit. 



12" A third possibility, that of producing interference by varying the distance of a 

 reflecting surface, is used in radio-altimeters but not in geophysics. 



