554 EXPLORATION GEOPHYSICS 



have given a much greater current penetration than any employed for 

 the steady-state work if the penetration of the transient current were 

 comparable to the steady-state current. Blondeau concludes that "the 

 large observed variations in near surface resistivity can hardly be attri- 

 buted to mineralization emanating upward over structure; and that the 

 number of anomalies found is so great that their direct association with 

 deep structure is highly improbable."^ 



Seismic-Electric Effect. — Seismic-electric effectf is the name given to 

 the possible phenomenon of the earth resistivity varying with elastic defor- 

 mation. This effect has been proposed as an entirely new method of pick- 

 ing up seismic energy.^ Various experiments have led to a controversy 

 rather than elucidation of the nature of such an effect. That there is a 

 variation in resistivity under an elastic disturbance is undeniable, but the 

 cause has been assigned to varying electro-chemical conditions at the elec- 

 trode surfaces, § to the so-called varying volume polarization, and to a 

 variation in specific resistivity. 



APPLICATIONS OF ELECTRIC METHODS 



The fields of applications of geoelectrical methods may be classified 

 as follows: (1) prospecting for highly conductive materials; (2) struc- 

 tural investigations, which include prospecting for placer deposits, 

 water supply problems, civil engineering construction problems, and oil 

 structural mapping. 



Prospecting for Highly Conductive Materials. — This application 

 includes prospecting for certain veins, lodes, and dikes. Results obtained 

 in vein deposit investigations will be discussed here.* 



Vein Deposits 



Geophysical methods are of special significance in the initial evalua- 

 tion of a prospect, because they yield important data regarding subsurface 

 conditions before purchasing or opening the property. 



Prospecting for ore bodies should be done with the viewpoint that a 

 great majority of the properties examined will be abandoned. This view- 

 point implies that the risk in examining geophysically and geologically 

 a large number of prospects is less than that in attempting to find an ore 

 body by extensive development work on one property. The soundness of 



t R. R. Thompson, "Seismic Electric Effect," Geofhysics, Vol. I, No. 3, October, 1936, p. 337; 

 Vol. IV, No. 2, March, 1939, p. 102. 



t L. W. Blau and Louis Statham, "Method and Apparatus for Seismic Electric Prospecting," 

 U. S. Patent 2,054,067, September 15, 1936. 



§ St. V. Thyssen, J. N. Hummel and O. Riilke, "Die Ursachen das Seismisch-Elektrischen 

 Effektes," Z. Geophs. 13, 1937, 112119; "Ueber das Wessen des Seismische-Elektrischen Effektes," 

 Beit. Angew. Geophys., 7, 1938, pp. 209-217. 



* A general discussion of the applicability of geoelectrical methods in this field is 

 given in the chapter entitled Geologic and Economic Background of Exploration 

 Geophysics. 



