228 



DEEP BOREHOLE SURVEYS AND PROBLEMS 



SuflEicient has been said to indicate that the electric or 

 electromagnetic field set up by the current directed to 

 the underground is investigated on the assumptions (1) 

 that the earth is a good conductor, (2) that the conductivity 

 of the object sought differs sufficiently from that of its 

 surroundings, and (3) that the object sought lies in such a 

 way in the current region that the deflection caused in the 



Fig. 156. — Schematic example of spontaneous polarization according to Schlum- 



berger. 



electric field or electromagnetic field can be detected on the 

 surface. Schlumberger says^ "the equipotential curves of 

 small radius belong to small spherical surfaces which lie at 

 small depths in the earth and are therefore unaffected by 

 deep-lying masses. The deep-seated masses thus only 

 influence curves of large radius." 



Schlumberger' s is distinct from any other method, 

 where the whole measuring processes are continually fed 

 by an electric current. It is executed by cutting out the 

 direct-current carrying electrodes and the measurement is 

 made on the surface by the polarization of the potential 

 distribution brought about in an ore body. If a constant 

 direct current flows through a mineral deposit, Schlum- 

 berger has ascertained that an electrolysis takes place in 

 the ground on the surface of the deposit (Fig. 156) if water 



1 "Etude sur la prospection electrique du sous-sol," 1922. 



