. Deep borehole surveys and problems. Boring. 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
. Deep borehole surveys and problems. Boring. 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 ; 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," Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Haddock, M. H. (Marshall Henry). New York, McGraw-Hill
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