USE OF GEOELECTRIC METHODS IN SEARCH FOR OIL 173 
natural electric currents in the earth which were then being initiated 
in the Department of Terrestrial Magnetism of the Carnegie Institu- 
tion of Washington. The method of measurement which was finally 
decided upon should be regarded as a direct-current method.! It was 
thought that with this method it should be easier to carry out sur- 
veys, especially when great depths of earth are to be included in the 
measurements, and that the interpretation should be simpler than 
for a number of other possibilities which were examined. This method 
is an improvement and extension of that developed by McCullom? 
and used by him in electrolysis surveys and which was in turn based 
on an alternating-current method suggested by Wenner.® The first 
preliminary survey, made in the early autumn of 1924 with the ap- 
paratus designed for large-scale measurements, showed that the 
method could be used for determining and locating structure under 
certain circumstances. Following the publication of a report on this 
work, the method was adopted in some form by several organizations 
which had previously made extensive use of equipotential methods. 
In response to requests in recent years, working drawings and other 
information required for the duplication of the apparatus have been 
supplied by the Department of Terrestrial Magnetism to a number 
of responsible individuals and research organizations. It appears from 
reports, published by some of these, that a few of the more essential 
features of the apparatus have at times been overlooked or mis- 
understood. The fact that this occurred, even where the work was 
in charge of capable physicists, indicates the need of the additional 
emphasis which a restatement of these features may give. 
There are situations where the detection of the desired structure 
is so simple that relatively crude apparatus will suffice. However, 
when the exploration is to extend to greater depths, in the hundreds 
and into the thousands of feet, and under conditions which are made 
difficult by the relationship between the conducting and non-conduct- 
ing strata, success is dependent upon careful attention to the follow- 
ing matters: (1) the polarization, contact potentials, and earth po- 
tentials of various origin-of the pick-up electrodes must be largely 
eliminated; (2) the effects of induction between the two circuits must 
be made negligibly small; and (3) the insulation between the rela- 
tively high-potential energizing circuit and the low-potential pick-up 
1 Terr. Mag., 30 (1925), pp. 161-88; Bull. Nat. Res. Council, 11, Pt. 2 (1926), 
pp. 86-91; U.S. Patent No. 1, 813, 845 (1931). 
2 “Measurement of Earth Currents,’’ Elec. Ry. Jour. (November 5, 1921). 
3 Bull. Bur. Standards, 12 (1916), pp. 469-78. 
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