ADVANCES IN OIL PROSPECTING 95 
States by 1925, when the Schlumberger Company was engaged to 
work for the Roxana Petroleum Corporation and the Shell Company 
of California. The work for the latter continued to about 1929. 
A number of oil companies have carried on extensive studies with 
their own electrical resistivity and potential equipment, such as the 
Sun Oil Company, the Pure Oil Company, and the Midwest Refining 
Company. 
In addition to the Schlumberger Company, several other geo- 
physical prospecting companies have since taken up studies, such as 
the McCollum Exploration Company, the Swedish-American Pros- 
pecting Corporation, the Radiore Company, the Elbof Company, the 
Geophysical Service Inc., and the International Geophysics, Inc. 
Very little has been published on the successes of structural resistivity 
prospecting; practically everything is by members of the Schlumberger 
Company (ref. list No. ITTy,10,12,16,18,20) 
One of the outstanding developments in resistivity-prospecting 
methods in late years was the perfection of the potential-drop-ratio 
method. This was suggested by three authors at almost the same 
time. Although a 3-contact ratio arm bridge was devised by A. B. 
Edge as early as 1925 and was used in northern Rhodesia during 
that year, and although an A. C. potential ratiometer was used, up 
to the end of 1929, by the I.G.E.S. in Australia, it was not until 
January; 1931, that the method was first described by Edge (ref. list 
No. IV,). Already before this publication appeared, Koenigsberger 
(ref. list No. IV;) had suggested a potential-ratio method in 1930. 
Two months after Edge’s article had been published, H. Lundberg 
and Th. Zuschlag published their description of the Racom (ref. list 
No. IV;). 
The potential-drop-ratio method, on account of its sensitivity 
and accuracy, gives very good results when suitable instruments are 
used, and it is believed that it is only in its initial stage of develop- 
ment. 
There follows now a discussion of (1) the resistivity of formations 
and methods for its determination; (2) a description of the technique 
of resistivity and potential-drop-ratio methods; and (3) a discussion 
of the results obtained in structural work. 
B, RESISTIVITY OF FORMATIONS AND ROCKS 
With the exception of the few rocks which contain some sort ofa 
metallic mineralization (mostly sulphidic), and the conductivity of 
425 
