ADVANCES IN OIL PROSPECTING 163 
given where the problem is to extend the structural information in 
_ a known area into adjacent territory. 
The chief obstacle which now faces the direct electrical location 
— of oil is that, with our present technique, we can not detect a perfect 
insulator any better than a stratum, the conductivity of which is only 
approximately 1o times smaller than that of the adjacent beds. How- 
ever, there is no reason why in the near future the methods can not 
be perfected to such an extent that, when a resistivity indication has 
been definitely established to be due to an oil bed in a certain depth, 
this indication can be traced into adjacent territory with the object 
of determining whether or not the oil is replaced by salt water. It is 
doubtful, however, whether or not the reverse will be accomplished 
so soon, as it is difficult, at the surface, to distinguish a dry resistant 
formation from an oil-bearing bed. In problems of the type just dis- 
cussed, that is, in tracing a formation with the object of determining 
whether its contents change from oil to salt water, the potentialities 
of the resistivity and potential-drop-ratio methods will probably be 
utilized to the greatest advantage when the resistivity indications 
are combined with results obtained from other geophysical methods 
which would give the structural data alone, such as the seismic re- 
flection method. 
REFERENCE LIST 
(Referenc== are arranged according to date of publication, under four headings.) 
I. RESISTIVITIES OF ROCKS AND FORMATIONS; DETERMINATIONS OF SAME 
1. R. O. E. Davis, ““The Use of the Electrolytic Bridge for Determining Soluble 
Salts,” U.S. Dept. Agriculture Circ. 427 (Washington, D. C., July, 1927). 
2. B. McCollum and K. H. Logan, “‘Practical Applications of the Earth Current 
Meter,” U.S. Bur. Standards Tech. Paper 351 (August, 1927). 
3. R. D. Harvey, “Electrical Conductivity and Polished Mineral Surfaces,” Econ. 
Geol., Vol. 23, No. 7 (November, 1928). 
4. W. Heine, Elektrische Bodenforschung, Gebr. Borntraeger (Berlin, 1928), 223 pp. 
5. J. Koenigsberger, ‘Field Observations of Electrical Resistivity and Their Prac- 
tical Application,’’ Geophysical Prospecting (Amer. Inst. Min. Met. Eng., New York, 
1929), PP. 221-37. 
6. M. W. Pullen, “Tentative Method for Making Resistivity Measurement of 
Drill Cores and Hand Specimens of Rocks and Ores,” U. S. Bur. Mines Circ. of Inf. 
6141 (June, 19209), 11 pp. 
7. J. G. Biddle, “Megger Ground Tester,” Cat. 1750 (Philadelphia, October, 1929). 
8. C. and M. Schlumberger, ‘“‘The Electrical Coring,” Soc. de Prospection Elec- 
trique (Paris, September, 1929). 
g. Karl Sundberg, “Principles of the Swedish Geo-Electrical Methods,” Gerland’s 
Beitraege Supp. Applied Geophys., 1, No. 3 (1931), pp. 298-361. 
10. , “Effect of Impregnating Waters on the Electrical Conductivity of Soils 
and Rocks,” Geophysical Prospecting (Amer. Inst. Min. Met. Eng., New York, 
1932), Pp. 367-91. 

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