10 L. J. PETERS AND J. BARDEEN 
covers a thick sediment with a large change in resistivity. Then the contours 
will roughly approximate the topographical contours on the sediment. Sup- 
pose that the sediment has a low resistivity. Then when it is near the surface, 
the apparent resistivity will be low. When the sediment is deeper, the ap- 
parent resistivity will be higher. 
The apparent resistivity map, of course, gives only a qualitative inter- 
pretation of the electrical data. The methods at present in use for interpret- 
ing Gish-Rooney surveys give only approximate depths to the points at 
which sharp changes occur in the resistivity of the earth. These approxima- 
. STATION F 

. STATION A STATION B . STATION © STATION OD STATION EW, 

LOW RESISTIVITY 
= ——— — — = 


—— — 
fontoet LOW RESISTIVITY 
oceTH 
Fig. 6. Qualitative interpretation of Gish-Rooney survey shown in 
Fig. 5, giving correlation with core drill. 
tions become worse and worse as the number of resistivity contrasts become 
greater. A rigorous solution of the problem of finding the variation of re- 
sistivity with depth from surface potential measurements under the assump- 
tion that the resistivity is substantially a function of the depth only could be 
based upon solutions of the potential problem given by the theory of images. 
However, these solutions are cumbersome to handle when the number of 
layers which must be dealt with exceeds two. The integral solution given in 
this paper is much easier to handle in the case of multilayer problems and 
should aid in placing the interpretation of direct current surveys on a more 
exact basis. 
III. ELEcTROMAGNETIC METHODS 
Electromagnetic methods depend primarily on the induction of currents 
in the earth by means of a primary current on the surface. Inhomogeneities 
in the earth will affect the resultant field on the surface. In oil prospecting 
these inhomogeneities take the form of practically horizontal beds with 
different conductivities. Many different schemes have been used with vary- 
ing success. The factors which should be taken into consideration are: 
(1) The primary field should be as simple as possible so that the associated 
theoretical problems may be solved. 
(2) The frequency should be chosen so that the inhomogeneities which are 
being followed will have a maximum measurable effect on the field. 
(3) The field components should be measured at points where the beds 
produce the greatest measurable anomaly in the field components. Thus 
both the magnitude of the change and the percentage change in the field 
should be considered. 
The quantities which may be measured are the magnitude and time phase 
of the components of the electric and magnetic vectors on the surface of the 
earth. 
154 
