. Early geophysical papers of the Society of Exploration Geophysicists . 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 cumb
. Early geophysical papers of the Society of Exploration Geophysicists . 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
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