ELECTRICAL METHODS 509 



Generally, the empirical "short cuts" just discussed are in error, 

 because they neglect the ever present distortion of the potential bowls 

 surrounding the source and sink of the current. For instance, in areas 

 where the resistivity increases with depth, the effective path of current 

 flow is always materially less than the I/3 value cited for the Wenner 

 configuration, t Usually the depth of measurement varies from about ^ 

 to % the distance between the power electrodes. Moreover, the depth of 

 measurement usually is not a constant fraction of the distance between the 

 power electrodes ; instead, it depends on such factors as the relative 

 conductivities of the component layers constituting the subsurface and 

 the lateral variations in conductivity. In general, therefore, the depth of 

 penetration is dependent upon many factors, only one of which is the 

 separation and configuration of the electrodes. 



Interpretation by Curve Correlation.- — This method of interpreta- 

 tion is based upon the fact that in an area of extended flat structures, any 

 given multi-layer structure composed of layers of different conductivity 

 usually produces a characteristic type of resistivity curve.t The general 

 shape of this curve is dependent upon the relative thickness, conductivity, 

 and sequence of the layers or components included in the measurements. 

 It has been found that each area usually has its own characteristic type of 

 curve. This curve will differ from that of another area if the subsurface 

 structure differs so as to produce electrical variations when the depth of 

 current penetration changes. 



Extensive field work has shown that in favorable cases a characteristic 

 pattern in one portion of the curve may often be followed through a series 

 of stations even though variations in thickness occur in portions of the 

 geologic section. In one application of the method for subsurface structural 

 mapping, a certain group of markers within a given depth interval is 

 correlated.* 



The measurements are made to include a given depth interval by 

 starting with the power electrodes at some fixed distance apart and 

 increasing their separation until the desired depth interval has been 

 measured. The similarity of the pattern at various stations in an area 

 will depend on the lateral uniformity of the subsurface layers. In areas 

 where rapid lateral changes are predominant, such as in the lenticular 

 and overlapped structures which prevail in the San Joaquin valley of 

 California, the curves will vary rapidly in character ; under such condi- 

 tions, it will be found that curve to curve correlation will be difficult even 



t H. M. Evjen, "Depth Factors and Resolving Power of Electrical Measurements," Geophysics, 

 1938, pp. 78-95. 



t J. J. Jakosky, C. H. Wilson and J. W. Daly, "Geophysical Examination of Meteor Crater, 

 Arizona," A.I.M.E. Geophysical Prospecting, 1933, pp. 63-97. 



J. J. Jakosky, "Continuous Electrical Profiling," Geophysics, Vol. 3, No. 2, Mar. 1938. 



* Correlations may be carried out even when the upper sedimentary beds over- 

 lying the group vary in thickness, because the variation usually is not sufficient to 

 mask the characteristics of the curve due to the lower part of the section. 



