Chap. 10) 



ELECTRICAL METHODS 



707 



V. RESISTIVITY METHODS 

 A. General 



The equipotential-line methods discussed in the preceding section are 

 best suited to the location of laterally limited geologic bodies, that is, 

 bodies with vertical or nearly vertical boundaries. If they are used on 

 horizontally stratified ground, only the spacing, but not the direction of 

 the lines, is affected. To determine differences in spacing, potential gra- 

 dients must be measured, that is, potential differences must be determined 

 at no less than two points. If such measurements are supplemented bj'' 

 observations of current in the circuit, they are referred to as resistivity 

 methods because (as stated on page 644) current measurements at two points 



Fig. 10-47. Lines of current flow in layered section. (Conductivity of lower medium 

 is fifty times greater than tiiat of upper.) (After W. Weaver.) 



and potential measurements at two others will give the resistivity of bodies 

 of almost any shape. 



The effect of vertical changes in conductivity on surface potentials is 

 illustrated in Figs. 10-47 and 10-48. The more conductive lower medium 

 (Fig. 10-47) results in an attraction of the current Unes toward it. The 

 current density in the upper medium is less than in the lower. Since the 

 equipotential lines are at right angles to the current lines, their spacing, 

 and hence the potential gradient, is likewise affected by the presence of 

 layers of different conductivity. As shown in Fig. 10-48, the effect on the 

 gradient varies vidth the conductivity, but it does not vary uniformly as 

 far as conductivity ratio is concerned. The effect of a layer a hundred 

 times as conductive as the surface layer is only about twice as much as 

 +hat of a layer five times as conductive (saturation effect). 



Measurements of potential differences in the vicinity of one power elec- 



