496 EXPLORATION GEOPHYSICS 



TABLE 17 



RATIO OF THE RESISTIVITIES AT VARIOUS ELECTRODE 

 SEPARATIONS t 



t Tagg, loc. cit. 



Substitution of the values given above for pi and Q2 into the last relation 

 yields 



P2 



~ n ooQ ~ ■38300 ohm-inches. 



u.zyo 



Resume. — The preceding sections have been concerned chiefly with 

 the mathematical relationships governing the current flow in the subsur- 

 face. Generally, however, a rigid mathematical treatment of resistivity 

 data obtained from measurements made at the surface of the earth is not 

 possible because many variable factors afifect the apparent resistivity. 



The measured values are influenced chiefly by two mutually depen- 

 dent phenomena : first, the actual path of the current flow, and second, 

 the surface potentials at the points where the potential electrodes make 

 contact with the surface of the earth. The current path is dependent 

 chiefly upon the distance between the current electrodes and the relative 

 conductivities of the strata constituting the subsurface. Although small 

 portions of the current spread out to great distances in all directions in the 

 earth, a considerable portion is confined to the subsurface volume included 

 between the two current electrodes. Furthermore, only a relatively small 

 amount of the current flows along the uppermost surface of the ground, 

 and the potentials measured in the resistivity methods are only those created 

 by this current flow at the surface. 



The relative magnitudes of the efifects produced by these factors are 

 discussed below. 



