1114 



Subsurface Geologic Methods 



When electrical energy is supplied to homogeneous and isotropic 

 ground at two points, the current flow lines and the equipotential lines 

 (at right angles to current lines) are symmetrically disposed about the 

 electrodes. The current flow lines and consequently the equipotential lines 

 are disturbed by the presence of either unusually low- or high-resistivity 

 bodies. In the equipotential-line method the paths of the lines of equal 



Figure 590. Results of telluric-current exploration at Haynesville, Texas, salt dome. 

 (After Boissonnas and Leonardon. Reproduced permission Geophysics.) 



potential are traced by probing the ground and locating points of the 

 same potential. The indicating instrument may be either a galvanometer 

 when direct current is used or an amplifier and head set when audio- 

 frequency alternating currents are used. By noting areas in which the 

 equipotential lines are abnormally disposed, predictions may be made as 

 to the presence of conducting or nonconducting bodies. The Russian and 

 Swedish geophysicists have had considerable success with this technique. 

 If current measurments are made in the power-electrode circuit and 

 potential measurements made at two other points, it becomes possible to 

 calculate the apparent resistivity of the ground for any electrode arrange- 



