Subsurface Methods as Applied in Geophysics 



1115 



ment. Measurements may be made with either direct current and non- 

 polarizable electrodes or with commutated direct current and metal elec- 

 trodes. One common electrode system is the Wenner configuration in 

 which the four electrodes are equally spaced at intervals a along a straight 

 line. For this particular arrangement the ground resistivity (apparent) 



V 

 may be calculated from the following relationship : p = — 27ra. The resis- 

 tivity obtained by such surface measurements applies to a volume of 

 ground that depends on the electrode spacing. The greater the spacing 

 between electrodes the greater the depth of penetration. As a rule of 

 thumb, the depth of penetration is approximately equal to the spacing a 

 in the Wenner-Gish-Rooney arrangement. As the spacing increases, appar- 

 ent resistivities are obtained from deeper and deeper strata. A plot of 

 apparent resistivity versus electrode separation will thus show different 

 values as the current reaches beds of different resistivities. Interpretation 



Figure 591. Resistivity map on base of red beds corrected for lateral changes in 

 conductivity, Monument oil field, New Mexico. (After England. (Reproduced 

 permission Geophysics.) 



