708 



ELECTRICAL METHODS 



[Chap. 10 



trode, or between two power electrodes, can therefore give information 

 in respect to the presence of subsurface formations of different conduc- 

 tivities. When these measurements are supplemented by measurements 

 of current, it is possible to determine the resistance of the circuit. By 

 applying a factor depending on the spacing of the electrodes, the ground 

 resistivity can be obtained. This is a true resistivity only if the medium 

 is homogeneous; if layers of different conductivities are present, it is an 

 apparent resistivity. It is customary to calculate the apparent resistivity 

 by the same formula that applies to homogeneous ground. 



The depth to which the resistivity is measured can be controlled by 

 var3dng the spacing between the electrodes. This gives rise to two appli- 



^^v/■^A■^^'»/A^^vy//^^v//A^^^ ' ^V/'A^^ T /. ^ .^ T .'/A^^ r ^^A^ ■ ^v//x^w//^ ' ^ ■ vV7>A.^v^/A^W//'A^^v/ 



40' 



er 



Fig. 10-48. Potential variations near electrode (outside and inside spread) for layers 

 of different conductivity, at 40 feet depth (after W. Weaver). 



cations of the method. In the first, the spacing is kept constant and the 

 arrangement as a whole is moved over the ground (resistivity mapping). 

 In the second, measurements are made at one location which is the center 

 of the measuring arrangement. From this center the spacing of the elec- 

 trodes is gradually increased. Thus, the depth penetration is increased 

 and the apparent resistivity is obtained as a function of depth (vertical 

 electrical drilling). 



While four equally spaced electrodes are the arrangement most gen- 

 erally used, other arrangements can be and have been applied. A deriva- 

 tion of the formulas for the more common arrangements is given in the 

 next article. 



