718 



ELECTRICAL METHODS 



[Chap. 10 



are due to local conditions, usually at the contact of the electrodes with the 



ground, and must be eliminated before any interpretation can be attempted. 

 4. Effects of vertical contacts. In Fig. 10-55, let AB be the surface of 



the ground and DE a fault plane dividing the region into two parts, of 



resistivities pi and P2 . If a f our- 

 3 electrode system is moved as a 

 whole or if the separation is in- 

 creased, a resistivity curve con- 

 sisting of five portions is obtained 

 (Fig. 10-56). These, in turn, 

 result from five possible positions 

 of the arrangement in reference 

 to the fault plane: (1) all elec- 

 trodes in the first medium; (2) 

 only one electrode across the 

 fault line; (3) two electrodes in 

 each medium; (4) three elec- 

 trodes in the second medium; 

 (5) all electrodes in the second 

 medium, A sixth case arises 



when measurements are made in either medium parallel to the strike 



of the contact. 



(o) Case 1 (four electrodes in one medium) corresponds to the condition 



a < 2d/Z, if a is the electrode separation and d the distance from the center 



Fig. 



10-54. Three-layer case (adapted from 

 Hummel). 



A 



d^§a 



Ct S P ^ Ct D Ci 



• « » — •= •J u^ 



I 



fi 



SL 



A 



Fig. 10-55. Four-terminal electrode arrangement on contact plane. 



of the electrode system to the contact. Assume a soujce of current / at 

 C\ and a sink — / at C2 . The potential at points Pi and P2 will depend 

 on the strength of 1 and — / and of their images at C'x and C'l resulting 

 from reflection on the interface BE. The potential at Pi due to the source 

 I at C\ is Ipi/2ira', that at Pi due to the sink — / at C2 is — Ipi/ira. The 

 potential resulting from the image at C( is klp\/2'jr{2d — 2a) and that 



