Chap. 10] 



ELECTRICAL METHODS 



749 



and veins. The P.D.R.'s for simple contacts with resistivities of pi on 

 one side and p2 on the other may be calculated from eqs. (10-40a) to (10- 

 4O7), which can be simplified, since only one source has to be considered. 

 Details of the calculations depend on whether the source is moved or kept 

 stationary with respect to the fault plane. The theory of this case has 



Disfancf firm Source flKfrodt 

 tt Center Stake 



(a) 



Distance from Source Electrode ie 1^ Stake 



(b) 



Fig. 10-78. Potential-drop-ratio curves for dipping bed: (a) constant spacing, 

 (b) expanding system. Depth is reckoned perpendicularly to bed. (After Jameson.) 



been discussed by Hedstrom.^^ Curves have been published by Hedstrom 

 and T. Zuschlag.^" Some of these are reproduced in Fig. 10-79. When 

 the power electrode is stationary, the greatest P.D.R. is obtained when the 

 center stake crosses the boundary. When the good conductor is the me- 



" H. Hedstrom, Min. Mag., April, 1932. 

 soA.I.M.E. Geophys. Pros., 48 (1932). 



