752 



ELECTRICAL METHODS 



C. Equipment; Procedure 



[Chap. 10 



Various types of equipment have been developed for P.D.R. surveying. 

 D C. bridges have been used for recording corrosion effects on pipes 

 (Schlumberger). Alternating current bridges are operated with audio 

 frequencies of from 250 to 900 cycles, which has the advantage that 

 phones can be used as indicating instruments. 



In the Imperial Geophysical Experimental Survey series-capacity 

 ratiometer, each arm consists of a condenser and a resistance in series. 



4ff 



"\ — \ — \ // — 7^ — 



\ \ * / / ' 



E A' 



rrh?: lillllllll 





m il l I r iiiiiiii • 



liiiiiiiiiiiiiiir lllllllllllllllli; 



fm^, 





^^.«^ 



■X^ 



iiiiiiiiHiiiiiiir 



Fig. 10-80. Elimination of surface geologic anomalies by ratio curves takori in 

 opposite directions (after Kihlstedt). 



The ratio of the potential drops Y\ and F2 between the equidistant pairs 

 of contacts AB and J5C, and their phase differences are determined by 

 adjusting the capacities and resistances in each arm to balance. Thus 

 72 = (Z2/Z1) Yi , where Z2 = VrI + Xl and Zi = VrI + Xl , Zi and 

 Z2 are the total impedances in the arms of the bridge, Ri and R2 resistances, 

 and Xl and X2 are capacitive reactances, with Xi = l/2irfCi and X2 = 

 l/27rfC2 . Hence, 



„ R2 cos tan~^ ^ 

 Vj _ ^1 



Ri cos tan 



-1 Xz 

 Ri 



) (10-43a) 



and the phase difference 



= tan ^ — - tan — . 



