762 



ELECTRICAL METHODS 



[Chap. 10 



dOOcps 



x^300cps± 



of transients. When current is sent into the ground, there result changes 

 in the concentration of electrolytic solutions which in turn give rise to a 

 counter (or polarization) e.m.f . It was noted before that this phenomenon 

 is responsible for corrosion potentials and that it is utilized in stimulating 

 the "activity" of ore bodies by passing current into the ground before a 

 self-potential survey. 



An arrangement for measuring these counter e.m.f. 's is shown in Fig. 

 10-88.^^ The upper portion represents a beat oscillator furnishing fre- 



I — |||||||||j_. lllllllh 1-5 cycles. The output 



is coupled to the ground 

 circui tthrough the trans- 

 former T and is recti- 

 fied on the secondary 

 side, so that impulses in 

 only one direction pass 

 through the ground. 

 The potential difference 

 between the electrodes 

 E\ and Ei , as well as 

 their time variation, is 

 recorded on the string 

 galvanometer or oscillo- 

 graph Gi . In parallel 

 with this circuit is an- 

 other one, consisting of 

 a second rectifier with 

 condenser in parallel, 

 going into the grid of an 

 amplifier with record- 

 ing galvanometer in 

 the plate circuit. With 

 proper selection of the 

 capacity C in parallel 

 with the second rectifier, the galvanometer Gz will record only the return 

 impulses because of the unilateral impedance of the rectifier tube. The 

 fluctuations of both the input e.m.f. and the polarization e.m.f. are recorded 

 on the same film with time marks every hundredth of a second. The 

 ratio of charging time to discharge time is always greater than 1 and is 

 a function of the electrolytic polarization properties of sub-surface forma- 



Transfbmtr 







£f ilj — -vwwwwv ||i'^ 



-A^AA/W W/)A 



1x5 



<i*t'm-» 1 — 



Fig. 



10-88. Circuit in electrochemical polarization 

 method (adapted from Mliller). 



' Ibid. 



