ELECTRICAL METHODS 



549 



^f^^ 



actuated measuring light; cabinet 2 contains the power control panel, 

 the field rheostat for the generator, ratio instrument, etc. ; cabinet 3 

 contains the direct current amplifier for the ground potentials; 

 cabinet 4 contains telephone 

 and signal apparatus, for com- 

 munication between mobile 

 and stationary electrode oper- 

 ators. The generators have a 

 2000 volt 2 ampere rating and 

 may be connected in series or 

 parallel. The control 11 gov- 

 erns the speed of the truck 

 motor; the latter drives the 

 generators through a conven- 

 tional power take-off. 



The recording camera, which 

 is similar in design to the 

 camera described below for 

 seismic work, employs two re- 

 cording traces and the dis- 

 tance measuring lines, as 

 shown in Figure ?)?)6. 



Transient Method. — When 

 a varying current is caused to 

 flow in the subsurface between 

 grounded current electrodes, 

 magnetic and electric fields 

 varying with time are pro- 

 duced at the surface. These 

 fields induce voltages in the 

 potential or measuring circuit, 

 which are added to the earth- 

 conductivity voltages. 



Consider, for example, that 

 the electrode configuration 

 comprises four electrodes with 

 the two potential electrodes located on the extension of an imaginary 

 line through the current electrodes.! Let A{t) denote the transient 

 voltage measured at the potential electrodes due to one ampere of 

 direct current suddenly applied between the current electrodes and 

 let A' {t) denote the derivative of A{t) with respect to time. {A{t) 

 and A' {t) are simply the voltage oscillograms that are obtained during 



Fig. 337.- — Complete equipment for continuous pro- 

 filing: (1) control cabinet for recording equipment, (2) 

 power control apparatus, (3) direct current amplifier 

 for earth potentials, (4) communication, (5) recording 

 camera, (6) and (7) high voltage direct current gen- 

 erators, (8) excitor, (9) commutator for pulse fre- 

 quency, (10) microphone for communication with 

 mobile electrode, (11) throttle control for truck engine 

 driving the generators. (Courtesy International Geo- 

 physics, Inc.) 



t Gifford White, "Application of Rapid Current Surges to Electric Transient Prospecting, 

 A.I.M.E. Geophysical Prospecting, Tech. Pub. 1216, 1940. 



