ELECTRICAL METHODS 



461 



Fig. 275. — Circuit employing commutated direct 

 current for mapping potential profiles. 



headphones when working in conducting regions where marked out-of- 

 phase conditions are encountered. These limitations are minimized by 

 using the very low frequencies developed by commutated direct current. 



A schematic diagram of a circuit 

 employing commutated currents is 

 shown in Figure 275. The commu- 

 tator C is connected to a direct cur- 

 rent generator or to a bank of heavy 

 duty batteries B. One lead from the 

 commutator is connected to one of 

 the power electrodes A; the other 

 commutator lead is series-connected 

 to the second power electrode A' 

 through a current meter / and 

 through the field coil of a galvan- 

 ometer G. (The galvanometer should 

 have a fairly long period, 2 to 5 seconds, to give smooth operation.) The 

 two search electrodes are connected to the movable coil of the galvan- 

 ometer. The current is reversed periodically by the commutator, which is 

 connected by suitable gears or belt drive to the generator. 



The chief disadvantage of this method is the necessity of connecting 

 the galvanometer field coil with the power supply, because this requires 

 handling extra wires during the field operations. From the viewpoint of 

 apparatus design, another undesirable feature is introduced : namely, the 

 necessity of insulating the moving or potential coil of the galvanometer 

 from the effects of the field coil. This is a marked disadvanta^^^e, because 

 it is difficult to prevent electrostatic and electromagnetic coupling between 

 the two circuits. 



Usually frequencies of from 5 to 10 cycles/sec. are employed. These 

 frequencies are sufficiently high that earth current variations, which ordi- 

 narily have a frequency of less than 1 cycle/sec. will not affect the read- 

 ings. Also, the frequencies are sufficiently low to preclude appreciable 

 phase shift phenomena. A direct current, direct-coupled amplifier may be 

 employed for greater sensitivity ; however, this is seldom necessary. 

 Ordinary iron electrodes are used for all contacts. 



A method of electrical prospecting using alternating current of rect- 

 angular or other non-sinusoidal wave form is described by Westf (page 

 551.) The null method, in which the detected electromotive force is bal- 

 anced against an adjustable electromotive force of a standard oscillator, 

 is used to determine changes in wave form caused by anomalous subsurface 

 structure. 



Low Frequency Voltmeter Method. — For shallow investigations 

 the most satisfactory form of apparatus for alternating current work com- 



t S. S. West, "Electrical Prospecting with Non-sinusoidal Alternating Currents," Geophysics, 

 Vol. Ill, No. 4, October, 1938, pp. 306-314. 



