THE LOOP METHOD IN EXPLORING BURIED STRUCTURES 



261 



axle trailer, which made it possible to speed up the whole cycle of obser- 

 vations. 



The increase in the length of the AB line and decrease in the effective 

 value of the e.m.f. received, except for the initial stage of the stabilizing 

 process, made it essential to increase the sensitivity of the measuring instru- 

 ment and to preamplify the e.m.f. received. 



N >»»> 



<(<*« M 



Fig. 12. The mutual disposition of the current and receiving collectors of the No. 2 



pulsator with lagging of the start of the e.m.f. record with respect to switching-on of the 



current (a > 0). 1 — insulating space of the collector MN (receiving); 2 — insulating 



space of the AB collector (feed); 3 — direction of rotation of the pulsator. 



With increase in the AB line from 200 to 4000 m, shifts were observed 

 in the phase of the electromagnetic field in the earth with respect to the E 

 and H fields of the feed current in the AB cable. This fact necessitated the 

 use of different methods for cancelling the mutual inductance between 

 both circuits. 



Compensation of the e.m.f. induced in the receiving circuit was obtained 

 either by moving electrodes perpendicularly to the AB line or by inclina- 

 tion of the frame circuit. The results of field observations (Fig. 13) were re- 

 corded on cine-film by means of the MPO-2 oscillograph. 



Visual observations were made with the same arrangement, but in this 

 case, instead of the oscillograph, the EP-1 potentiometer was connected. 



