618 



EXPLORATION GEOPHYSICS 



circular 3-foot frame, connected to an alternating current bridge curcuit. 

 A three-stage amplifier and headphones are used for null determinations. 

 The phase relationship between the fields aflfecting the two coils may be 



Movincf Coi/ 



Fig. 385. — Turatn method for mapping lines of equipoten- 

 tial, phase and amplitude. 1, amplitude control; 2, phase 

 control; 3, audio-frequency amplifier; 4, headphones. (Hed- 

 strom, A.I.M.E. Geophysical Prospecting, Tech. Pub. 827.) 



determined by changing the inductance or capacity in one leg of the bridge, 

 while the ratio of field strengths may be determined by use of the conven- 

 tional potentiometer method. 



Fig. 386. — Equiphase curves in electromagnetic 

 field over ore body. (Hedstrom, A.I.M.E. Geophys- 

 ical Prospecting, Tech. Pub. 827.) 



Interpretation is based upon phase differences and ratio of field strengths. 

 The results usually are plotted as contours comprising equi-phase curves 

 or equi-field-strength curves. (Figure 386.) 



