628 EXPLORATION GEOPHYSICS 



planning of the field work. Shifting the position of the energizing 

 coil will usually differentiate between topographic and subsurface con- 

 ductor effects. Thus, in any area where "indications" are mapped, they 

 should be checked by moving the energizer to a substantially different 

 location and repeating the measurements. Although the high frequency 

 methods are convenient and rapid for reconnaissance work, their "indica- 

 tions" must be checked by the low frequency or direct current methods 

 having greater penetrating power, in order to differentiate between a sur- 

 face effect and a subsurface conductive zone effect. 



Strike Angle of Detector Coil 



The azimuth angle or direction of the direction-finding coil when in a 

 vertical plane is called the "strike" and represents the resultant direction 

 of the fields cutting the direction-finding coil with reference to a hori- 

 zontal plane or plan projection. The relative positions of the energizing 



DIRECTION -FINDING 

 COIL 



PLAN VIEW 



Fig. 394. — Relative positions of energizing and 

 direction-finding coils for maximum and minimum 

 signals. (A.I.M.E. Geophysical Prospecting, Tech. 

 Pub. 134.) 



coil and the direction-finding coil for maximum and minimum signals are 

 shown in Figure 394. It will be noticed that the strike of the direction- 

 finding coil is toward the energizing coil only when it lies in the plane of 

 the energizing coil. When the direction-finding coil is not in the plane 

 of the energizing coil, the directions for strike are as indicated. Thus, 

 if the apparatus is mounted so as to "point" toward the direction-finding 

 apparatus, the receiving operator knows that any strike other than toward 

 the energizing equipment is caused by some distorting influence. This 

 disturbing factor may be a distortion of the primary field wave front 



