624 



EXPLORATION GEOPHYSICS 



turns of rubber-insulated stranded wire bound together into a single cable 

 to facilitate handling. The flag seen in the illustration is placed at the 

 electrical center of the coil and is used for proper alignment of the direc- 

 tion-finding coils. The energizing coil should be oriented substantially 

 parallel to the strike of the vein system. Any convenient means may be 

 employed for supporting the cable during operation. In the work illus- 

 trated, two 15-foot 3-inch X 2-inch hardwood supports were employed, 

 properly guyed at each end. The coil had dimensions of approximately 15 

 feet by 45 feet. An alternator of the type shown in Figure 389 may be used 

 for exciting the coil where work is done at shallow depths. For deeper 

 work, it is preferable to employ a gasoline engine-driven alternator of 

 at least 1000 watts output. 



Field Operations for Vertical Energizing Coil Method. — If the area 

 under investigation is homogeneous, the electromagnetic field produced at 

 the surface by the vertical coil will be polarized in a substantially hori- 

 zontal direction, provided the other fields acting at the surface are negli- 

 gible relative to that of the energizing coil.t. Hence, when a direction- 

 finding coil is placed in such a position that its axis of rotation lies in the 

 plane of the energizing coil, the minimum signal will be obtained when the 

 two coils are at right angles to each other, i.e., when the direction-finding 

 coil is horizontal and the energizing coil is vertical. On the other hand, a 

 maximum signal will be obtained when the direction-finding coil is in the 

 same plane as the energizing coil. 



Referring to Figure 392, it will 

 be noticed that the axis of rota- 

 tion for the direction-finding coil 

 is horizontal only when the direc- 

 tion-finding coil is at the same ele- 

 vation as the energizing coil. Ini- 

 tial setting up of the inductive 

 energizing and direction - finding 

 equipment therefore involves two 

 operations : ( 1 ) proper alignment 

 of the energizing coil so that its 

 plane is always vertical and passes 

 through the axis of rotation of 

 the receiving coil, and (2) align- 

 ment of the direction-finding coil 

 so that its axis of rotation passes 

 through the center of the energiz- 

 ing coil. To allow this second step 

 to be done accurately and quickly, 

 alignment sights and a ball-socket joint are provided on the head of the 

 direction-finding coil, as described previously. 



t J. J. Jakosky, "Method and Apparatus for Locating Unknown Conductive Bodies," U. S. 

 Patent 1,811,547, issued June 23, 1931. 



AXIS or ROTATION y 



OF DIRECTION-FINDING COIL 



Fig. 392. — Sketch illustrating possible relative 

 orientations of direction-finding coil and ener- 

 gizing coil for minimum signal. (A.I.M.E. Geo- 

 physical Prospecting, Technical Paper 134.) 



