806 



ELECTRICAL METHODS 



[Chap. 10 



ISO' 

 160' 



(a) 



Y 



to 



0.5 



-^^9y 



9r^'- 



. Cabk 



9 



160* 

 140* 



45 feet; base 133 feet) and 

 by Mason/"^ with their 

 plane parallel with the as- 

 sumed strike. The I.G.E.S. 

 employed a Bieler- Watson 

 coil to determine the quad- 

 rature field in terms of the 

 loop field. 



The lack of penetration 

 associated with the use of 

 audio frequency in vertical 

 loops may be overcome by 

 excitation with higher fre- 

 quency. The I.G.E.S. em- 

 ployed frequencies around 

 60 kc, the Radiore Com- 

 pany about 50 kc. On the 

 other hand, there is a def- 

 inite upper limit to fre- 

 quency, due to the tendency 

 of noncommercial conduc- 

 tors (water-bearing fissures, 

 and the like) at shallow 

 depths to become energized. 

 In application, a vertical transmitting loop is set up with its plane ap- 

 proximately parallel with, and (if possible) directly above a suspected con- 

 ductivity zone. A certain distance away a receiving coil is placed with 

 its axis of rotation horizontal, pointing toward the transmitting loop. The 

 field of the transmitting loop at the location of the receiving coil is hori- 

 zontal if the centers of both are at the same elevation. The magnetic field 

 of the transmitter induces 

 currents .along the edge of '^'^^ 

 a subsurface conductor. _--'-- 

 These currents, in turn, are 

 surrounded by an electro- 

 magnetic field. This field 

 combines with the loop field 

 into a resultant vector, 

 whose direction may be de- 

 termined by tilting the re- 



(b) 

 Fig. 10-122. Horizontal component and phase 

 of horizontal component for beds of different 

 depths in Vienna basin (after Friedl). 



W*'/AViV-9W.'/9^^y4'y 



,^^ Sidtsurhxe Conduchr 



1 Drillhole 

 ^ Salt 



Q . . 10 00m 

 harizontnl Scale 



Z50m 



Verfko/ 5ca/e 



i«*A.LM.E. Geophys. Pros. 

 13 (1929). 



Fig. 10-123. Equivalent conductor determined 

 by application of inductive methods at Hawkins- 

 ville salt dome, Texas (after Sundberg). 



