Chap. 10] 



ELECTRICAL METHODS 



805 



phase curves. Fig. 10-123 indicates the position of an equivalent deep 

 conductor on the Hawkinsville salt dome. By the mapping of such beds, 

 the outline of a salt plug, in addition to the structure of the flank forma- 

 tions, maj'^ be determined. Sundberg^"^ has further described the results 

 obtained at the Moore's Field dome. Ft. Bend Co., Texas, illustrating the 

 usefulness of the inductive method for mapping details of formations 

 around a dome, and for locating faults. A survey of the Bruner field, near 

 Luling, Caldwell Co., Texas, (Balcones fault zone), has been discussed by 

 Zuschlag.^"* 



\\\\m\t"^"'''"" 



Brosvenor 



Workings 



Workings 



y//////l\^ 



m m 300 400 Fttt 



Hot 

 t 

 £le(. 



200 M 600 000 Turns 



Scales 



Fig. 10-121. Bieler-Watson vectors of quadrature horizontal component at Lead- 

 ville, New South Wales (after Edge and Laby). 



2. Vertical-loop methods. Vertical transmission loops are used in some 

 inductive methods with audio- and high-frequency excitation. As was 

 previously stated, they afford a closer coupling with vertical or steeply 

 dipping ore bodies, as well as reduction of interference from highly con- 

 ductive surface beds. This advantage is offset by the difficulty of erecting 

 low-frequency vertical loops in the field and by their lack of magnetic field 

 strength. The range of vertical audio-frequency loops is limited and of 

 the order of 500 feet. Triangular loops were used by the I.G.E.S. (height 



»" Inst. Petrol. Technol. J., 17(92), 376-380 (June, 1931). 

 »»* A.I.M.E. Geophys. Pros., 156-157 (1932). 



