804 



ELECTRICAL METHODS 



[Chap. 10 



and the inductive methods as here discussed and using phase difference 

 and amplitude ratio for interpretation, it has been possible to locate not 

 only good but also poor conductors. 



The Imperial Geophysical Experimental Survey carried out a number 

 of surveys with the Bieler-Watson method. Fig. 10-121 shows direction 

 and amplitude of the horizontal quadrature field for a survey at Leadville, 

 N.S.W. The ore bodies, 100-200 feet long by 15-20 feet wide, consist 

 of pyrite, chalcopyrite, galena, and zincblende, and occur on the contact 

 of porphyry and highly altered sedimentary rocks. Oxidation is fairly 



Fig. 10-120. Observations of phase differences and vertical intensity ratios with 

 calculations of in-phase {Zp) and quadrature (Zq) components across a conductor 

 (adapted from Hedstrom). 



deep. At the Grosvenor workings, the depth is from 80-100 feet; in the 

 Extended Workings, about 70 feet. The most pronounced indication was 

 obtained over the Eastern Lode (60 feet wide) of the Extended Workings. 

 A similar survey was conducted at Renison Bell, Tasmania, on flatlying, 

 outcropping lodes of pyrrhotite on the contact of tuffs and slates. 



Figs. 10-122 and 10-123 illustrate the appHcation of the Sundberg in- 

 ductive method in stratigraphic investigations. Fig. 10-122 represents 

 results obtained with the earher technique when ampUtude and phase 

 profiles of the horizontal component were measured. They demonstrate 

 clearly the effects of differences in depth on the gradients of intensity and 



