ELECTRICAL METHODS 465 



potential survey in Pinal County, Arizona. The ground was energized 

 with a 25-cycle gasoHne-driven alternator. The iron power electrodes 

 were positioned at locations £1 and Eo. The lines of equipotential were 

 traced with two probing electrodes connected to the voltmeter type of 

 equipment illustrated in Figure 276. 



The topography in this area is moderately rough. The formations 

 are well exposed and the surface contacts can be traced easily. Within 

 the area covered by the survey are rocks of diabase, hornblende porphyry, 

 and quartzite schist. The area is cut by numerous faults, and the ore 

 usually occurs in the north-south fault or fracture system. The ore is 

 associated with quartz, which fills the fractures. The ore is galena accom- 

 panied by sphalerite, tetrahedrite, chalcopyrite, and bornite. The chal- 

 copyrite and bornite are usually found in the diabase close to the fracture 

 zones. The galena ore is an excellent electrical conductor. 



The location of the fractured mineralized zones was known from 

 geological and exploratory work. The chief purpose of the equipotential 

 studies was to ascertain the zones of high electrical conductivity, because 

 it was predicted that the best commercial values would be found in such 

 zones. 



Three types of electrical studies were conducted: (1) equipotential, 

 (2) high frequency inductive, and (3) resistivity. The high frequency 

 studies were conducted first, and the "indications" followed the main 

 fracture system. The strongest effects were obtained between stations 

 1 to 8. The electrodes £1 and Eo for energizing the ground for the 

 equipotential studies were located so as to allow the current path to 

 include the main fracture system. The lines of equipotential A to R 

 showed their maximum deviation from the normal along contours G to 

 L, in the vicinity of stations 4 to 8. The anomalies in the contours may 

 be seen best by holding the book so the plane of Fig. 277 is level with the 

 eye and sighting along the contours. 



Because both the high frequency and the equipotential methods are 

 "indicative" only, it was decided to obtain more detailed information 

 regarding the probable depth of the mineralized area by use of resistivity- 

 depth studies. The centers of three resistivity-depth stations were located 

 at Rb, Ra and Re, with the lines oriented parallel to the fracture zone 

 (approximately in a north-south direction). Subsequent drilling estab- 

 lished the existence of a good grade commercial ore zone at a depth of 

 approximately 400 feet. 



RESISTIVITY METHODS 



The resistivity methods allow quantitative electrical data to be obtained 

 from the field measurements. Calculations may be made of the average 

 resistivity of the portion of the subsurface included in the measurements. 

 The methods have a greater resolving power than the regular potential 



