1112 Subsurface Geologic Methods 



The electrial methods may be classified as: 

 I. Natural-current methods 



1. Currents due to electrochemical effects 



2. Telluric currents 



II. Controlled-current methods 



1. Conductive or galvanic methods 

 (a) Direct-current m.easurements 



(6) Alternating-current measurements 

 (c) Transient methods 



2. Inductive methods. 



Natural Earth Currents 



Certain of the sulphide ore bodies and concentrations of a few other 

 minerals set up spontaneous electrical currents that may be detected at 

 the surface. Specific conditions must exist before this natural phenomenon 

 may occur. The ore body must be continuous and lie both within the 

 oxidizing zone and the reducing zone, oxidation in general occurring above 

 the water table and reduction beneath. A drop in potential is noted along 

 the surface of the earth as the ore body is approached. The negative- 

 potential center over the body may be of the order of 1,000 millivolts or 

 more. To determine the shape and intensity of the field, measurements are 

 made of the potential variation over a network of surface points. The 

 only instrument required is a potentiometer with nonpolarizing electrodes. 



Figure 589 shows the results of a self-potential survey over the 

 Malachite- ore body in Jefferson County, Colorado. The ore consists of 

 the following primary sulphides: chalcopyrite, pyrrhotite, pyrite, and 

 chalcocite, which assayed 0.05 ounces of gold per ton and 3.5 percent 

 copper. Diamond drilling has outlined the existence of about 34,000 tons 

 of ore lying at an average depth of about fifty feet. An anomaly of about 

 160 millivolts defines the ore body.^® 



Telluric currents are irregular natural currents which flow through 

 the earth in vast sheets. According to Boissonnas and Leonardon,^'^ telluric 

 currents consist of four vast current whorls covering the entire globe. 

 Earth currents are intimately related to magnetic variations, and both are 

 apparently due to solar activity. Whereas most electrical methods are 

 limited as to depth of penetration, the telluric-currents technique gives 

 information to great depths. Measurements are made by comparing 

 the potential gradient along perpendicular lines at a fixed station and at 

 mobile stations. As potential gradient is related directly to current density, 

 the direction and intensity of the subsurface current flow at any instant 

 may be observed. It is noted that the telluric field at a given station 

 varies in intensity and direction with time; however, the variations of 



^' Heiland, C. A., Tripp, R. M., and Wantland, Dart, Geophysical Surveys at the Malachite Mine, 

 Jefferson County, Colorado: Am. Inst. Min. Met. Eng. Tech. Pub. 19-17, pp. 2-4, Feb. 1946. 



*' Boissonnas, E., and Leonardon, E. G., Geophysical Exploration by Telluric Currents, with Special 

 Reference to a Survey of the HaynesviUe Salt Dome, Wood County, Texas: Geophysics, vol. 13, no. 

 3, p. 389, July 1948. 



