ELECTRICAL METHODS 453 



Artificial conductors such as car rails, pipe lines (especially those 

 extending from the surface to underground workings), etc., are all possible, 

 and usually probable sources of earth currents. In areas where mining 

 operations are in progress and direct current is used for power, difficulty 

 will usually be met due to leakage and "ground-return" power circuits. 

 Oftentimes, these leakage currents are encountered at distances of a mile 

 or more from the mine and cause erratic self-potential data. In one case, 

 failure to correct for leakage currents led to an erroneous conclusion. 

 (Fortunately, the error was disclosed on checking the area by resistivity 

 measurements prior to development of the "indication.") 



Various methods have been proposed for interpreting self -potential 

 field data, all making certain simplifying assumptions. Stern based a 

 method on formulas derived from assuming the ore body to be a polarized 

 bar.f Heiland assumed a vertically polarized sphere in his method. $ 

 DeWitte§ provided a method based on the theory for surface distribu- 

 tion of potentials worked out by Petrovsky.ff 



DeWitte's method provided interpretation for location, depth and dip 

 of the ore body making use of an equipotential line pattern. The ore body 

 is assumed to be a polarized sphere, and for the theory, an image of this 

 body is assumed to be located symmetrically above the horizontal boundary 

 (surface of the earth). The unique points of this theory involve deter- 

 mining a positive maximum of potential, as well as a negative minimum, 

 and the "mid-value" point of potential. The value of the angle of dip is 

 then determined from theoretical curves, while the depth is determined 

 from the curve involving various angles of dip. A final set of curves 

 readily yields the vertical projection of the center of the ore body. 



FIELD RESULTS 



Surveys of Sulphide Ore Bodies.^ — A well known self-potential sur- 

 vey is that made by Schlumberger in 1913 on the Sain-Bel ore body.$$ 

 (Figure 271.) The potential difference between the negative center and a 

 distant point of the surrounding terrain is of the order of 220 millivolts. 

 The hatched area represents a cross section of the ore body at a depth of 

 about 300 feet. It will be noted that the equipotential curves outline the 

 area of mineralization quite accurately. The dotted profiles show the varia- 

 tion of the potential in the transverse direction across the ore body. At A 

 there was located a negative center which was later proved to lie over a 

 lens of fine pyrite. 



t W. Stern, A.I.M.E. Transactions. Vol. 164, 1945, p. 189. 



% C. A. Heiland, Geophysical Exploration, p. 673, Prentice Hall, New York, N. Y. 

 § L. DeWitte, "A New Method of Interpretation of Self-potential Field Data," Geophysics, Vol. 

 XIII, No. 4, October, 1948, pp. 600-608. 



ft A. Petrovsky, Philosophical Magazine 5 (1928), pp. 334, 914, 927. 



tX C. Schlumberger, Etude de la Prospection Electrique du Sous-Sol. (Gauthier-Villars, Paris 

 1920). 



