Chap. 10] ELECTRICAL METHODS 675 



upper end. Then the potential for the upper (negative) source or sink 

 is Fi = — p//27rri and that for the lower source is V2 = pl/2irr2 , so that 

 the total potential 



y = -^f- - -V (10-206) 



27r \ri r2/ 



Substituting 



ri = Vx' -\-y^ + hl and ra = V(x - a)' + / + hi, 



the potential is 



F = - ^ {(x' + y' + h\r" - [(x - af + y' + h'^'"]. (10-20c) 



To what extent the second term is effective depends on the length of the 

 ore body. The negative center becomes displaced from a position above 

 the negative pole in proportion as the body becomes shorter and the dip 

 becomes less; until, for a flat-lying body, a positive and negative anomaly 

 of equal strength will be observed. As before, depth rules can be calcu- 

 lated for various angles of dip of the bar. Such calculations are simplified 

 by moving the point P into the xz plane {y = 0). 



The effect of a dipping polarized sheet may be derived from the one 

 previously treated by assuming polarized lines instead of point sources at 

 the upper and lower ends of the body. Calculations and curves are given 

 by Edge and Laby. 



D. Corrections 



Compared with other electrical methods, few corrections and inter- 

 ferences occur in self -potential surveying. (1) For very accurate surveys 

 a correction for polarization of electrodes may be determined and deducted. 

 (2) In hilly country, corrections may arise from uphill currents (due to the 

 fact that in the earth's electrical field, localities of higher elevation are at a 

 different potential). The topographic effect is not so pronounced in the 

 self-potential as in the equipotential-line method (in which the entire 

 ground is energized and, therefore, the distribution of potential is influenced 

 by topography). 



E. Results 



The self -potential method has been appUed to the location of: (1) sulfide 

 ore, (2) anthracite coal, (3) metals in placer deposits, (4) formation 



«« Op. ciL, p. 244. 



