ELECTRICAL METHODS 



445 



In practical cases, the negative center is directly over the ore body only 

 if the topography of the region under investigation is relatively flat. 

 (Compare Figures 266 and 270.) 



An interesting theory proposed by Kelly* in explanation of the cause 

 of self-polarization involves the pH content (measure of acidity) of surface 

 solutions in comparison to solutions at 

 depth. In laboratory experiments and 

 field tests in South America, Kelly 

 found experimental evidence that dif- 

 ferences in potential are largely due to 

 dififerences in pH between solutions at 

 near surface and depth. The field tests 

 were carried out on a sulphide body 

 of short vertical extent and results 

 showed a pH of about 2 (highly 

 acidic) of the waters bathing the apex 

 and a pH of about 8 (slightly basic) 

 of the waters issuing from the bottom. 



This evidence has not disproved the 

 statement that oxidation can be the 

 cause of spontaneous polarization, since 

 oxidation increases the acidity of the 

 near-surface waters. However, it does 

 indicate that the difference of pH is a 

 possible cause for spontaneous polar- 

 ization potentials. 



Additional evidence which may 

 tend to support the theory is found in measurements over deposits of 

 graphite and pegmatite, both totally lacking in sulphides, yielding high 

 spontaneous polarization potentials. In such cases, the postulated low pH 

 near the surface could arise from humic acids or carbonic acid from rain 

 water taking up carbon dioxide from the atmosphere and vegetable matter. 



Negsfive Center 



Fig. 266. — Equipotentials and negative center 

 over a vertical ore body. 



FIELD EQUIPMENT AND PROCEDURE 



Location of Points of Equal Surface Potential. — The apparatus for 

 this work comprises : (a) indicating meter or galvanometer, (b) electrodes 

 for making contact with the earth,' and (c) two insulated cables, one about 

 twenty feet and the other about two hundred fifty feet in length. 



Galvanometer. — Various types of sensitive direct current galvanom- 

 eters may be employed. The galvanometer should be of rugged construc- 

 tion and have a sensitivity of 0.25 to 2 microamperes per readable unit of 

 scale. The resistance of the indicating meter should be 1000 ohms, or 



* S. F. Kelly, personal communication, May 3, 1949. 



