ELECTRICAL METHODS 451 



INTERPRETATION OF SPONTANEOUS POLARIZATION 



STUDIES 



The field work involved in spontaneous polarization measurements is 

 relatively simple, yet its interpretation is oftentimes complicated due to 

 several factors. In some cases, it is of interest to compare observed potential 

 data with theoretical potential profiles. f Final interpretation should give 

 careful consideration to: (1) effects of topography, (2) geological and 

 structural conditions, (3) spurious earth currents, (4) ore occurrence 

 in the district, and (5) regional gradient. 



In areas where rugged topography prevails, interpretation is usually 

 complicated by an irregular distribution of surface potentials. In many 

 cases, the negative center will be shifted, with a resultant shift in the pre- 

 dicted location of the oxidizing ore zone. This effect is apparent when 

 the topographic contour map of the area is studied in conjunction with the 

 potential map. As a general rule, the negative center in fault studies will 

 be shifted toward the hanging-wall side of the ore body. 



Interpretation should never be attempted without proper geological 

 control. Areal maps should be drawn to the same scales as the geophysical 

 potential contours. Contacts of dififerent materials containing ground waters 

 of different chemical properties often give rise to earth potentials which 

 may not be related to ore occurrence. In addition, different geological 

 formations usually possess different electrical conductivities; hence, they 

 cause a distortion of the normal regional ground currents with a resultant 

 redistribution of the surface potentials. Fault zones filled with wet clay 

 gouges, or other conducting materials, cause severe distortion of the 

 surface potentials. 



The efifects of topography and dififerent materials may best be seen by 

 reference to Figure 270 which shows the surface potentials existing over 

 a pyrite vein formation in Arizona. The sulfides exist at a depth of 

 approximately 45 feet. A general cross section of the vein is shown in 

 the upper portion of the figure. The mineralization occurs at the contact 

 of the Hmestone and a monzonite intrusion. The limestone has an electrical 

 resistivity of 85,000 ohm-cm. while the monzonite has a value of 30,000 

 to 50,000 ohm-cm. As shown on the plan view, the mineralization is 

 localized between two cross faults. The faults are filled with a clay gouge 

 material which when wet has a very low resistivity, approximately 8,000 

 ohm-cm. Laboratory measurements of the pyrite vein material gave values 

 of 200 to 6,000 ohm-cm. The equipotential lines are shown dotted in 

 the plan view in the lower portion of the figure. It will be noted that the 

 current flow was concentrated chiefly between the faults in the lower resist- 

 ance monzonite. The negative center was found to be approximately 40 

 feet to the east of the actual vein itself. 



t Theoretical potential profiles produced by (1) a polarized rod and (2) a polarized sheet are 

 discussed by Broughton Edge and Laby, Geophysical Prospecting (Cambr. Univ. Press, 1931), 

 pp. 243-246. 



