444 EXPLORATION GEOPHYSICS 



SPONTANEOUS POLARIZATION OR SELF-POTENTIAL 



METHOD 



This method utiHzes the natural flow of current in the earth. The 

 general subsurface distribution of the natural earth current is determined 

 from studies made of the lines of equipotential at the surface of the ground. 

 From this information, and a knowledge of the geology of the district, 

 predictions can be made regarding the presence of an oxidizing ore body 

 at depth. This method is one of the simplest, and perhaps the oldest, geo- 

 physical process utilizing electrical phenomena. In recent years, the 

 spontaneous polarization method has been used to locate corroding pipe 

 lines and other extended metal structures in contact with the earth.f 



OPERATING PRINCIPLE 



The method operates on the fundamental premise that an ore body 

 undergoing oxidation is a source of electric current. Water seeping 

 downward from the surface carries absorbed oxygen. Such water coming 

 in contact with a sulfide ore body creates, as a result of the oxidizing 

 process, a natural large-scale galvanic cell, with the top or upper portion 

 of the ore as the positive pole and with the weak acid formed in the 

 oxidation process as the electrolyte. The potential differences at the sur- 

 face of the earth resulting from the chemical activity of the ore body 

 vary with the electrolytic properties, the size and configuration of the 

 ore body, and with the depth of the body below the surface. In some cases 

 this potential difference may be as great as 500 to 1000 millivolts. The 

 electric current flow is usually downward within the ore body, and then 

 outward and upward through the surrounding earth. The return cur- 

 rents spread outward for considerable distances, due to the relatively high 

 resistance of the earth. At the surface of the earth, the current flow is 

 toward a point, usually above the ore body, which is called the "negative 

 center." 



The negative center may be found by measurements over the surface 

 of the ground: (1) by locating points at the same potential; (2) by meas- 

 uring the earth potentials at regularly spaced intervals and drawing the 

 equipotential contours; (3) by obtaining potential profiles in a direction 

 across the ore body. 



In the theoretically ideal case of a vertical cylindrical ore body sur- 

 rounded by a homogeneous medium having a uniform distribution of 

 moisture or subsurface water, the equipotential curves'^ measured on the 

 surface of the earth would be a set of concentric circles and the center of 

 the circles would be the nesfative center. 



t H. C. Hayes, "Electrical Prospecting," U. S. Patents No. 2,368,217 and 2,368,218, Jan. 30, 

 1945. 



* An equipotential curve or an equipotential is a curve such that every point on it 



is at the same potential. 



