CHAPTER FIVE 

 ELECTRICAL METHODS 



Electrical prospecting is the technique of measuring certain properties 

 of electrical fields of force and then utilizing such data to predict the 

 subsurface deposits or structures. Usually, the electrical methods depend 

 for their operation upon the effects produced at the surface of the earth 

 by the flow of an electric current through subsurface formations. 



There are several methods by which the electric field may be created. 

 Natural electrochemically-generated ground currents create a field which 

 may be utilized, as in the so-called self-potential method. More commonly, 

 artificial means are employed wherein an electric current is conductively 

 or inductively caused to flow in the portion of the subsurface to be in- 

 vestigated. Electric field properties which may be measured include: 

 potential distribution at the earth's surface, ratio of surface potential to 

 energizing current, phase shift, electromagnetic field strength and direc- 

 tion, distortion of wave front and polarization effects. 



Often, the electrical properties of rocks will vary to a different degree 

 than the other physical properties, and then the electrical methods may be 

 of advantage under conditions where other methods cannot be employed 

 successfully. Also, they may be utilized to obtain data which will supple- 

 ment the data given by other methods. Conversely, electrical methods will 

 not give useful data when the electrical properties of the subsurface are 

 not sufficiently different to create detectable or measurable differences in 

 the electric field. 



ELECTRICAL PROPERTIES OF ROCKS 



The magnitude and the distribution of current flow in the subsurface 

 depend upon the effective resistivity, or its reciprocal, conductivity, of the 

 subsurface materials. The resistivity of a material is defined as the re- 

 sistance in ohms between opposite faces of a unit cube of the material. 

 The units of resistivity commonly employed are the ohm-centimeter 

 and the ohm-meter.t The corresponding conductivity units are the 

 mho/centimeter and the mho/meter. Different rocks exhibit marked differ- 

 ences in resistivities, and in a great majority of rocks, a variation in elec- 

 trical resistivity will be accompanied by a discernible variation in lithology. 



The flow of an electric current in non-metallic rocks is chiefly electro- 



t Compare S. F. Kelly, "A Uniform Expression for Resistivity," A.I.M.E. Geophysical Pros- 

 pecting, 1932, pp. 141-1«3. 



