26 METHODS OF GEOPHYSICAL EXPLORATION [Chap. 2 



drop-ratio methods; inductive methods as apphed to the mapping of 

 horizontal beds are comparable to reflection methods but lack their re- 

 solving power. 



In electrical prospecting, three kinds of current conduction are sig- 

 nificant: (1) electronic conduction in solids (metallic minerals and ores); 

 (2) electrolytic conduction (by ions); (3) dielectric conduction (by dis- 

 placement current). Conductivity of rock minerals plays a part only in 

 metallic ores; in most igneous and sedimentary formations "mineral" con- 

 ductivities are insignificant. Their conductivity is a function of the pore 

 volume and of the conductivity and amount of the water filling the pores. 

 For all practical purposes it is sufficient to assume that sodium chloride is 

 the only substance in solution. 



Self-potential method. The self-potential method is the only electrical 

 method in which a natural field is observed; its causes are spontaneous 

 electrochemical phenomena. These phenomena occur on ore bodies and 

 on metallic minerals and placers; they are produced by corrosion of pipe 

 lines and on formation boundaries in wells by differences in the conduc- 

 tivity of drilling fluid and formation waters. Ore bodies whose ends are 

 composed of materials of diiferent solution pressure and are in contact 

 with solutions of different ion concentration, act as wet cells and produce 

 an electrical field which can be detected by surveying equipotential lines 

 or potential profiles. 



To prevent interference from electrode potentials set up by contact of 

 metallic stakes with moist ground, nonpolarizable electrodes are used. 

 These consist of porous pots filled with copper sulphate into which a cop- 

 per rod is immersed. For the mapping of equipotential lines, a high-resist- 

 ance milliammeter is connected to two nonpolarizable electrodes. One is 

 kept stationary and the other is moved until the current vanishes. At 

 that point the electrodes are on an equipotential line. 



Potential profiles are run by measuring potential differences between 

 successive electrode locations (sec Fig. 2-12). Interpretation of self- 

 potential surveys is qualitative; the negative potential center may be 

 taken with sufficient accuracy to be the highest location of an ore body. 

 Approximate depth determinations can be made by observing the dis- 

 tance from the point of maximum potential to the half-value point in the 

 potential curve. Interpretative advantages are often gained by plotting 

 the results in the form of current-density curves which are obtained from 

 the potential curve by graphical differentiation. 



Equipotential-line and potential-profile methods. When a source of 

 electrical energy is grounded at two points, an electrical field is produced. 

 Distortions of this field result from the presence of bodies of different 

 conductivity; good conductors will attract the lines of flux, and vice versa. 



