628 ELECTRICAL METHODS [Chap. 10 



with the presence of conductive bodies. Two commercial treasure finders, 

 the "metallascope" (Fisher) and the "terrometer" (Barret), operate on 

 this principle. The terrometer is reported to be sensitive enough to detect 

 mineral disseminations in shallow placers to a depth of fifteen or 

 twenty feet. 



11. ELECTRICAL PROPERTIES OF ROCKS 



It is the function of electrical prospecting methods to measure the 

 distribution of natural and artificial potentials, of electromagnetic fields, 

 and of the propagation of radio waves. This variety of electrical phe- 

 nomena causes not merely one but a number of physical properties of 

 geologic bodies to be significant. They are: (1) electrochemical properties, 

 giving rise to (a) spontaneous and (5) polarization potentials; (2) electrical 

 conductivity; (3) dielectric constant; and (4) magnetic permeability. Of these, 

 the electrical conductivity is undoubtedly the most important. Strictly 

 speaking, none of the above properties are constants but depend on other 

 factors, mainly frequency. 



A. Electrochemical Properties 



Electrochemical effects are responsible for the electrical field surrounding 

 chemically polarized ore bodies; they give rise to interference potentials 

 when metallic electrodes are placed in contact with moist ground; they 

 produce electrical potentials when solutions of different concentrations 

 come in contact with one another in wells or when a solution is forced 

 through a porous medium; and, lastly, they give rise to counter e.m.f.'s 

 when current is applied to the ground. The first three of these effects do 

 not depend on extraneous electrical fields and are therefore called spon- 

 taneous potentials, or, more specifically, (a) electrode potentials, (6) diffu- 

 sion potentials, and (c) electrofiltration potentials. Potentials caused by 

 the application of an electrical field are called polarization potentials. 



\. Spontaneous potentials, (a) Electrode potentials. When a metallic 

 electrode is placed in a solution it acquires a potential difference against 

 the solution which, however, cannot be measured except by placing a 

 second electrode in the liquid. If the metals of the two electrodes are 

 alike and if the concentrations of the solution at the two contact points 

 are the same, no potential difference between the two electrodes is observed. 

 A potential difference occurs, however, if either the metals or the concen- 

 trations are different. Practical application of this principle is made in 

 electrical prospecting (1) in the so-called activity of ore bodies, which is 

 due (a) to their contact with solutions of different character and concen- 



