624 ELECTRICAL METHODS [Chap. 10 



confined to the location of sulfide ores. Soon this was supplemented by 

 structural investigations. Lately the field has been extended to the loca- 

 tion of such poor conductors as gold quartz vems and the determination 

 of gold content in placer deposits. In the field of civil engineering, appli- 

 cations of electrical methods have been ever increasing in number, applica- 

 tions including determinations of depth to bedrock on dam and tunnel 

 sites; harbor investigations; location of materials for highway, railroad, 

 and dam construction ; location of water-bearing formations and of buried 

 metallic objects, pipes, corrosion, ammunition, and the like. 



B. Classification of Electrical Methods 



The classification adopted here distinguishes three groups of methods. 

 In the first, ground potentials and, in the second, the electromagnetic 

 fields of the ground currents are determined. A third group includes radio 

 methods and treasure finders. 



1. Potential methods may be divided into (a) self-potential, (6) D.C. and 

 A.C. equipotential-line and potential-profile, (c) resistivity, (d) potential- 

 drop-ratio, and (e) electrical-transient methods. 



(a) In the self-potential method the electrical field is furnished by the 

 electrochemical polarization of ore bodies and other geologic formations. 

 The electrical field is investigated by surveying lines of equal-potential or 

 potential profiles. It has been found that not only sulfide ore bodies but 

 also metals in placer deposits, faults, corroded pipe lines, and the migration 

 of subsurface waters cause such electrochemical phenomena. Spontaneous 

 potentials likewise occur when solutions of different character (for ex- 

 ample, drilling fluid and formation water) come in contact with one 

 another. Electrofiltration potentials are produced by the movement of 

 water in porous formations and are used, together with the concentra- 

 tion potentials just mentioned, to indicate the porosity of beds in electrical 

 logging. 



(b) In this group direct or alternating current is impressed on the ground. 

 The primary electrodes may be pointed or linear. The potential distribu- 

 tion between them is studied by measuring equipotential lines or by sur- 

 veying potential profiles. D.C. methods require the use of depolarized 

 electrodes and potentiometers. In A.C. methods equipotential Hues, 

 strictly speaking, do not exist but can be surveyed when out-of-phase 

 components are not too large. A more exact method is the determination 

 of potential differences according to magnitude and phase by a bridge 

 compensator in which a reference voltage is carried to the instrument 

 from the generator, is varied in phase and amplitude, and is balanced 

 against the unknown voltage difference. Sulfide ores may be located and 

 structural and stratigraphic conditions may be studied by these methods. 



