Chap. 10] ELECTRICAL METHODS 623 



frequency and radio work is greatly handicapped by terrain because of the 

 refraction of the wave front on the ground surface. There is no satis- 

 factory way of correcting for terrain effects in potential, high frequency, 

 and radio methods except by small-scale model experiments. Electro- 

 magnetic and inductive methods are comparatively free from terrain 

 effects. Field components can be measured in reference to the terrain 

 surface, and its disposition relative to subsurface conductors can be taken 

 care of geometrically in the interpretation of the results. Interferences of 

 a geologic nature that may seriously affect the interpretability of electrical 

 results include mineralized soTutions in formations and on fissures, and 

 rocks impregnated with noncommercial minerals, such as graphite and 

 pyrite. 



In some electrical methods (such as the spontaneous potential, the 

 equipotential-line, and those electromagnetic methods in which only the 

 direction of the field is determined) interpretation is merely of a qualitative 

 nature; that is, it is concerned only with locating areas of anomalous indi- 

 cations. Depth determinations with these methods are generally not 

 possible except in simple cases where the depth may be estimated or calcu- 

 lated from the shape of the anomaly curve. However, absolute depth 

 determinations are possible where vertical changes in conductivity are ob- 

 tained by varying the spacing between transmitting and receiving units 

 (resistivity and potential-drop-ratio methods). In some inductive meth- 

 ods, depth calculations are made indirectly by comparing the field data 

 with type curves calculated for various possible depths of subsurface con- 

 ductors. More often, however, recourse is had in the interpretation of 

 electrical results to small-scale model experiments. 



Progress and development in most geophysical methods have been 

 largely the result of preceding developments in geophysical science. In 

 gravitational, magnetic, and seismic methods field procedure and methods 

 of observation are closely allied to those used in pure geophysics. Elec- 

 trical methods lacking this background have followed their own course of 

 development. 



Electrical prospecting methods have three fields of application: oil ex- 

 ploration, mining, and engineering geology. In oil exploration, surface 

 potential, resistivity, potential-drop-ratio, "Eltran," inductive, and elec- 

 trochemical methods have been used to delineate structure. The most 

 widespread use of electrical methods is made in oil exploration in the 

 process of "electrical logging." ^This is a modified resistivity method and 

 involves running a continuous resistivity record in uncased wells with an 

 electrode assembly of fixed spacing. 



Since their early stages of development, electrical methods have been 

 applied in mining exploration. At first this work was almost entirely 



