702 



ELECTRICAL METHODS 



[Chap. 10 



When complete observations are made by resolving the potential into its 

 in-phase and out-of-phase components, the effect of the leads is readily 

 recognized and may be separated from the in-phase anomalies. As a 

 matter of fact, the out-of-phase potentials near the cable may be of great 

 diagnostic value (see page 801). 



5. Effect of topography. Barring the existence of highly conductive 

 layers of irregular composition or thickness near the surface, the inter- 

 pretation of equipotential line surveys in level terrain is generally not too 

 difficult. However, with the exception of northern glaciated countries 

 such as Canada or Sweden, ore prospecting work is usually carried on in 

 mountainous areas where considerable interference may result from 

 changes in topographic conditions. 



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Fig. 10-42. Potential anomalies on model ore bodies (after Sundberg). Solid 

 lines: equipotential lines, traced by phones. Dotted lines: in-phase equipotential 

 lines, constructed from amplitude and phase measurements. 



Topography is, first, of geometric influence. If a line connecting the 

 two electrodes is not parallel with the surface, the ore bodies are not 

 located vertically below their surface mdications but are on a normal to the 

 line connecting the electrodes.'* The second effect of topography is of an 

 electrical nature. The earth's surface represents the boundary between 

 a conductive and a nonconductive medium; hence, the current lines, 

 tending to follow the surface of the conductive medium will reflect, in 

 vertical section, the contours of the surface, and the equipotential lines 

 will show corresponding distortions. 



To correct for topography by analytical methods is not practicable. 

 The usual procedure is to plot topographic contours, together with equi- 

 potential lines, and to eliminate from consideration equipotential lines 



" W. Heine, Elektrische Bodenforschung, p. 107, Fig. 53 (1928). 



