Chap. 10] 



ELECTRICAL METHODS 



703 



tending at right angles to the topographic contours (see Fig. 10-43). 

 Where this would involve discarding an entire survey, the only procedure 

 left is a model experiment. Notwithstanding the marked effects of topo- 

 graphic irregularities, it is observed in most applications of the equipo- 

 tential line method that irregularities in surface geological features are of 

 remarkably little effect. The only exceptions appear to be areas where 

 highly saline beds occur at or near the surface. 



'HOLANDA(stVILL^MINC 



30 MO isom 



Fig. 10-43. Equipotential line survey in which most distortions are caused by 



topography (after Heine). 



E. Discussion of Results 



Equipotential-line and potential-profile methods have been applied to 

 (1) location of ore, (2) structural studies, and (3) military and civil engi- 

 neering problems. 



1. Ore location. Probably the most extensive work with equipotential- 

 line and potential-profile methods has been done in the Skellefte district 

 in northern Sweden. Close to 120 square miles have been surveyed there 

 systematically with electrical methods and more than twenty ore fields 

 have been found. Conditions for electrical prospecting are exceedingly 

 favorable because of a comparatively shallow blanket of glacial moraine 

 and frequent ore indications in the form of ore boulders and float. Al- 

 though these had been known for centuries, it was not until 1918 that 

 systematic electrical prospecting was started. 



The ores in the Skellefte district occur in a leptite formation which 

 corresponds in age to the Keewatin. It consists largely of volcanic rocks 

 which were intruded, in Archean times, by large bodies of granite; this led 

 to considerable metasomatic alteration and to deposition of ore. The 



