CHAPTER X 



GEOPHYSICAL METHODS OF INVESTIGATING 

 BOREHOLES 



Introductory Note. — Geophysical methods of locating 

 mineral fields and particular ore bodies and minerals 

 are already well established and can be divided into six 

 main groups, viz., gravitational, electrical, magnetic, seismic, 

 thermal, and radioactive methods. Of these, so far, only 

 electrical and seismic or elastic methods have been applied 

 to coring and borehole problems. 



By a geophysical method we mean one in which some 

 established physical property of matter, i.e., ore body, 

 oil, coal, etc., is investigated, excited or otherwise examined 

 in contrast with its surroundings and the features betrayed 

 measured at a distance. At present bodies at depths down 

 to about 600 ft. have been successfully located by these 

 means. In many cases only one particular method can be 

 applied, e.g., seismic methods (artificial ground shocks) 

 alone suit the geological features connected with the deep 

 oil zones of Iraq, while in Sweden electrical methods are 

 most favored. We have detailed these methods elsewhere,^ 

 so will not enlarge upon them here. 



Electrical Methods.— The transmission and distribution 

 of electrical energy currents of various kinds which are sent 

 out from artificial or natural sources form the basis of the 

 most widely adopted methods of scientifically investigating 

 the earth's crust. There are four chief divisions: 



1. Methods in which a current is purposely generated 

 and introduced into the ground. 



2. Methods in which the currents generated in bodies 

 themselves in the ground are harnessed. 



^ Colliery Guardian, May to September, 1927, and Haddock, M. H.^ 

 "Location of Mineral Fields," Chap. VI, Crosby, Lockwood & Sons, London. 



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