44 EXPLORATION GEOPHYSICS 



physical work will be more economical during the reconnaissance stage 

 while obtaining an overall picture of the area. 



Some of the practical problems encountered in the various phases of 

 mining geophysics may be described briefly as follows : 



Metal Mining : locating of new ore bodies, extensions of old ore 

 bodies; depths of oxidized zones; location and determination of general 

 extent of sulphide deposits below oxidized cappings ; determination of 

 length and width of mineralized areas ; extensions of partially exposed ore 

 bodies; location of faulted segments of veins; delineation of ore shoots in 

 veins, etc. 



Non-Metallic Mining: determination of thickness of overburden; pres- 

 ence and location of faults and other structural features ; extent of par- 

 ticular formations of rock masses ; depth, size, and extent of gravel 

 deposits ; etc. 



Depth of Application. — The depth to which reliable geophysical work 

 may be conducted in the exploration for minerals is governed almost 

 entirely by the size and configuration of the ore body and the physically 

 measurable dififerences between the ore in situ and the country rocks. For 

 instance, a medium-size massive sulphide ore deposit could produce a 

 detectable gravitational anomaly (in an area where the necessary cor- 

 rections may be made for topography and changes in formation) at depths 

 of 700 to 1,000 feet. On the other hand, this same quantity of sulphide 

 ore existing as a stringer of mineralization along a vein can seldom be 

 detected by gravity work to a depth of even a hundred feet. However, 

 such a long extended ore body could easily be detected by the electrical 

 methods, which should be able to show its presence to depths of perhaps 

 400 feet. Generally speaking, mining geophysical work should seldom be 

 attempted to depths exceeding 300 to 400 feet from the point of measure- 

 ment. The use of geophysical studies should therefore be confined to 

 these depths, unless there are very good reasons for believing that the 

 local conditions are favorable enough to allow the work to be conducted 

 to depths exceeding that range. 



Even with this limitation, there are very large fields of application 

 for geophysical work in mining exploration. In Canada, about 90 per- 

 cent of the pre-Cambrian shield is covered with shallow water and over- 

 burden. In the United States and Mexico there are many hundreds of 

 square miles covered with alluvial fill, lake beds, and other masking mater- 

 ial, underneath which commercial mineralization may exist. 



Choice of Methods for Particular Geologic Problems. — In general, 

 the selection of geophysical methods for mining exploration is governed by 

 considerations similar to those that influence the selection for petroleum 

 exploration; namely, geological information available, type of mineral 

 occurrence, resolving power of the geophysical method, etc. Magnetic, elec- 

 trical, seismic, gravitational and thermal methods have been employed 



