42 EXPLORATION GEOPHYSICS 



GEOPHYSICAL METHODS IN MINING 



General Field of Application. — The application of geophysics to min- 

 ing exploration may be divided into two general categories. First is its 

 application to operating mines or previously mined areas where detailed 

 studies are necessary. Second is its application to wider areas, where 

 reconnaissance studies are conducted to delineate the areas of special inter- 

 est, which then may be subjected to intensive investigation. 



The largest application of mining geophysics has been to operating or 

 previously worked properties. In detailed studies of such areas, mining 

 exploration differs from petroleum exploration chiefly in the scale and 

 complexity of the geological problems involved. Whereas in petroleum 

 exploration the geophysicist deals with large areas and simple structures, 

 in mining he is concerned with complex local structures of small areal 

 extent, generally in more rugged terrain. Consequently, greater detail is 

 necessary in the mining geophysical observations, and closer correlations 

 are required between geophysical data, surface geology, and history of 

 local ore occurrence. Usually, therefore, costs per acre for mining geo- 

 physical exploration are higher than those for petroleum exploration. Also, 

 the relatively small size of the ore bodies imposes depth limitations on 

 the various geophysical methods, and increases costs of surveys. The 

 feasible exploration range on a given property may not even include the 

 entire depth range which may be amenable to economic ore exploitation in 

 any one mining district. 



These factors have acted to delay a more general adoption of geo- 

 physics in mining as compared to petroleum exploration. Another signifi- 

 cant factor, however, is that property lines and claim corners far too often 

 are important legal limitations in a survey, and the problem of the geo- 

 physicist becomes one of locating an ore body within the confines of a 

 particular property. Under such conditions, geophysical work for mining, 

 is, in fact, usually a distress measure, resorted to as a last attempt to 

 find ore and stave off shut-downs. With the required goal of locating 

 commercial ore, it has little chance of achieving success, due simply to the 

 fact that there are relatively few so-called "extensions" of ore bodies, or 

 new bodies, in previously worked areas. However, when used in such areas 

 as a means of evaluation and a guide for operational planning, geophysics 

 can serve a most useful function. If a new ore body is indicated, continued 

 mining can be projected. If not, an intelligent decision can be made as to 

 when to discontinue operations, avoiding the useless expenditure of further 

 capital. A wider appreciation of the value of geophysics in providing an 

 answer to the question of "when to stop" would stimulate its application in 

 the field of mining. 



The complex geology of mining districts and the variable nature of 

 mineral deposits offer an extensive and fertile field for the application of 

 geophysical methods. The more prominent modes of mineral occurrence, 

 particularly metal deposits, form abrupt discontinuities in local geology. 



