GEOLOGIC AND ECONOMIC BACKGROUND 43 



and the magnitude of the associated changes in physical properties of the 

 rocks and ores may be correspondingly large. The electrical conductivity 

 of a pyrite vein may be over a thousand times greater than that of the 

 adjacent country rock. Such large differences in physical properties can 

 often offset the adverse influence of structural complications, irregular- 

 ities, and small size. 



The second category of application, geophysical exploration over wider 

 areas, has in the past few years become increasingly important.f In a 

 survey of this type, the program must be directed primarily toward obtain- 

 ing an overall regional picture, and at the lowest cost per property or per 

 acre. It is thus more akin to petroleum geophysics. In many cases this can 

 be accomplished by the following steps : (1) geological studies, with refer- 

 ence to structure, modes of ore occurrence, and mineralogical guides in 

 the area; (2) reconnaissance geophysics, over the geologically favorable 

 areas; then (3) detailed geological and, if applicable, detailed geophysical 

 examination of the favorable zones ; followed by (4) direct exploration, 

 such as trenching, shaft, tunnel, or drill hole. 



By proper planning, geologically-guided geophysical work may be 

 applied as economically as haphazard prospecting or geologically-guided 

 direct exploration. Quite often a comparison is made between the cost of 

 a geological examination and a geophysical examination of a mining 

 property. This comparison is not too enlightening, because geological 

 work alone in the usual complex mining area is limited in its ability to 

 predict the subsurface conditions. Dips and strikes of exposed rocks must 

 be extended underground with extreme caution, in the usual folded and 

 faulted area. As the experienced mining engineer and geologist also knows, 

 geological prediction is greatly handicapped until enough development has 

 been done to establish local habits of ore genesis and occurrence. Geo- 

 logical work then must be supplemented with direct exploration such as 

 tunneling and drilling. The cost of such geological work and the necessary 

 direct exploration usually is far greater, as to expenditure of both time and 

 money, than geologically-guided geophysical work. 



Whether geologically-guided direct exploration or geologically-guided 

 geophysical reconnaissance is the better procedure will depend upon local 

 conditions and the information desired. For instance, the cost of a single 

 300-foot drill hole is equivalent to the cost of a number of days of geo- 

 physical work. The drill hole may give detailed and specific information 

 about conditions along the exact path traversed by the drill. On the other 

 hand, the geophysical work may give general information over a relatively 

 large area. Obviously, there can be no fixed rule as to which is the more 

 expedient technique. In general, however, the geologically-guided geo- 



t Hans Lundberg, "Mining Geophysics," A.I.M.E., Min. &■ Met., Vol. 29, No. 494, pp. 88-92, 

 February, 1948. 



V. G. Gabriel, "Geophysical Prospecting — Its Part in American Mining," Eng. and Min. Jnl., 

 April, 1939. 



