54 GEOPHYSICAL METHODS IN MINING [Chap. 5 



Methods ordinarily used for the location of salt domes are applicable, 

 especially gravimetric and seismic methods. 



C. Salt 



Salt is obtained commercially from salt brines, salt beds, or salt domes. 

 The literature on the location of salt brines is meager and deals only with 

 the type occurring in salt mines. As far as salt domes are concerned, the 

 geophysical problem is the same as in oil exploration (see Chapter 4) ; for 

 depth determinations of salt beds, seismic and electrical resistivity methods 

 have been used. 



D. Nitrates, Phosphates, Potash 



It is doubtful whether any commercial geophysical work has been done 

 on these deposits (owing to the abundant supply of nitrate in Chile, and of 

 phosphate in the v/estern United States). Since the commercially impor- 

 tant potassium minerals occur with salt, the geophysical methods discussed 

 in the preceding paragraph have found frequent application in the indirect 

 location of potash deposits. Sineriz' second report'^ contains numerous 

 examples for depth determination of salt beds in Spain. Underground, it 

 appears possible to diflferentiate between potash beds of different age by 

 measurement of their penetrating radiation. 



E. Building and Road Materials 



Applications of geophysical methods in this branch of the nonmetallic 

 field overlap with those in engineering and include cement materials, 

 gravel, sand and clay, and building and road stone. 



Cement materials possess a number of physical characteristics (for 

 example: high elastic wave speed and density of gypsum, anhydrite, and 

 limestone; electrical resistivity of lime, chalk, and the like) which indicate 

 the best possibilities for electrical and seismic methods. 



Electrical, seismic, and (in case of crystalline bedrock) magnetic methods 

 are suitable for the location of gravel, sand, and clay. Further details 

 are given in Chapter 6 under the heading of engineering applications. 



A few geophysical surveys have been made for the location of building 

 or road materials. Electrical exploration appears to hold the greatest 

 possibilities, since resistivities of formations depend on their degree of 

 alteration and moisture content. Magnetic methods are often applicable, 

 since rocks preferable from the standpoint of roadbuilding (igneous rocks) 



2 J. G. Sineriz, "La interpretacion geologica de las mediciones geoficicas applica- 

 das a la prospeccion," Inst. Geol. y Minero Espana Mem. (1933). 



