GEOLOGIC AND ECONOMIC BACKGROUND 53 



cretionary beds ; (2) disseminations in igneous rocks ; (3) pegmatite dikes ; 

 (4) guano deposits; (5) replacements of limestone; (6) residual concre- 

 tions. A summary of prior literature dealing with exploration for various 

 non-metallic materials is contained in an early publication. § 



Applications of geophysical methods in the non-metallic field are governed by 

 the factors outlined for other mineral resources earlier in this chapter. For the 

 sake of brevity, the following discussion will comprise a resume of applications to 

 bedded and residual deposits. The applications are primarily of the indirect type, 

 i.e., the mapping of structure associated with the occurrence of commercially valuable 

 deposits. The location of faults, dikes, and folds and the determination of thickness - 

 of overburden constitute the important problems. For the solution of these problems, 

 magnetic, electrical, seismic, and gravitational methods are all used. 



Sedimentary Rocks. — Examples of sedimentary rocks or rock ma- 

 terials which are of commercial importance are limestone, sandstone, 

 clay, sand, gravel, etc. Electrical methods are ordinarily the most ap- 

 plicable in exploration for deposits of these materials. 



In addition to structural studies, it is possible in some cases to lo- 

 cate the various deposits directly by utilizing differences in electrical 

 conductivity relative to adjacent rocks. The degree of alteration or 

 weathering, which is often important in evaluating limestones and other 

 rocks employed for building purposes, may be determined by electrical 

 methods. Oftentimes, sand and gravel deposits may be located directly 

 by electrical methods, t 



Magnetic methods may be useful ( 1 ) for determinating structural 

 conditions governing deposits of these types of rock materials and (2) 

 for locating buried stream gravels having an appreciable content of 

 magnetite. 



Saline Residues. — Saline residues are accumulations of certain min- 

 erals which have been deposited as the result of evaporation in closed 

 or partially closed basins. Some of the commercially important minerals 

 or ores in this class are rock salt, gypsum, anhydrite, sodium nitrate, 

 borax, and potash. 



Under favorable conditions, distinctive physical properties may permit 

 direct detection of these deposits. For example, salt, relative to some 

 sedimentary rocks, has a low density, poor electrical conductivity and 

 transmits seismic waves at a high velocity. These properties have been 

 utilized in exploration for salt domes by gravitational, electrical, and 

 refraction seismic methods, respectively. To date, however, utilization 

 of these properties in the direct location of salt and other saline deposits 

 has been extremely limited. 



§ C. A. Heiland, "Geophysical Prospecting in the Non-Metallic Field," A.I.M.E. Geophysical 

 Prospecting, 1934. 



t M. King Hubbert, "Results of Earth Resistivity Survey on Various Geologic Structures in 

 Illinois,^" A.I.M.E. Geophysical Frospectins. 1934. 



Karl S. Kurtenacker, "Some Practical Applications of Resistivity Measurements to Highway 

 Problems," A.I.M.E. Geophysical Frospectins, 1984. 



S. N. Wilcox, "Sand and Gravel Prospecting by the Earth Resistivity Method," Geophysics, 

 Vol. 9, No. 1, Jan., 1934. 



