54 EXPLORATION GEOPHYSICS 



Magnetic and electrical methods have been employed to a limited extent 

 in determining local structural conditions. An example is the search for 

 borax deposits in California. The magnetic and electrical work located 

 faults and determined the subsurface distribution of lake bed sediments 

 and buried lava flows that are genetically related to the accumulations 

 of borax in the Mojave desert area. 



Residual Deposits. — The most common non-metallic ores or rock 

 materials which occur as residual deposits are clay, barite, phosphate, 

 and bauxite. Geophysical methods have not been extensively applied 

 to these deposits, but the field for indirect application of the methods is 

 similar to that for other non-metallic resources. Stearn t has given an 

 example of the indirect use of magnetic methods in the search for 

 bauxite deposits in Arkansas. 



Coal and Lignite. — Measurements have been conducted in diflferent 

 parts of the world to investigate the applicability of geophysical methods 

 in exploration for coal deposits. Due to the variable chemical and 

 physical properties of coal, as well as the variety of structural condi- 

 tions associated with the deposits, its direct location by geophysical 

 methods has not been very successful. In a few cases of shallow 

 occurrence, experimental electrical surveys have directly indicated the 

 presence of coal seams, t Hawkins § reports generally unsatisfactory 

 results in attempts to locate lignite directly by resistivity measurements. 



The geophysical work that has been done in exploring for coal in- 

 dicates that direct location of coal beds may be accomplished only under 

 especially favorable conditions of coal composition and structural set- 

 ting. Anthracite deposits in some cases exhibit high electrical conductivity 

 relative to the surrounding sediments. Under such conditions, these 

 deposits undoubtedly can be located directly by electrical methods. Simi- 

 larly, the relatively poor conductivity of bituminous beds in certain areas 

 suggests the possibility of locating such deposits by electrical methods. 



The resistivity method has been successfully used to find the depth to a 

 shallow coal seam (about 75 feet) in connection with a proposed strip 

 mining operation. It very effectively supplemented more costly drill 

 exploration. 



In general, however, it may be concluded that the most successful 

 applications of geophysics in coal exploration will be in the field of 

 structural and stratigraphic determinations. Electrical, seismic, gravi- 

 tational, and magnetic methods have all been tried. At the present time, 

 electrical methods promise to be the most valuable. 



t N. H. Stearn, loc. cit. 



t Maurice Ewing, A. P. Crary, J. W. Peoples and J. A. Peoples, Jr., "Prospecting for Anthra- 

 cite by the Earth-Resistivity Method," A.l.M.E. Geophysical Prospecting, Tech. Pub. 683. 



§ R. H. Hawkins, "Application of Resistivity Methods to Northern Ontario Lignite Deposits," 

 A.l.M.E. Geophysical Prospecting, 1934. 



