GEOLOGIC AND ECONOMIC BACKGROUND 55 



Miscellaneous Non-Metallic Deposits. — Various kinds of igneous 

 rocks are used commercially in the form of building stones, road building 

 materials, etc. Magnetic methods are usually the preferred choice in 

 exploration for these materials because of the relatively high magnetic 

 susceptibility of most igneous rocks. Electrical methods are useful also — 

 particularly to outline the extents of the rock deposits, locate contacts, and 

 give qualitative information regarding the amount of weathering or 

 alteration. 



In some cases, geophysical methods are useful in indirect explora- 

 tion for gems and precious stones by reason of their genetic relationship 

 to certain types of rocks or formations. Stearn t gives an example of 

 the location of diamond-bearing peridotites by magnetic methods. Gems, 

 semi-precious stones, and certain non-metallic minerals of industrial use 

 occur in pegmatites. The possibility of locating a pegmatite dike by 

 use of electrical methods has been mentioned in a preceding section. 



GEOPHYSICAL METHODS IN WATER SUPPLY 

 ENGINEERING 



Engineering, economic, and geological problems arising in the general 

 field of water supply are very extensive and can be discussed here only 

 briefly. $ The sources of water are: (1) surface water in lakes, streams, 

 rivers, etc., and (2) subsurface water (ground water). Control, conser- 

 vation, storage, and use of surface water is accomplished by building 

 dams and other engineering works. Similar utilization of ground water 

 requires storage in subsurface basins and the drilling of wells in proper 

 locations to produce water by natural flow (artesian wells) or by pumping 

 wells. The original source of the ground water for which drilling explora- 

 tion is conducted is run-ofif water and rainfall. The available supply of 

 ground water in any area is therefore mainly determined by : amount of 

 precipitation, character of topography, extent of water-sheds and drainage 

 basins, etc. 



Local accumulation of ground water, however, is controlled primarily by the geo- 

 logical factors of local structure, petrology, porosity, etc. Detailed geological infor- 

 mation is essential to aid the engineer in the proper location of water wells. He should 

 know the extent, thickness, depth, composition and relative position of water-bearing 

 strata or subsurface basins, the depth to the ground water table, the location of 

 buried stream channels, faults, dikes and other subsurface structural features. Often- 

 times some of this information may be obtained by surface geological observations. 

 In recent years, geophysical methods have been applied in an increasing extent to 

 secure much of the subsurface geological information which is necessary for the 

 intelligent direction of subsurface water-supply development. 



Information concerning accumulation and distribution of ground water may be 

 desired for several purposes: (1) to locate a supply suitable for domestic uses, irri- 

 gation, mining, or other purposes; (2) to determine ground water conditions insofar 



t Noel H. Stearn, loc. ctt. 



t C. F. Tolman, Ground Water (McGraw-Hill Book Company, New York, 1938). 



