CHAPTER II 



GEOLOGIC AND ECONOMIC BACKGROUND OF 

 EXPLORATION GEOPHYSICS 



Exploration for mineral resources, which are bountifully but always 

 irregularly distributed within the earth's crust, has engaged the attention 

 of man since earliest times,f and the art of mineral exploration has 

 advanced step by step.J- Formerly explorers searched for those mineral 

 deposits whose presence was suggested or indicated by evidence at the 

 surface of the ground. The present-day search is for mineral substances 

 which are deeply buried below the earth's surface. Modern exploration 

 methods have made it possible to discover these hidden deposits even 

 though indicative surface outcrops are completely absent.* 



The science and art of exploration has in general kept pace with the 

 advance of civilization and the improvement in methods for refining and 

 conserving mineral resources. Before the advent of geophysical explora- 

 tion, the chief hope of maintaining a continuing supply of these necessary 

 mineral resources was provided by improvements in exploitation and refin- 

 ing, which permitted increasingly greater recovery and utilization of the 

 raw materials and minerals. The recent appHcation of geophysical explora- 

 tion methods on a world-wide scale, while in no way insuring a permanent 

 supply of metals and oil, has resulted in a vast increase in estimated known 

 reserves. Much more yet needs to be done to relieve the critical shortage 

 of metals. 



General Factors Governing Application and Choice of Geophysical 

 Methods. — The application of geophysical methods to the various geolog- 

 ical problems involved in the search for mineral resources** is governed 

 primarily by two conditions : ( 1 ) the technical factors which are associated 

 with mode of geologic occurrence and type of geologic structure, and 

 (2) the economic factors pertaining to cost of operation. 



Irrespective of the specific field of exploration in which it is proposed 

 to do geophysical work, there are certain general considerations which 



t T. A. Rickard, Man and Metals, 2 vols., Whittlesey House, McGraw-Hill, New York, 1932. 

 t A. J. EHis, "The Divining Rod— A History of Water Witching," U.S.G.S. Water Supply 

 Paper No. 416. 



* An example of the successful culmination of modern exploratory effort is the 

 recent discovery and production of oil from strata buried over 13,000 feet below the 

 blanketing alluvium of the San Joaquin Valley, California. H. W. Hoots, "Additions 

 to California Oil Reserves," California Oil World, April, 1939. See also the March, 

 1938 issue of the Petroleum World. 



** The term mineral resources is used here to include any substance whose natural 

 habitat is the earth's crust and whose beneficial exploitation requires the use of the 

 various well-known mining, drilling, and engineering methods. Practically, the term 

 includes metallic and non-metallic minerals, petroleum, and water. 



