INTRODUCTION 15 



opment and research in the several methods of exploration geophysics. 

 The contributions of many individuals to the progress and the enviable 

 success achieved by these methods are recorded throughout the following 

 text. More than 4,000 patents and literature references indicate the divers- 

 ity of thought which has contributed to modern geophysics. Although 

 geophysical prospecting techniques had their beginnings abroad, most of 

 the important developments of the past twenty-five years have been made 

 in the United States. It has been estimated that over 95 per cent of the 

 world's geophysical work is being done by U. S. organizations.! 



TRENDS IN DEVELOPMENT OF FUTURE METHODS 



The generally dependable character of the geological data obtained by 

 geophysical methods and the advantage of operating cost which these 

 methods have over exploration by drilling or other direct exploration have 

 established the worth of geophysics in evaluating prospective lands. Geo- 

 physical exploration will decline in scope, however, unless the resolving or 

 detecting power of its methods is increased in proportion to the diminishing 

 size and increased depth of the undiscovered deposits. This may be illus- 

 trated by a brief review of the several techniques that have been employed 

 in the exploration for oil. 



Early prospectors who located the first oil wells were guided entirely 

 by the occurrence of surface seepages. Soon after the discovery of the first 

 fields by this method and by even more haphazard prospecting, it became 

 recognized that the occurrence of oil and gas is commonly limited to the 

 structurally high parts of formations. The anticlinal or structural theory, 

 advanced to explain this occurrence, soon became established as sound and 

 of tremendous value in the search for oil. Throughout subsequent years 

 geologists searched feverishly for favorable anticlines exposed at the sur- 

 face, until the supply became largely exhausted. 



Petroleum geologists later recognized the probable existence of addi- 

 tional favorable anticlines, hidden beneath extensive areas of shallow 

 alluvium, or with poor surface exposures. The necessity for extending 

 knowledge of structure into such areas led to pit-digging and trenching, a 

 very old method of prospecting. As knowledge increased, it became appar- 

 ent that suitable structural conditions may exist in the older strata uncon- 

 formably underlying those exposed at the surface. To map such hidden 

 structures, core drilling was introduced in Oklahoma in 1919. This method 

 of exploration was new to the petroleum industry in this country. Core 

 drilling gave the ultimate in exactness of data, but its application was 

 limited by its high cost, with resultant restriction to relatively shallow 

 depths. Core drilling was extended to most of the petroleum districts of 

 the country and is still used in areas where structural relief is small, or 

 where less direct methods are not efifective. 



The use of core drilling for structural mapping had made considerable 



t E. A. Eckhardt, ibid. 



