GEOLOGIC AND ECONOMIC BACKGROUND 35 



near-surface magnetic deposits (alluvium, lava flows, etc.) exist. Gravita- 

 tional methods are greatly hampered in areas of near-surface lateral density 

 variations and in areas of large topographic relief. 



The relative applicability of the various geophysical methods in the 

 location of folded structures often depends more on the geologic setting 

 (stratigraphic and lithologic properties of a petroliferous province) than 

 upon the type of structure. For example, the seismic reflection method has 

 given excellent results in most of the petroliferous provinces of the United 

 States. However, in some provinces the relative success of this method has 

 been moderate to mediocre. Difficulty has been experienced (a) in areas 

 of steep dips and faults, as in California, (b) in areas covered by thick 

 surface deposits of alluvial gravel or glacial fill, as in certain parts of Cali- 

 fornia, Iowa, Illinois, Michigan, and (c) in areas in which lithologic con- 

 tinuity is poor and formations of high elastic wave velocity intervene 

 between the surface and the oil-bearing structure, as exemplified by the 

 salt beds of parts of Kansas and the Permian basin and the highly indur- 

 ated limestone which caps the Edwards Plateau area of Texas. In certain 

 instances of such unfavorable near-surface conditions, the drilling of deep 

 shot holes might improve the success of this method, but the increased costs 

 usually are not justified. Areas of this type often may be more satisfac- 

 torily explored by long distance refraction work or by slim-hole drilling. 



Certain areas and problems found to be difficult for reflection seismic 

 work sometimes have proved amenable to continuous electrical profiling, 

 the outstanding cases being the successful mapping of structure near the 

 top of the San Andreas Lime and at the base of the salt in the Permian 

 basin, and the mapping of faults in California and South Texas. Electrical 

 methods have also been successful in mapping faults and folded structure 

 beneath a mantle of 500 to 800 feet of unconsolidated alluvium in the San 

 Joaquin valley of California, and have been employed to determine struc- 

 ture beneath the glacial drift in parts of Iowa, Illinois, and Michigan. A 

 small amount of electrical work has been conducted in the Edwards Plateau 

 area, with some evidence that the surface cover of Edwards limestone may 

 not prevent the mapping of underground structure by this method. The 

 usefulness of electrical methods in petroleum exploration appears to be 

 confined to: (1) detecting the location of faults, (2) mapping of shallow 

 structure, and (3) detailed structural mapping to depths of about 5000 

 feet in certain districts where underground stratigraphy is favorable. 



Salt dome structure may be classed as a special case of folded structure. 

 The outstanding early success of refraction seismic and gravitational 

 methods was in the location of relatively shallow salt dome structure, and 

 this still constitutes a special field of applicability of refraction seismic 

 methods. Reflection seismic methods, however, are the first choice in 

 modern exploration for deep-seated salt domes. 



The discovery of homoclinal oil-bearing structures by geophysical 

 methods generally has been only moderately successful. (It is of interest to 



