44 GEOPHYSICAL METHODS IN OIL EXPLORATION (Chap. 4 



The refraction method has definite possibilities in virgin areas as a means 

 of stratigraphic analysis of a section. As a means of finding salt domes, 

 its use has decreased considerably on the Gulf coast in favor of gravity and 

 reflection-seismic work. Some oil companies still retain the use of the 

 refraction method for work preceding or accompanying reflection work if 

 the section to be followed out does not exceed 6000 feet in depth. Beyond 

 the distances required for such depth penetration (about seven miles) 

 difficulties in transportation, handling of dynamite, and therefore operation 

 costs generally increase out of proportion to the results achieved and detail 

 of information obtained. 



For detail, the reflection method is unquestionably the most widely used 

 geophysical technique ; it works best on low-dip structures but encounters 

 difficulties with beds of complex faulting, folding, and steep dip. Com- 

 pared with many other geophysical methods, it has the advantage of giving 

 positive depth information. Because of the fact that several reflecting 

 beds may be mapped in one area, this method gives data in regard to 

 lateral variations of the thicknesses of formations and permits the follow- 

 ing of unconformities. 



As aptly expressed by Barton,^ "Geophysical orientation in regard to an 

 area is as necessary to the geophysicist as geological orientation is to the 

 geologist." The geologist must acquire a knowledge of the general stra- 

 tigraphy and regional structure of an area and must learn the relation of 

 particular features to the regional geology before he can begin to evaluate 

 the significance of local structure in respect to its potential productivity. 

 Likewise, the geophysicist should first investigate the general magnetic, 

 gravitational, seismic, and electric characteristics of an area from a regional 

 point of view before he can begin to evaluate the local significance of 

 geophysical anomalies. Moreover, the regional geological features must 

 be correlated with the regional geophysical features. It often takes 

 considerable time until certain magnetic and gravity anomalies, seismic 

 velocities, reflection travel times, and electrical logging indications can be 

 definitely tied up with specific formations or groups of formations. The 

 geophysicist frequently must wait for the driller before he can complete his 

 correlations. 



The task of choosing methods for reconnaissance and for detail which 

 will give the most reliable information at least expense is facilitated if it is 

 definitely known what type of geologic structure will be encountered in a 

 given area, and if geophysical measurements have been made on a known 

 field. When geophysical methods were first introduced in oil prospecting, 

 practically every method was first tried on known structure. In exploring 



* A. E. Dunston, et al., The Science of Petroleum, Oxford University Press (1938). 



