2 EXPLORATION GEOPHYSICS 



ogy. Due chiefly to stiff commercial competition, this branch of applied 

 geophysics is a dynamic art continually changing and bristling with per- 

 sonalities. Many different methods are being used, and it is the purpose of 

 this book to outline the fundamental theory and general application of these 

 various exploratory methods. 



The demand for metals in the latter part of the nineteenth century 

 contributed much to the development of mining geophysics. The chief 

 incentive to modern exploration geophysics, however, was the search for 

 oil during the early years of the twentieth century. No longer can the 

 geologist go forth to look for oil using only such signs as gas bubbles on 

 water, petroleum seepage along creek beds, outcropping oil-bearing strata, 

 or other surface evidence of subsurface oil accumulation. To these methods 

 of finding oil have been added surface geology, cartography, micropaleon- 

 tology, lithology, electrophysics, geophysics, and geochemistry. 



Most underground structures can be diagnosed if detectable variations 

 in these structures exist. Thus the four leading geophysical techniques 

 relate to the four most common characteristics which can be determined 

 from the surface : magnetism, density, electrical conductivity, and elasticity, 

 which are investigated respectively by magnetic, gravitational, electrical, and 

 seismic methods. Variations discovered by these methods are then assessed 

 in geologic terms. Sometimes the desired results may be achieved in- 

 directly : if the mineral being sought lacks properties detectable from the 

 surface, another body associated with it and possessing these properties 

 may be mapped.* 



During the period of early petroleum production only six per cent of 

 the wildcat wells were successfully completed as producers. An increased 

 demand for oil and the inadequacy of the haphazard methods of the early 

 "wildcatter" in meeting this demand forced the development of the new 

 science of petroleum geology, and with it the effective tools of geophysical 

 exploration so well adapted to the study of the remaining unexplored areas. 

 Even with these aids, only about 21 per cent of the exploratory wells now 

 drilled are completed as producers. During the entire life of the American 

 petroleum industry an average of about 180,000 barrels of oil have been 

 discovered for each dry hole drilled. During 1947, with geophysical explo- 

 ration as the guide in a large portion of the wells drilled, discovery of oil 

 was made at the rate of about 528,000 barrels for each dry hole. This 

 increased efficiency must be credited to the better geophysical-geological 

 techniques now in use. 



Geophysical methods have been successful in the search for oil primarily 

 because of their ability to determine the geologic structure of buried forma- 

 tions, mapping conditions which are favorable for the accumulation of oil 

 or gas. Although each method is undergoing constant revision to improve 

 its accuracy and broaden its field, several have had phenomenal success in 



* For example, highly magnetic magnetite associated with gold in a placer, or a 

 geologic structure related to ground water. 



