Subsurface Methods as Applied in Geophysics 1039 



being focused on areas where outcrops are rare or unknown. The responsi- 

 bility for providing these tools has fallen on the geophysicist and geo- 

 chemist. Problems faced by the mining geophysicist were discussed at a 

 recent meeting of the Society of Exploration Geophysicists.^ Geophysics 

 applied to mining in the United States is in the same position today that 

 petroleum geophysics was a quarter of a century ago. The addition by 

 the United States Bureau of Reclamation of geophysicists to its engineer- 

 ing and geologic staff indicates interest in applications of geophysics to 

 civil-engineering problems. 



It is beyond the scope of this chapter to discuss the detailed techniques 

 involved in geophysical exploration, and only a general discussion of the 

 topic is presented. For a complete treatment, the reader is referred tc 

 the several text books on geophysical methods of exploration.^ ^ ^ 

 Statistics 



During 1947 the drain on our oil resources was greater than ever 

 before, yet, according to Eckhardt ^ the industry was able not only to 

 maintain its reserves but to increase them. Lahee '^ shows that of the 

 3,471 new-field wildcat wells drilled in 1947, 394 became oil producers. 

 It may be interesting to compare the effort of the various exploration 

 techniques in these unproved areas: 10.5 percent of the holes drilled only 

 on geologic information were producers; 16.7 percent drilled only on geo- 

 physical information were successful; 17.7 percent drilled on combined 

 geophysical-geologic information were producers; and only 3.8 percent 

 of the wells drilled on nontechnical information became productive. It is 

 little wonder that the petroleum industry spent a minimum of $105,000,000 

 on geophysical exploration during 1947, not including the millions spent 

 on research. 



The number of geophysical crews in the field has shown a steady in- 

 crease, averaging about 555 crews during 1947. This is double the number 

 reported in 1942. Rapid expansion of seismic programs is chiefly account- 

 able for the increase, as the number of magnetic and gravity parties in the 

 field has decreased somewhat during the past several years. (See fig. 546.) 



Geophysical Exploration 



Geophysical exploration may be defined as "prospecting for mineral 

 deposits and geologic structure by surface measurement of physical quan- 

 tities." ^ In order to be considered a usable geophysical method the fol- 

 lowing five criteria must be satisfied:^ 



^ Symposium on Mining Geophysics: Geophysics, vol. 13, no. 4. pp. 535-583, Oct. 1948. 



' Heiland, C. A., Geophysical Exploration, New York. Prentice-Hall, 1940. 



' Jakosky, J. J.. Exploration Geophysics. Los Angeles, Times-Mirror Press, 1950. 



5 Ne'f^e'on. L. L.. Geophysical Prospecting for Oil. New York. McGraw-Hill Book Co.. Inc.. 1940. 



^ Eckhardt, E. A., Geophysical Activity in the Oil Industry in the United States in 1947: Geo- 

 physics, vol. 13, no. 4, p. 529, Oct. 1948. 



' Lahee, F. H., Statistics of Exploratory Drilling in 1947: Am. Assoc. Petroleum Geologists Bull., 

 vol. 32, no. 6, pp. 851-868, June 1948. 



^ Heiland, C. A., op. cit., p. 3. 



^ Rust, W. M. Jr., Evaluation of New Geophysical Methods: Geophysics, vol. 10, no. 3, p. 331, July, 

 1945. 



