48 GEOPHYSICAL METHODS IN OIL EXPLORATION [Chap. 4 



at all, in view of the available appropriation and anticipated returns, and, 

 if so, which method will be the most economical. Frequently the con- 

 sideration of cost may lead to the adoption of geological surveying or 

 shallow drilling, where applicable, to replace or at least reduce the geo- 

 physical work. Time may be another factor in view of competitive 

 activities or expirations of leases and options, and it may readily lead to 

 the selection of a faster but more expensive, in place of a slower and less 

 expensive, method. Certain terrain conditions, such as swamps and 

 water, will often eliminate some of the methods, for example, torsion 

 balance, gravimeter and magnetometer, from consideration. Fortunately 

 the most important reconnaissance and detail methods— seismic refraction 

 and reflection — can be operated on both land and water. Rugged topog- 

 raphy precludes the application of torsion balances, but seismic reflection 

 work is still possible unless conditions are extreme. Adverse surface geo- 

 logic conditions will handicap many methods, such as heterogeneous glacial 

 beds in torsion balance work, igneous and metamorphic rocks in magnetic 

 exploration, and high-speed surface formations in refraction and reflection 

 shooting. Some types of surveys, for example, those with gravimeters 

 and magnetometers, may be made along public roads without permits, 

 while others requiring the most favorable terrain setups, as torsion balance 

 and some reflection work, will necessitate going on private land. Remark- 

 ably free from this restriction is the refraction method, inasmuch as plots 

 of ground for which permits are not obtained may be placed between shot 

 and detector location. In respect to secrecy, operation of gravimeters or 

 magnetometers is much more favorable than is torsion-balance or seismic 

 exploration. Finally, restrictions in regard to the use of dynamite or 

 patented processes may be decisive factors in the selection of a method. 



Specific applications of geophysical methods to oil exploration problems 

 will be found in Chapter 7 on pages 157-162, 272-286; in Chapter 8 on 

 pages 422-433; in Chapter 9 on pages 499-501 and in Figs. 9-46, 9-74, 

 9-85, 9-92, 9-93; in Chapters 10, 11, and 12 on pages 706, 735-739, 752, 

 835-837, 856, 863-865, 869, 898-901, and in Figs. 10-122 and 10-123. 



The role of geophysics in oil exploration is reflected in a statement by 

 De Golyer who says: "With good, not average, practice the cost of oil 

 finding in the early twenties was 20 to 25 cents per barrel. Today on 

 volume operation, the cost of oil finding for good practice is 10 to 12 cents 

 per barrel." It is estimated that geophysics has been responsible for the 

 discovery of over 5 billion barrels of crude oil to 1939.^ 



» E. De Golyer, Mining and Metallurgy, 20(391), 335 (1939). 

 ' G. Egloff, Colo. School of Mines Mag., 29(6), 277 (1939). 



