1040 



Subsurface Geologic Methods 



1. The method must involve physical measurements of some prop- 



erty of the earth. 



2. The measurements must give reproducible results. 



3. The process must be able to compete economically with existing 



methods. 



4. The method must have a significant relation to the occurrence of 



oil (economic minerals) . 



5. The method must have a teachable interpretation. 



The major methods of exploration may be roughly divided into those 

 which measure variations in one of the earth's potential fields, magnetic 



450 



400. 



350 



"300 



250 



"200 



150 



loo 



50 



Figure 546. Number of seismograph and gravimeter parties in United States from 

 1938 to 1947. (After Eckhardt. Reproduced permission Geophysics.) 



and gravitational; or into those in which a measurable reaction is pro- 

 duced at the surface by the application of an artificial force field, seismic 

 and electrical. Inference may be made as to subsurface geologic condi- 

 tions only when detectable differences in physical properties exist below 

 the surface. The physical basis for the use of the magnetic methods is 

 thus a variation in magnetism or susceptibility, for gravitational methods 

 a variation in density, for seismic methods a variation in velocity, and for 

 electrical methods a variation in electrical conductivity.^*^ Generally speak- 

 ing, the geophysicist is unable to give specific information concerning the 



^" Does not apply to the self-potential method. 



