1060 Subsurface Geologic Methods 



shown in figure 558. This area was flown by the United States Geological 

 Survey at an altitude of 1,300 feet. The results of a conventional ground 

 survey accompany the map of the air-borne survey to show the similarity 

 between the methods. This area is located in Greer County, Oklahoma, 

 near the Wichita Mountains. ^^ 



In summary, it can be seen that application of the simple depth 

 rules will in many instances give a clue to the probability of the asso- 

 ciation of a magnetic anomaly with actual structural uplift. However, 

 the magnetic method should be used in cooperation with other sub- 

 surface exploration techniques. 



Gravitational Prospecting 



Gravitational prospecting, like magnetic prospecting, involves the 

 measuring of variations in one of the earth's natural fields of force. 

 Since the introduction of the torsion balance on the Gulf Coast in 1922, 

 gravity surveying has been a popular exploration tool, the number of 

 gravity parties in the field reaching a maximum of about 175 crews in 

 1945. Primarily the gravity methods have been used as reconnaissance 

 tools, although a number of successful locations have been made on grav- 

 ity prospects, particularly on the Gulf Coast. The first salt dome and oil 

 structure discovered in the United States by any geophysical method was 

 the Nash.dome in Brazoria County, Texas, which was located on the basis 

 of torsion-balance work in the spring of 1924. 



It is interesting to observe that twenty salt domes were discovered in 

 Mississippi during a twenty-week period by two gravity meters at a total 

 cost which was less than the cost of discovery of a single salt dome by the 

 seismograph.^^ It is estimated by Jakosky -^ that the total ultimate re- 

 covery from fields located by gravitational methods will exceed one billion 

 barrels of oil. 



Basic Principles 



Sir Isaac Newton formulated the universal law that every particle of 

 matter in the universe attracts every other particle. If two objects of 

 mass m\ and m2 are separated by a distance r, and r is considerably larger 

 than the dimensions of either object, there is a force of attraction be- 

 tween the two objects which is related directly to the masses and in- 

 versely as the square of the distance r. This expression may be written 

 as follows: 



p _ m^m2 



^' Balsley, J. R., The Airborne Magnetometer: U. S. Geol. Survey Geophys. Inv. Preliminary Rept. 

 3, p. 1, 1946. 



2S Eckliardt, E. A., A Brief History of the Gravity Method of Prospecting for Oil: Geophysics, vol. 

 5, no. 3, p. 232, July 1940. 



-® Jakosky, J. J., Exploration Geophysics : p. 6, Los Angeles, Times-Mirror Press, 1940. 



