8 



EXPLORATION GEOPHYSICS 



equipment to oil prospecting was apparently made by the Marland Oil 

 Company in Kansas and north-central Oklahoma ( 1925-26). f More recent 

 developments are described later in the text. 



Initial tests in 1922 over the Spindletop dome in Texas gave a definite 

 gravity maximum, but subsequent work over various prospects gave indefi- 

 nite results, until a survey of the Nash area in Texas showed a good 

 anomaly. A well was drilled in 1924, proving the presence of the structure. 

 Oil was found on the flank of this structure in 1926, probably the first oil 

 pool to be discovered by geophysical methods in America. Large oil fields 

 mapped by the torsion balance method which have now been developed 

 include: Thomson (Rabbs Ridge), Sugarland, Tomball, Manvel, Dickin- 

 son, Hastings, Anahuac, Cedar Point, Fairbanks, Friendswood, Mykawa, 

 Roanoke, Iowa, Gillis, and English Bayou. It is estimated that the total 

 ultimate recovery from fields located by gravitational methods will exceed 

 one billion barrels of oil. 



GRAVIMETER PARTIES 



INJ 

 THE UNITED STATES 



Fig. 1. Gravimeter parties operating in the United States, 1938-1947 

 (E. A. Eckhardt, Geophysics, XIII, 4, Oct. 1948.) 



As may be seen by Figure 1, the number of gravimeter parties reached 

 a peak of 170 during 1945, declining to about 120 in 1947. Approximately 

 one-quarter of the gravity work during that year was done in the coastal 

 areas of Texas and Louisiana. | The geographical distribution of crews is 

 illustrated in Figure 2. 



Electrical Methods. — As early as 1720, Gray and Wheeler made elec- 

 trical studies of rocks and tabulated their electrical conductivities. Watson, 

 in 1746, discovered that the ground was a conductor and noted that current 

 passed between electrodes imbedded in the ground at a separation of two 

 miles acted erratically and in a different manner than when wire was used 

 to complete the circuit. 



The ore-finding problems of the mining industry led to the early study 

 of electrical phenomena in relation to mineral deposits. Robert Fox in 1815 



t E. A. Eckhardt, ibid. 



% Eckhardt, Geophysics, XIII, 4, October, 1948. 



