Chap. 7] 



GRAVITATIONAL METHODS 



281 



type were first confused with the cap effects of moderately deep domes. 

 DriUing these produced only negative results. As shown in Fig. 7-114, 

 interpretation of the Esperson anomaly gave 6000-8000 feet for the top 

 of the salt. It was actually encountered later at a depth of 7000 feet. 



In oil exploration the torsion balance has been equally successful in 

 surveys of nonsalt dome-type structures, although fewer results have been 

 publisliod. It was recognized at an early date that anticlines and domes 



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Fig. 7-111. Torsion balance gradient survey of Hoskins Mound salt dome 



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(after 



with heavier material in their cores would give rise to a positive torsion 

 balance anomaly. Fig. 7-115 shows the results of a survey made in 1915 

 to 1916 on the Egbell anticline by Eotvos and his associates."* The oil 

 occurs in the Sarmatian formations near the surface; denser Eocene and 

 Paleocene formations are assumed in the core of the folds. While the 

 maximum on the Egbell anticline is well defined, the Sasvar dome shows 

 only as a terrace superimposed on the regional gradient. Subsequent 



1" See footnote 130. 



