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EXPLORATION GEOPHYSICS 



The lower half of the salt core, due to its relatively lower density, 

 produces a gravity minimum. The amplitude of the minimum depends on 

 the diameter of the salt core and the downward flare of the flanks. In the 

 Gulf Coast it usually varies between 2.5 and 3.5 milligals, but may reach 

 7.0 milligals. The values of the gradients of these salt dome minima are 

 of the same general magnitude as those of regional features. Hence, the 

 center of the minimum may be shifted considerably. Most shallow domes 

 show both the maximum and the minimum ; one or the other, however, 

 may be too small to be detected. 



A deep salt dome produces a gravity minimum similar to that pro- 

 duced by the lower half of a shallow salt dome. The gravity anomaly 

 produced by the cap of a deep dome is not detectible. 



Fig. 211. — Isogams of Esperson salt dome minima. (Barton, The Science of Petroleum.) 



Figures 210 and 211 show the gravity maximum of a shallow salt dome and the 

 minima of a deep dome respectively. In Figure 210, the gradient arrows are super- 

 imposed on the structural contours on the top of the cap-salt. The Nash dome is of 

 particular interest as the first Gulf Coast salt dome to be discovered by geophysical 

 methods. It was predicted from the gradient arrows shown. The two heavy dashed 

 lines are 500 — 900 feet and 4,000 — 5,000 feet contours on the cap-salt predicted prior 

 to any drilling. The convergent gradient arrows well away from the dome are presum- 

 ably due to a large minimum to the west, north, and east rather than to the Nash dome. 



Figure 211 shows isogams due to (a) the minimum of a deep salt dome, Esperson, 

 the top of whose salt core lies at a depth of 7,000 feet; (b) part of the minimum 

 around a shallow dome, South Liberty, Texas, the top of whose cap is less than 320 feet 



