GRAVITATIONAL METHODS 



361 



EXAMPLES OF TORSION BALANCE SURVEYS 



Salt Domes. — A major salt dome usually produces a large, clearly 



defined density anomaly. Salt domes in the Gulf Coast usually com- 

 prise a frustrum of a cone of salt capped by a cylinder or thimble-like mass 

 of lime rock-anhydrite-gypsum, intruded into 20,000 to 30,000 feet of Ter- 

 tiary and Cretaceous sands and clays, t The diameter of the top of the salt 

 core is usually between 1 and 3 miles ; the height of the salt core above 

 its base is 3 to 6 miles ; and the difiference between the densities of the 

 salt and the surrounding beds range from to +0.2 grams per cubic 

 centimeter at the surface to — 0.5 grams per cubic centimeter (estimated) 

 at a depth of 20,000 feet. The vertical thickness of the cap is usually be- 

 tween 200 and 500 feet, but on a few domes it varies between 900 and 

 1000 feet. The difiference between the density of the cap and the surround- 

 ing sediments is + 0.5 to + 0.7 grams per cubic centimeter. 



well into salt 



wcll stopped in cap-rock 



PRODUCTIVE OILWELU 

 JOCCEOTVOS UNITS) 



1000 METCRS 



Fig. 210. — Gravity maximum over a shallow salt dome (Nash dome) lo- 

 cated in the Gulf Coast, Brazoria and Fort Bend Counties, Texas. (Barton, 

 A.I. MM. Geophysical Prospecting, 1929.) 



A characteristic shallow salt dome in the Gulf Coast produces a com- 

 posite anomaly consisting of a small maximum within a large minimum. 

 The cap, due to its relatively large positive density, produces a gravity maxi- 

 mum. The amplitude of the maximum, which is of the general order of 

 0.6 milligals, is large relative to the regional variations of gravity. Also, 

 the maximum lies directly above the top of the dome and is only slightly 

 wider than the cap. 



t D. C. Barton, "Gravitational Methods of Prospecting," Science of Petroleum, Vol. 

 374-375. 



I. 



