278 



GRAVITATIONAL METHODS 



[Chap. 7 



I. 



II. 



torsion balance in the early spring of 1924. The results of the reconnais- 

 sance torsion balance survey were published by Barton. ^'^ The dome is 

 of the ''shallow" type, its cap coming to within 750 feet of the surface, 

 and it is characterized by a positive gravity anomaly. In the same year 

 the Long Point dome was found with the torsion balance; in 1925 the 

 Allen, Clemens, and Fannet domes were located. A study of a previously 

 known dome, Hoskins Mound, was made in 1926 with the object of deter- 

 mining the shape and approximately the sulfur reserves of the cap, which 



comes within 700 feet of the surface and is 

 about 500 to 600 feet thick. The inter- 

 pretation of the torsion balance data (Fig. 

 7-111) by D. C. Barton"^ is shown for 

 various cap rock sections in Fig. 7-112, 

 The estimated accuracy of prediction was of 

 the order of 10 per cent; larger deviations 

 may occur, since the interpretation furnishes 

 a smooth cap-rock surface while the depth 

 given by drill may correspond to a depres- 

 sion or crevice in the cap. On the Belle Isle 

 dome, St. Mary's Parish, Louisiana,'*^ agree- 

 ment between prediction and drilling was 

 not nearly so good as at Hoskins Mound 

 because of unfavorable (marshy) terrain. 

 According to Barton, the prediction of depth 

 to top of cap rock was 33 per cent correct 

 and prediction of its thickness only 58 per 

 cent correct. The specific gravities assumed 

 in the calculations were: 2.2 for the salt and 

 1.9 to 2.5 for the sediments down to 1600 feet 

 depth; from 1600 to 4400 feet, the density 

 of the salt was assumed to be the same as that 

 of the surrounding sediments. Torsion bal- 

 ance results have been published for several 

 other shallow Gulf coast domes. The Blue Ridge dome shows the familiar 

 pattern of gradients, indicating a gravity maximum over the cap rock, 

 which comes within 100 feet of the surface. ^^^ The Moss Bluff dome,^*^ 

 whose cap is at a depth of 650 feet while the salt is at a depth of 1170 



^so 



300 



350m 



t^g.'O' 



250 



iOO 



350 



Fig. 7-108. Relation of grav- 

 ity anomalies and salt depths 

 in Solikamsk district, Russia 

 (after Numerov). I, north of 

 Solikamsk; II, south of Solik- 

 amsk; III, near Berezniaky. 



»"D. C. Barton, A.I.M.E. Geophysical Prospecting, 416-466 (1929). 



"8 A.I.M.E. Tech. Publ., 719 (1936). 



"9 D. C. Barton, A.A.P.G. Bull., 16(11), 1335-1350 (Nov., 1931). 



"0 Jack Logan, Oil Weekly, Oct. 11, 1929. 



1" J. B. Eby and R. P. Clark, A.A.P.G. Bull., 19(3), 356-377 (March, 1935). 



