272 



GRAVITATIONAL METHODS 



[Chap. 7 



Fig. 7-103 illustrates the results of very careful measurements made by 

 B. Numerov*^^ on Lake Shuvalovo near Leningrad, which proved to be a 

 suitable study object since its maximum depth is only 13 meters. In its 

 middle a sand ridge comes within 1 meter of the surface. There is an 

 almost complete identity of the isogams and isobathic lines. Since in the 

 formula Agr = 2kir8h, the factor 2Trk is equal to 42 E.U., the variation in 



(Oo£ 

 Fig. 7-1036. Isogams and gradients (in 0.01 milligals) on Lake Shuvalovo. 



depth of the lake bottom may be predicted from Ah = A A^- 10/425. 

 The density difference was one. An anomaly of 1.37 milligals corresponded 

 to the sand bank previously mentioned. 



2. In oil exploration the torsion balance has found undoubtedly its 

 greatest commercial application. Its possibilities for mapping subsurface 



'" B. Numerov, Zeit. Geophys., 5, 276-289 (1929). 



