242 



GRAVITATIONAL METHODS 



[Chap. 7 



10 



to its highest value, a negative peak occurs over the foot and a positive 

 over the upper edge of a continental shelf in isostatic equilibrium. For a 

 slope of about 1° inclination, Helmert calculated a positive peak of 53 

 milligals above the upper edge of the shelf, dropping to about 25 milligals 



100 km inland. Hence, a 

 I' coastward gradient is pro- 



duced along a wide zone 

 which for the slope men- 

 tioned is of the order of 3 

 E.U., increasing to 5 and 

 more units near the coast. 

 For shelves of steeper slope 

 the effect is correspondingly 

 greater. Like other regional 

 effects the coast effect may 

 be determined by direct ob- 

 servation or calculation and 

 may be deducted (vectori- 

 ally) from the observed gradi- 

 ents. 



5. Corrections for fixed 

 masses (other than terrain), 

 effects of underground open- 

 ings (tunnels and the like). 

 Corrections for fixed masses 

 other than terrain are re- 

 quired only in exceptional 

 cases when setups near them 

 cannot be avoided. Fre- 

 quently it is sufficient to cal- 

 culate their effects by assum- 

 ing point masses, linear 

 masses (trees), slabs, or para- 

 llelopipeds. The formulas 

 given for these in the section 

 on interpretation (pages 258- 

 265) are applicable. If several 

 fixed masses are present, it is often possible to reduce or virtually avoid 

 a correction by setting up the instrument so that their effects are com- 

 pensated. On gradients two or more equal masses in symmetrical dis- 

 position cancel; on curvatures three symmetrical, equal masses compensate. 

 In underground work a similar precaution reduces corrections for tunnel 



Fig. 7-88. Lines of equal gradients in infinite 

 rectangular tunnel section (after Meisser). 



