GRAVITATIONAL METHODS 



347 



values are at the crests of anticlines and over the edges of salt domes 

 where the curvature contrasts are maximum. The gravity value as it relates 

 to curvature quantities is a function of the degree of density contrast 

 present in the subsurface. 



ANTICLINE 



FINAL 

 PLOTTING 



FINAL 

 PLOTTING 



-6; 2U,u 



2Ux 



Fig. 198. — Plotting the R-line value for two stations, Station A taken 

 over an anticline and Station B over a syncline. This shows the change in 

 direction by 90° of the R-line value in the two cases. 



The reversal of the direction of the R-line by 90° is a point of 

 importance, as it furnishes a criterion as to whether a station is above the 

 anticlinal portion of a structure or above the synclinal portion. Take, for 

 example, two stations A and B, the first over an anticline and the second 

 over a syncline with the following values for the ^a quantities. 



n 



i^r 



fa) SHALLOW STRUCTURE 



(fa) DEEP STRUCTURE 



(c) UNSYMMETRICAL 



STRUCTURE 



Fig. 199. — Illustrating the effect of (a) a shallow structure, (b) a deep struc- 

 ture, and (c) an unsymmetrical structure, on the amplitude and shape of profiles of 

 the gradient of gravity. 



For station A the U^ = -8 E. and for station B the V ^ = +8 E. For 

 both stations it is assumed that the 2Uxy values = 0. As demonstrated in 

 Figure 198 (A) and (B), the reversal by 90° in the direction of the R-line 

 value takes place. 



As illustrated by Figure 199 (a) and (b), symmetrical subsurface 



