GRAVITATIONAL METHODS 



289 



Around the edges of a salt dome, as given in Figure 148, the R-line 

 values radiate outward and are, in general, at right angles to the surface 

 trace of the edge. Stations located in the vicinity of the trace show large 

 values, but stations situated above the dome itself and inside the edge 

 trace are usually small and indeterminate in direction. This is logical, since 

 the top of a salt dome is, in general, rather flat wnth little curvature dif- 

 ference for different sections of the equipotential surfaces conformable to 

 it. The edge stations, however, express the sharp difference in curvature 

 between the more or less flat top of the dome and its very steep sides, 

 resulting in a large curvature quantity. 



The presence of a dense cap rock over a salt dome may not necessarily 

 alter the diagnostic characteristics of the curvature quantities considered 

 above. The R-line values express 

 differences in the curvature of the 

 equipotential surfaces of gravity 

 which follow the outlines and shape 

 of subsurface geologic structures, 

 such as anticlines, domes, etc. More 

 strictly considered, the equipotential 

 surfaces reproduce with subdued 

 outlines the density contrast surfaces 

 or planes due to the structures. 



The above points may be further 

 illustrated by a traverse of R-line 

 values across a fault, Figure 149. 

 The position of the fault is indicated 

 by a 90° change in direction of the 

 R-line. Such a reversal indicates that 

 for stations above the down-throw side the equipotential surfaces are 

 warped down (synclinal conditions), while for stations over the upthrow 

 side, these surfaces are warped up (anticlinal conditions or with R-Hnes 

 parallel to the strike). 



The R-line value, like the gradient, is given in two components from 

 torsion balance measurements. The resultant curvature quantity from the 

 north-south and the east-west components is plotted with its center at the 

 station on final maps, as shown in Figures 147 to 149 inclusive. The unit 

 in which differential curvature is measured is 1 • 10"^^ radians per centi- 

 meter, which expresses the second term in the right hand side of Equation 

 27. This quantity is multiplied by g, which is approximately 10^ c.g.s in 

 magnitude, so that R-line values come out in units of 10~®. This is numer- 

 ically, though not dimensionally, the same as the Eotvos unit. The R-line 

 quantities are plotted therefore in numerical equivalents of Eotvos units. 



The gradient and the curvature quantities illustrated in Figures 141 to 

 149 serve as a background of basic type cases. Actually, such simplified 

 structures are seldom encountered, and the regional effects may mask or 



Fig. 149. — R-line values for stations across a 

 fault. The presence of the fault is indicated by 

 a rotation of 99° in the direction of the R-line 

 quantity. 



