GRAVITATIONAL METHODS 



267 



across a granite ridge would show a gravity maximum over the crest of 

 the ridge. The general shape of the Ag'^o curve would correspond to the 

 shape of the (heavy or more dense) subsurface ridge. This is shown in 

 Figure 126. 



Across a syncline the Ag'^o curve follows the shape of the syncline, with 

 a gravity minimum over the synclinal axis. In like manner, a fault which 



TRAVERSE DISTANCE 



TRAVERSE DISTANCE 



','<'"./' 



' / / 't ' j ) ' / '' , I ' '' 1 /J^*^ * *J 

 ►■•■ V^'^^^^>'*^?'^^'^~'SVNCLINE ,. '. 



Fig. 126. — Ag"o profile across a granite 

 ridge: maximum gravity values over crest 

 of ridge. 



Fig. 127. — Ag"o profile across a syncline; 

 minimum value of gravity on synclinal axis. 



/ 



TRAVERSE DISTANCE 



/.FAULT 



TRAVERSE DISTANCE 





Fig. 128. — Ag"o profile across a fault. 

 Gravity values follow outline of subsurface 

 heavy mass. 



Fig. 129. — Ag"o profile across a salt 

 dome which has no cap. This geologic fea- 

 ture shows as a gravity minimum. 



brings a dense rock closer to the surface, on the up-throw side, would show 

 a profile of t>.g" o that follows the shape of the dense basement rock quite 

 closely. These situations are illustrated in Figures 127 and 128. 



A salt dome, which is composed of salt with a density of about 2.2 in 

 a host rock which usually has a density of 2.4 or more, would create a local 

 deficit in gravity force and would show as a gravity minimum on the 

 Ag" curve. See Figure 129. The case here considered is for a salt dome 

 without a heavy caprock, which sometimes modifies the overall effect, as 

 will be described later. 



