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EXPLORATION GEOPHYSICS 



ridge in Figure 126, is related to subsurface configuration of materials of 

 different densities. Specifically, the gradient reflects the shape and slope 



of the contrast surface of light versus 

 heavy material at depth, i.e., the granite 

 against the sediments in the illustration. 

 A plan view of the distribution of the 

 gravity gradients in vector form for a 

 traverse across a granite ridge is shown 

 in Figure 141. The gradient arrows 

 across a syncline and across a fault 

 appear in Figures 142 and 143. 



In the case of a salt dome, as illus- 

 trated in Figure 144, the gradients 

 radiate outward toward the relatively heavy sediments surrounding the 

 dome. If, as is often the case, the salt dome has a heavy cap rock, the 

 direction of the gradient arrows may be controlled by it; they may then 

 be reversed, and converge to a point over the center of gravity of the salt 

 plug and cap rock. As indicated in Figure 145, at stations well removed 



Fig. 141. — Arrangement of gradient ar- 

 rows for torsion balance traverse across a 

 granite ridge. Note zero gradient at crest of 

 ridge. 



Fig. 142. — Showing distribution of grad- 

 ients of gravity across a syncline. Gradient 

 arrow is like a dip symbol, pointing up-dip. 



Fig. 143. — Gradient arrows for traverse 

 across a fault. The gradient shows a maxi- 

 mum value for the station over the fault. 

 At this position, the maximum rate in change 

 of gravity occurs. 



Fig. 144. — Gradient arrows radiating out 

 from the crest of a simple salt dome. They 

 point toward the relatively dense sediments sur- 

 rounding the dome. 



Fig. 145. — Gradients over a salt dome with a 

 dense cap rock. Above and near the dome, the 

 gradients are directed to the center of the dome 

 due to the controlling influence of the cap rock. 

 At a distance from the dome, the gradients 

 reverse and point to the more dense surrounding 

 sediments. 



