GRAVITATIONAL METHODS 



419 



These values of gravity are added, and then divided by the number of 

 points read to obtain an average (or regional) value. This value can be 

 ascertained for as many specific map locations as are deemed necessary, or 

 for each survey station if desired. 



The observed gravity values are corrected by the average regional 

 values to obtain the residual or anomaly quantity. These latter values are 

 put on a new map of the area and contoured at an interval best suited 

 to express the resulting variation in the residual gravity. 



The above procedure is rigorous only in so far as the numerical averag- 

 ing itself is concerned. Tests can be made to select the best number of 

 gravity values to be averaged and their distribution, as well as the map 



Fig. 249. — Residual gravity map of South Houston field using 

 (1) unit residual figure: he.xagon of 2 inches per side; (2) residual 

 value every 1 inch; (3) contour interval, 0.2 millidyne. (W. R. 

 Griffin, Geophysics, Vol. 14, No. 1, Jan., 1949.) 



spacing of the locations where the regional gravity values are to be deter- 

 mined. Griffin found that it was safe to include gravity values within a 

 5-mile radius around a point where the regional effect was to be found. 

 Beyond that distance the presence of neighboring subsurface bodies may 

 alter the result. This finding applied to a salt dome area. 



Two residual gravity maps for the South Houston area are given in 

 Figures 249 and 250. In Figure 249, average or regional gravity values 

 were calculated, using a hexagon as the unit residual figure. Its dimen- 

 sions were 2 inches, or approximately 2 miles on a side. Regional correc- 

 tion values were determined at points 1 inch (1 mile) apart over the 

 area. 



