420 



EXPLORATION GEOPHYSICS 



In Figure 250, the sides of the hexagon were 3 inches to the same 

 map scale, and as in the previous case average values w^ere calculated for 

 points at a 1-inch spacing. The two residual gravity (or local anomaly) 

 maps show^ certain differences, relating, however, more to details than to 

 the overall picture. They verify the earlier statement that the details of 

 the residual anomaly are influenced chiefly by the choice of dimensions of 

 the figure used. 



Fig. 250. — Residual gravity map of South Houston field using 

 (1) unit residual figure: hexagon of 3 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.) 



Smooth Contour Method. — One technique for differentiating between 

 the regional and the residual anomaly is a visual inspection procedure and 

 consists of drawing "smooth" gravity contour lines on the survey map, in 

 a manner and at a contour interval which appears to best represent the 

 regional gravity picture. After such a contour map of the regional gravity 

 has been prepared, the departure or difference between the observed gravity 

 contours and the regional contours is determined. If the "smooth" contours 

 are substantially correct, these departures are the residual values for the 

 points chosen, and become the local anomaly gravity values. The data 

 should then be transferred to another map and contoured. Both the regional 

 and the residual contour maps should be kept, for later restudy. 



In the preparation of the "smooth" contour regional gravity map, the 

 direction and spacing of the contours can be guided, in part, by the con- 

 figuration of the contours of the original survey for those portions of the 



