418 



EXPLORATION GEOPHYSICS 



figure) was 1 inch, the maximum residual gravity value* was found to be 

 0.3 millidynes. Plot maps of approximately one inch to the mile were 

 used. In the second case, for a decagon of 1.5 inches radius (1.5 mile), 

 the maximum residual of gravity was —0.6 millidynes. For a 3-inch 

 radius (3 miles) for the decagon, it was —1.51 millidynes. 



Fig. 248. — Superposition of family of concentric octagons, as 

 applied to Figure 247. (W. R. Gritfin, Geophysics, Vol. 14, No. 1, 

 Jan., 1949.) 



These variations in the maximum residual gravity value illustrate the 

 importance of choosing the proper dimensions for the distribution figure. 

 A cut-and-try procedure may be necessary to obtain what appears to be 

 the most reasonable value. The decision as to the number and spacing 

 of the map locations at which regional gravity calculations or averages 

 are to be made usually is not a major factor in the final results. It is of 

 about equal importance with the number of gravity values entering into 

 the average. 



The particular geometric figure selected for the number and the distri- 

 bution of gravity values may be drawn to scale on a transparent sheet, as 

 indicated in Figure 248. This sheet may be positioned at different points 

 on the original gravity map of the area, and the point value of gravity may 

 be read from the contours which will be visible through the overlay sheet. 



* The residual (gravity) value at a station is the Bouguer gravity at the station 

 corrected for the regional effect. Therefore for a set of stations corrected for regional 

 gravity the maximum residual value would depend as to amount and sign on the par- 

 ticular regional correction applied. 



