GRAVITATIONAL METHODS 423 



for elevation, Bouguer effect and latitude as usual, but no regional correc- 

 tion has been applied. The map shows the outline of the Wellington Oil 

 Field as traced by the lowest closing structural contour of the preceding 

 figure. 



It is apparent from the observed gravity map that the exact location and 

 shape of the Wellington Anticline is masked to a large extent by the 

 regional effect. The anticline is expressed by a persistent flexure or wrinkle 

 in the observed gravity contours. This flexure appears to run from north 

 to south across the survey area. The values also show a steady increase 

 in gravity force toward the south or toward a known large regional 

 gravity maximum. The Wellington structure lies on the north flank of 

 this regional gravity feature. The regional gravity effect is superimposed 

 upon and tends to mask the local gravity anomaly arising from the smaller 

 (oil field) subsurface structure. 



To evaluate the regional effect, averages were taken at equally spaced 

 stations (section corners) across the map, along the north-south and the 

 east-west section lines. Contour values were used where stations were 

 missing at the regular spacing. These average values were plotted in profiles, 

 as shown in Figure 252. If desired, straight line approximations may be 

 used, as shown by the dotted lines. In some cases, curved line smoothing of 

 the profiles is advantageous. The profiles indicated that the regional gravity 

 in this area increased to the south at the rate of 2.2 milligals per mile. The 

 east- west component of the regional effect was zero. 



The resulting residual gravity map, after the above regional effect had 

 been subtracted at each station, is presented in Figure 253. It reveals 

 a closed local gravity anomaly which is in good general agreement, as to 

 shape, size, position, and the strike of its axis, with the known Wellington 

 Anticline disclosed by drilling. It is noteworthy that the highest closing 

 gravity contour is very nearly in the same position as the highest closing 

 structural contour. This example shows the advantage of correcting for 

 the regional gravity effect. 



As evidenced by the examples given, it is apparent that local gravity 

 anomalies are related to local subsurface structures, and that the regional 

 gravity effects express the larger scale regional subsurface conditions. It 

 has been shown that often the regional effect can be approximated and 

 applied as a correction to permit the residual anomalies of gravity to depict 

 more clearly the local subsurface features. 



It should be recognized that a lateral change in the density of sub- 

 surface formations or in the basement rocks can also give rise to a gravity 

 anomaly. Such a lithologic density variation may or may not be meaningful 

 in terms of local gravity anomalies or structures. Wilsonf notes that the 

 marked increase in gravity southward from the vicinity of the Wellington 

 Field agrees with a large regional gravity maximum. This maximum, on the 



t J. H. Wilson, loc. cit. 



