GRAVITATIONAL METHODS 



421 



area which are considered to be outside the local gravity anomaly. Judg- 

 ment must be exercised in selecting the limits of the area which is held 

 to be uninfluenced by the local anomaly. If too large an area is included, 

 beyond the local feature under study, other adjacent subsurface anomalies 

 may influence the observed gravity values, and hence the contours. For 

 example, in areas where anticlinal structures are narrow, such a feature 

 might be a nearby anticline ; or in a salt dome region, a neighboring dome. 

 If too small an area is used, the regional efifect will not be properly shown. 



No rigid rules can be set up to govern the drawing of such contours, as 

 each case must be judged on its merits. The general size and dimensions of 

 structures which are geologically reasonable in a locality will serve as a 

 guide to the probable extent of a local gravity anomaly. On this basis, the 

 width of the area outside the local anomaly and not influenced by it could 

 be assumed with fair safety to be roughly equal to that of the anomaly 

 itself. 



It is apparent that the personal r 69 w r ea w 



equation and the experience of the 

 interpreter will play a large part in 

 the application of the "smooth" con- 

 tour method. The character and use- 

 fulness of the local anomaly obtained 

 would thus depend to a considerable 

 degree on the skill and judgment of 

 the author of the "smooth" or hypo- 

 thetical regional gravity map. The 

 method is quite simple and has con- 

 siderable merit. It does not take an 

 undue amount of time, and in many 

 cases a more elaborate analysis of 

 gravity data is not justified. 



Profile Method. — A third way of 

 finding the regional gravity in an area 

 makes use of profiles of average 

 gravity along section lines in the 

 locality investigated, and is therefore 

 adapted to sectionized country.f In 

 unsectionized regions an arbitrary 

 grid pattern would be selected. 



In this profile method, the region- 

 al gravity effect is expressed in milli- 

 gals per mile, as determined from the 



* -f . , , , 1.1 Fig. 251.— Wellington Oil Field, Larimer 



profiles, m the north-south and m the County, Colorado, subsurface structural map 



. .J- .• <-ri . e contoured on Muddy Sand. (J. H. Wilson, 



east-west directions, ihe amount of Geophysics. Vol. 6, No. 3, July, 1941.) 



t J. H. Wilson, "Gravity Meter Survey of the Wellington Field, Larimer County, Colorado", 

 Geophysics, Vol. 6, No. 3, July, 1941, pp. 264-269. 



