424 



EXPLORATION GEOPHYSICS 



basis of the known geology of the area, is thought to relate to density varia- 

 tions in the basement rock. 



It follows from the above that if a gravity anomaly of regional signifi- 

 cance can be caused by a density change in the basement or in the sub- 

 surface section, so can a local gravity anomaly be caused in the same manner. 

 The difference, in general, between regional and local gravity features or 



R 68W 



N 6'q 



eUTLlNC or WU.LINCTON STdUCTI 



Fig. 253. — Adjusted gravity contoured at 0.2 mg. interval. Regional grad- 

 ient of 2.2 mg. per mile south has been subtracted from observed gravity, 

 Wellington Field, Larimer County, Colorado. (J. H. Wilson, Geophysics, 

 Vol. 6, No. 3, July, 1941.) 



structural features is largely a matter of size and areal extent. There is 

 no reason, geologically, why density changes in shallow beds should not 

 cause local gravity anomalies. This possibility should be kept in mind 

 during interpretation of data in areas where there is a geological basis 

 for expecting such subsurface lateral changes. 



