1064 



Subsurface Geologic Methods 



igneous rock or an increase in lime content of a sedimentary rock is gen- 

 erally accompanied by an increase in density. Older rocks generally have 

 a higher density than younger rocks. This probably is caused by a higher 

 lime content and greater compaction. Density shows a more or less consist- 

 ent increase with depth of burial. Barton ^° found that the density increased 

 at the rate of 0.07 grams per cubic centimeter per thousand feet of depth 

 to about 8,000 feet on the Gulf Coast. This increase with depth is due 

 in large part to greater compaction of the sediments with increasing load. 

 Several exceptions to the foregoing should be noted. A decrease in 

 density with depth will be encountered when (1) dense limestones are 

 underlain by sands and shales as in the Edwards Plateau region, (2) thick 

 salt beds are present at depth as in the Gulf Coastal Plain, and (3) when 

 diatomaceous material is encountered in the geologic section as in the 

 San Joaquin Valley, California. 



Density contrasts in sedimentary sections may vary from zero to 

 about 0.50 grams per cm.^, an average figure frequently used in gravity 

 calculations being 0.25. 



One of the principal differences between magnetic and gravity inter- 

 pretation is that magnetic anomalies, with few exceptions, originate 

 either at or in the basement rock, whereas gravity anomalies may represent 

 horizontal variations in density at any point within the geologic column, 

 including intrabasement variations. Gravity anomalies due to local struc- 

 ture are in many cases superimposed on larger anomalies caused by 

 variations of density within the basement complex. 



Earth's Gravitational Field 



Any measurements of gravity or its derivatives involve the normal 

 terrestrial gravitational field, with an additional effect caused by variations 

 in density within the earth's crust. Since geologic inference may be drawn 

 only from gravity variations associated with density changes in the under- 

 go Barton, D. C, The Science of Petroleum, New York, Oxford Univ. Press, vol. 1, p. 374, 1938. 



