GRAVITATIONAL METHODS 263 



Geodesists have made searching investigations to determine the causes 

 of the deviation from normal of the plumb line and the variation of gravity 

 along parallels of latitude. It was readily realized that the land masses 

 above sea level and the deficiencies in density due to water in ocean basins 

 as compared with equal volumes of surface rock would cause irregu- 

 larities. But even after corrections are applied for the effects of these 

 excesses and deficiencies of mass, there are anomalies. The causes of these 

 irregularities must lie in the outer portion of the earth because stations 

 only short distances apart are affected differently. Consequently the 

 causes of these effects must lie at depths comparable to the distances 

 between the stations. In other words, there must be abnormal densities 

 of materials in the outer portion of the earth. These abnormal den- 

 sities at relatively shallow depths are the variables of interest in gravi- 

 tational surveying. 



Geodesists have made computations on the theory that at some depth 

 below sea level the pressure for a unit area is the same, i.e., does not 

 depend on the density of the rocks near the earth's surface. The compu- 

 tations have confirmed this assumption of isostasy to a remarkable degree. 

 Thus, it has been shown that at least a greater portion of the abnormal 

 densities occurs in the outer 60 miles of the earth. By these isostatic com- 

 putations the anomalies in the goedetic data have been reduced to from 

 1/7 to 1/10 of what they would have been if there were no such com- 

 pensation as the theory of isostasy postulates. These small residual anom- 

 alies are believed to be caused by rapid changes in density of the materials 

 at a shallow depth and therefore are properly the object of investigations 

 in applied science, because the variations in density producing these anom- 

 alies are caused by the juxtaposition of the different rocks close enough 

 to the surface to interest the geophysicist and geologist. 



DENSITIES OF MATERIALS 



The applicability of the gravitational methods depends on the existence 

 of detectable differences in density between a subsurface body and the sur- 

 rounding medium. The density in grams per cubic centimeter of some of 

 the materials found in the earth's crust are given in Table 7. The table 

 affords only a general survey. Densities of materials in a particular region 

 are preferably obtained from measurements made on samples taken from 

 that region. 



In a majority of petroliferous areas the density increases with depth. 

 The increase in the Gulf Coast of the United States is 0.07 per thousand 

 feet to depths of about 8,000 feet.f In applying gravimetric work to locate 

 possible oil structures, it has been found that superimposed upon this gen- 

 eral increase of density with depth, there are local variations that may be 

 greater or less than the normal values. For instance, a decrease in density 

 is found: (1) if a massive salt series is present at considerable depth; (2) 



t D. C. Barton, "Gravitational Methods of Prospecting," The Science of Petroleum (Oxford 

 Univ. Press, 1938), Vol. I, p. 374. 



