70 GRAVITATIONAL METHODS [Chap. 7 



However, differences in its direction between locations may be determined 

 ("deflections of the vertical"). The Eotvos torsion balance is used for 

 measuring the rate of change, or gradients, of gravity and of its horizontal 

 components in horizontal direction. In more common terminology, this 

 measures (1) the north gradient of gravity, (2) the east gradient of gravity, 

 (3) the difference in the maximum and minimum curvatures, and (4) the 

 direction of minimum curvature of an equipotential surface of gravity. 

 For the measurement of the vertical gradient, various instruments have 

 been suggested but they have not come into practical use. There are no 

 methods in present use to measure the potential of gravity; the pendulum 

 and gravity meters measure its first derivative and the torsion balance 

 its second derivatives. 



Instruments employed in pendulum exploration rather closely resemble 

 those developed for scientific purposes. The development of a geologically 

 useful gravimeter, attempted repeatedly since the turn of the century, is 

 largely the result of the efforts of commercial geophysicists. On the other 

 hand, the torsion balance, long known to physical science, has been adopted 

 by exploration geophysicists in the form developed by Eotvos, with com- 

 paratively minor changes. 



The pendulum and gravimeter have been used predominantly in oil 

 exploration to outline large regional geologic features, to determine base- 

 ment-rock topography, and to locate buried ridges. Owing to its greater 

 accuracy and rapidity, the gravimeter is being used for more local problems 

 of oil geology, for example, location of domes, anticlines, salt domes and 

 general structure. Attempts have also been made to use the gravimeter 

 in mining, primarily for large near-surface ore bodies, to supplement data 

 secured by other geophysical methods. 



The torsion balance has been used predominantly in oil exploration for 

 the determination of general geologic structure, mapping of basement 

 topography, and location of buried ridges, anticlines, domes, terraces, 

 faults, volcanic dikes, intrusions, and salt domes. In mining exploration, 

 the following problems have been attacked by the torsion balance : location 

 of iron, copper, and lead ore bodies, faults, dikes, veins, meteors, lignite 

 and barite deposits, and salt domes (exploration for sulfur and potash). 

 Indirectly the torsion balance has been of use in mining in the determina- 

 tion of the thickness of the overburden, mapping of buried channels, and 

 the like. 



II. ROCK DENSITIES 



A. Determination of Rock Densities 



A direct determination of formation densities in situ is possible from 

 gravitational measurements if the dimensions and depth of a geologic 



