Subsurface Methods as Applied in Geophysics 



1063 



exceedingly small force, but it may be indicated by sensitive instruments 

 such as the torsion balance. Gravity prospecting is accomplished with 

 instruments that either compare the force of gravity between points on 

 the earth's surface, the gravity meter or the pendulum; or measure the 

 derivatives of gravity in a horizontal direction (gradient and curvature 

 quantity), the torsion balance. 



The unit of measurement in gravity prospecting is the milligal 

 which is roughly one-millionth part of the total gravity field. The measure 



■7 7 7 ■'——7—7—7' 



8= 1.7 



"^ ' 7 "^ ' 7 T-—7—ZP 7 T- 



Top Soil 8=1.7 



Top Soil 



Shale 



6=2.2 



X Basement "Sima" 8=3.3 X X X X X 

 XXXXXXXXXXXX 



X 



X 



X X 



Figure 560. Gravity vectors over a normal area compared with a disturbed area 

 showing change in magnitude and direction of vectors. (Size greatly exaggerated.) 



of gravity gradient is the eotvos unit, which represents a rate of change 

 of gravity equal to 10"^ gals per horizontal centimeter. 



Rock Properties 



Because geologic application of the gravity methods demands a hori- 

 zontal variation in rock density, many oil companies are making sys- 

 tematic studies of this property from cores and outcrop samples. In gen- 

 eral, it has been found that the density varies with geologic age, depth 

 of burial, and lithology. For example, a decrease of silica content in an 



