1062 



Subsurface Geologic Methods 



terrestrial gravitational force plus or minus the Newtonian attraction of 

 the disturbing mass, depending on the densities involved. 



The absolute direction of the gravity vector may not be measured 

 in the field although comparative differences in direction may be de- 

 duced by field methods. Relative measurements of the "deflections of the 

 vertical" could theoretically be used as an exploration tool ; to be of local 

 geological value, however, measurements would have to be made to an 

 accuracy beyond the scope of portable equipment. 



It should be recognized that measurements of the deflections of the 



Basement "Sima 6= 3.3 



Top Soil 



Basement Simo 



6=1.7 



8=3.3 



Figure 559. Gravity effects over a normal area in which no horizontal variation 

 in density occurs compared with gravity effects over a disturbed area. 



vertical are of importance in geodetic and large-scale geologic studies. 



The force of gravity may be measured directly by the use of pendu- 

 lums, although extreme care must be used in such observations to obtain 

 even moderate accuracy. The absolute gravitational force has been meas- 

 ured with the greatest accuracy at Potsdam, Germany, and the value de- 

 termined there has been carried to other points throughout the world by 

 comparative-gravity measurements. 



As the gravity vector is deflected by variations in distribution of 

 mass, it is evident that the total force of gravity may be resolved into 

 vertical and horizontal components. The horizontal component is an 



