290 



EXPLORATION GEOPHYSICS 



greatly modify the results. The map showing the gradient and curvature 

 values over an anticline in northern Mexico is given in Figure 150 as a 

 summary example. 



- CONTOURS ON LIMESTONE 



- GRADIENTS OF GRAVITY 



- VALUE OF CURVATURE 



- FAULT 

 PRODUCING WELL 

 DRY HOLE 



SECTION 



Fig. 150. — Gravity gradient and curvature values over an anticline in northern Mexico. 

 (Courtesy of the American Askania Corporation.) 



Density Determinations. — The importance of density of rocks and 

 formations and of density contrast surfaces has been brought out in the 

 preceding section. The density of any material is defined as its mass divided 

 by its volume. One common method of determining relative densities, in 

 the laboratory, is to weigh the sample first in air, and then in water to 

 find its volume. The difference in weight in air and in water gives the 

 weight of the volume of water displaced, hence the volume. (1 cubic cen- 

 timeter of water weighs 1 gram.) 



To apply the terrain correction nec- 

 essary at a torsion balance station, the 

 density of the surface material must 

 be determined or estimated with fair 

 accuracy. The equipment used in one 

 very simple method of measuring den- 

 sity of the surface material in the field 

 is shown in Figure 151. 



(a) 



Fig. 151. — (a) Metal cylinder with han- 

 dle, used ill taking sample of soil for density 

 determination, (b) Cloth sack used to find 

 volume of material. 



A light steel cylinder or tube, about 18 

 inches long and 6 inches in diameter, fitted 

 at its top end with a handle-bar, is turned 

 and pushed into the ground to a depth of 



