344 EXPLORATION GEOPHYSICS 



Both of these sets of type figures are pertinent in relation to interpretation, 

 and it is suggested that they be restudied at this time. 



Torsion Balance Traverse Across an Anticline. — A plan view of a 

 traverse of stations across an anticline showing the gradient and the curv- 

 ature values is given in Figure 196a. (Note: the stations are set in a 

 straight line, but in the figure they are staggered, so that each individual 

 gradient arrow may be seen.) 



The hypothetical structure depicted is a symmetrical anticline. The 

 gradient arrows point up the dip on the flanks of the structure and show a 

 zero value at the crest. The curvature quantity, in contrast to this, exhibits 

 a maximum at the crest of the anticline, where its direction is parallel to 

 the strike of the structure, and changes in direction by 90° where the 

 station is ofif structure in the adjacent syncline. For stations in the syncline 

 the magnitude of the curvature value is less than at the top of the anti- 

 cline. These diagnostic variations in gradient and curvature quantities can 

 be shown best by profiles. 



The magnitude of the gradient of gravity at a station expresses the 

 amount of the density difiference between light and heavy subsurface 

 masses. It also expresses the degree of slope of the subsurface density 

 contrast surface. Gradients are maximum over steep slopes (steep-sided 

 anticlines) and approach minimum where subsurface features are nearly 

 flat. Gradient arrows are analogous to geologic dip symbols, except that 

 they point up-dip. 



Profiles of Gradient.^ — The values of the gradient of gravity along 

 a traverse can be shown in the form of a profile. (Figure 196b.) The 

 gradient at a station is projected onto the line of profile, and the amount 

 of the projection is plotted as an ordinate at the station location. A certain 

 direction for the gradient is chosen as representing a plus value, and for 

 gradients in the opposite direction the ordinate is designated as minus 

 and so plotted. The gradient changes sign when an anticline is crossed 

 by the line of profile, as shown in Figure 196b. 



The ordinates on gradient profiles designated as 8 gr/^s represent the 

 rate of change of the gradient (symbol gr.) in respect to the profile direc- 

 tion s as indicated in the figure. 



Profiles of Curvature Quantity. — Profiles of curvature quantity 

 are made in a somewhat similar manner to the gradient profiles. A 

 form of projection is used to determine the amount of the ordinate at a 

 station. This amount is based on the formula : 



U^=\R\ cos2 <if (110) 



In the above |i?I is the magnitude of the R-line value at the station, and 



