348 EXPLORATION GEOPHYSICS 



features yield symmetrical gradient profiles. The deeper structures give 

 gradients of smaller values. Lack of symmetry in a subsurface structure 

 is manifested by a greater amplitude to the gradient profile on the steep 

 dip side of the subsurface feature and by a shift of the zero in the gradient 

 toward the gentle dip side of the structure. (See Figure 199c.) 



It has been shown in Figures 196 and 197 that curvature values are 

 parallel to the strike of subsurface heavy masses when stations are above 

 these masses. The R-line values however are at right angles to the strike 

 when stations are away from uplifted portions of such heavy masses. This 

 idea can be better expressed by saying that R-line values parallel the 

 strike when the curvature of the equipotential surfaces, conforming to 

 subsurface heavy features, is positive. They are perpendicular to the strike 

 when the equipotential surfaces are negative, as discussed on page 288. 



General Rules of Interpretation. — As illustrated in Figure 196, the 

 gravity gradients point toward the highest elevation of the heavy masses 

 or structures in the subsurface. The maximum gradient usually corresponds 

 to the steepest outline of these masses (or of the boundary surface of light 

 and heavy masses). Zero points of gradients, or points where their direc- 

 tion changes 180°, are places where the efifect of the left side of a heavy 

 mass equals that of the right side, i.e., usually above the center of the mass. 

 However, localities of maxima in gradient or of zero points may shift 

 considerably over the deeper heavy masses, and also over unsymmetrical 

 masses. (See Figure 199, a, b, and c.) 



Theoretically the curvature values would be zero above a fiat density 

 contrast surface as far as the contribution of that feature itself is con- 

 cerned. In fact the outlines of heavy masses 

 — *■ — *- —*■ — ^ that are level for considerable distances have 



.■:;:.: ^^;:;■.•;^.^>;^•;^:^■>;>>^-'a^' 1^0 clTect on Ag'^o, gradient, or curvature values. 

 :^:;:v';v^ii!:i^^^^ If a density contrast surface is flat, but has 



j1>HJ7^^-^> '■{";'^','Jv~4.^-jV a dip as in a monocline, it would produce a 

 / '-, ' -^ '/ -/ -^ ' -^ - (^ - ' gradient and a change in the value of ^g'^o, 

 Fig. 200.— Theoretical case for a ^y^ theoretically it would Still uot influence 



flat density contrast surface with -' . , . . ... 



uniform dip. No R-line value the curvatui'e quantity. This situatiou IS pic- 



results from this type of feature, , . ~^. or>/-\ 



considered alone; gradients only turcd m T IgUre ZUU. 



are caused, as indicated. 



Regional Effects. — The measured value 

 of the gradient and the curvature quantity at a torsion balance station or a 

 series of such stations is the sum of all the gravitational elTects acting on the 

 instrument. It will include the local terrain, the distant topography and 

 even such items as buried boulders near the station site. It will be influenced 

 by the configuration of shallow density contrast surfaces and also of deep 

 ones, possibly a number of thousands of feet deep. 



The deep density contrasts may represent regional effects such as the 

 slope of the flank of a buried granite ridge or the floor of a basin. Where 

 regional efifects are strong, the local structural features may be obscured 



