352 



EXPLORATION GEOPHYSICS 



20 30 40 50 



GRADIENT IN E07V0S UNITS 



Fig. 203. — Conversion of torsion balance gradients into contour inter- 

 val of 0.0005 dynes for various scale maps. To construct new curve, 

 measure number of inches on particular map being used. Multiply this 

 number by values in right hand side of above tabulated values, and plot 

 against those on left hand side of table. 



If so desired, a scale may be constructed from the curve which gives 

 distance between contours but is graduated in terms of gradient units. 

 (Courtesy of E. V. McCollum.) 



Effects Produced by Simple Geometrical Forms and Structures. 



— A great number of petroliferous structures resemble approximately some 

 one of the simple bodies shown in Figure 204. A horizontal layer of 

 infinite extent alters the absolute value of gravity, but produces no effect 

 on the relative gravity, the gradient, or the differential curvature. Hence, 

 the presence of such an infinite layer tangent to the top or bottom of one 

 of the basic forms will not affect the observed anomalies ; i.e., the "equiva- 



