GRAVITATIONAL METHODS 



353 



lent forms" of Figure 204 produce anomalies which are identical with 

 those of the corresponding basic or primary forms. 



The gradient and differential curvature anomalies produced by certain 

 simple structures are shown in Figure 205 where the curves are drawn 

 for the case that the structures are denser than the surrounding media. 

 (If the anomalous geologic structures are less dense, the anomalies will 

 have the same form but will be inverted.) 



PRIMARY rORMS 



^^ ^^^???^ '^^^ 



z^^. 



EQUIVALENT FORMS 



r - r r / yj U //t t 



tr >>> f flf/f/z^/n 





" Lj-L^ y//>^ 





""^^ 





f/// 7 V//> 



Fig. 204. — Simple geometric forms which correspond to many common types of geologic structure. 

 (Barton, The Science of Petroleum.) 



Convenient rules of thumb for determining the approximate depth to 

 the top of the geologic structure are given by the following two relations : 

 (1) for fairly symmetrical anomalies, e.g., type A, the horizontal distance 



mHIi" 



<iZ^^^22^ 



''^Z^^^ 



Fig. 205. — Anomalies produced by bodies of simple geometric form. 



A. Relative gravity and gradient profiles for a plate-like prism. 



B. Relative gravity and gradient profiles for a prism of triangular cross section, apex up. 



C. Relative gravity and gradient profiles for prism of triangular cross section, apex down. 



D. Differential curvature profile for horizontal plate-like prism. 



E. Differential curvature profile for prism of triangular cross section, apex up. 



F. Differential curvature profile for prism of triangular cross section, apex down. 



(Barton, The Science of Petroleum.) 



