CuAP. 7] GRAVITATIONAL METHODS 143 



1 or 2 milligals; for less accurate work and large anomalies, 5 or 10 milli- 

 gals. The results may also be represented as profile curves, preferably 

 in conjunction with a geologic section (Ag curve). 



G. Theory of Subsurface Effects; Methods of Interpretation 



1. General procedure in interpretation. The interpretation of gravity 

 anomalies has as its objective the determination of density, shape, and 

 depth of subsurface bodies. Theoretically this is an impossible task, as 

 the number of unknowns is greater than the number of equations. In 

 other words, an infinitely great number of mass arrangements may produce 

 the same anomaly in the gravitational field. In practice, however, con- 

 ditions are much more favorable. Some of the unknown quantities may 

 be determined (1) by the application of another geophysical method, pref- 

 erably one with depth control (seismic or electric) ; (2) by a consideration 

 of the geological possibilities in the area. Even after some of the unknowns 

 are thus determined, it still remains difficult to compute directly from the 

 results the shape or depth of subsurface bodies. The "direct" method is 

 applicable only (1) when geometrically regular forms are involved; (2) 

 when the gravity anomaly is of regular shape; and (3) when only one 

 disturbing body exists. The same considerations apply here as in the 

 interpretation of torsion balance anomalies. 



Usually a tentative or qualitative interpretation is first made by con- 

 sulting theoretical curves for a number of common geologic features, such 

 as vertical steps, anticlines, cylinders, and the like. For routine inter- 

 pretation it is of advantage to have an extensive file of such curves on 

 hand not only for various subsurface bodies but for as many variables 

 in each case as possible. Formulas and tables are given on pages 148-150 

 and 152-153, respectively, for such calculations and estimates. By con- 

 sulting curves based on these and by considering the geologic possibilities, 

 a fairly good picture of the subsurface conditions may generally be 

 obtained. 



This qualitative procedure may be supplemented by quantitative in- 

 terpretation which likewise is an indirect and a trial-and-error method. 

 Definite shapes, densities, and depths of subsurface bodies are assumed 

 and their effects are calculated. The results are compared with the data 

 obtained in the field, and the theoretical assumptions are changed until 

 a satisfactory agreement is obtained. Naturally, the complexity of these 

 computations increases with the number of geologic bodies which must be 

 considered in the interpretation of any given anomaly. 



For the calculation of subsurface effects, three methods are available: 



" For further details, see page 250 ff. 



