250 GRAVITATIONAL METHODS [Chap. 7 



and others) for tracing the curve J f(x) dx, if the curve y = f(x) is given. 

 The error of closure in this method naturally depends much on how 

 closely the integrated gradient curve approaches the true gradient varia- 

 tion. In any event, some sort of an adjustment of the errors of closure 

 is required, as in the other two methods. In areas where gradients are 

 very erratic and near-surface anomalies overshadow deeper effects, a 

 better isogam picture may be obtained by arranging the stations in clusters 

 of three or four, calculating the average vector for the center of gravity 

 of the station polygon, and using this average vector for the isogam 

 construction. 



F. Theory of Subsurface Effects, Interpretation Methods 



1. Interpretation methods. Interpretation of torsion balance results is 

 based on gradient and curvature maps, isogam maps, or gradient and 

 curvature profile curves constructed as described in the last section. 

 Whether all of these maps and curves or only some of them are used de- 

 pends entirely on the nature of the geologic objects under survey. In 

 oil exploration, a representation of torsion balance results in the form of 

 a gradient, a curvature, and an isogam map is best suited for interpreta- 

 tion. If such a gravitational survey ^reveals definite geologic units (such 

 as an anticline, a salt dome, or an intrusion), it is better to plot the results 

 as profile curves, since they lend themselves better to quantitative analysis. 

 Gradient profiles are usually more reliable than curvature profiles. In 

 rugged country the latter are often disregarded entirely. 



Interpretation methods are qualitative, semiquantitative, or quantita- 

 tive, depending upon the nature of the survey, the complexity of the 

 geologic situation (number of effective geologic bodies), and, generally, 

 the amount of geologic information available. Naturally the first step 

 in almost every torsion balance survey is preliminary interpretation by 

 inspection. For this purpose gradient and isogam maps are most suitable. 

 From the appearance of the indications it is possible to determine whether 

 the geologic features mapped are extensive or of local significance, whether 

 they are of three-dimensional proportions or extended in the direction of 

 strike, whether they occur at great or at shallow depths, whether their 

 outline is well defined or gradual, and so on. An observer acquainted 

 with the regional geologic possibilities will be able to arrive rather rapidly 

 at a preliminary interpretation of the geologic significance of the anomalies. 



The reliability of these preliminary findings is enhanced considerably 

 if local geologic or geophysical information is available from outcrops and 



I'l J. Koenigsberger, A.A.P.G. Bull., 14(9), 1222 (Sept., 1930). 



