Chap. 7] GRAVITATIONAL METHODS 251 



wells, or from magnetic, seismic, and electric surveys. Even the pre- 

 liminary interpretation of a torsion balance map requires close cooperation 

 of the physicist and the geologist, or else the geologist must acquire a good 

 working knowledge of the theory of subsurface effects (that is, he must be 

 able to appreciate the physical possibilities), and the physicist must be 

 familiar with the geologic possibilities to avoid misinterpretation of the 

 results. In any event, it is advisable, wherever possible, to start a survey 

 in an area where geologic information from outcrops, well records, or 

 underground workings is available. 



The preliminary interpretation of an isogam or a gradient map will 

 often indicate the need for a revision of the map by allowing for a regional 

 gradient. How this correction is applied depends entirely upon the 

 geologic situation. It is a trial-and-error proposition and may require a 

 considerable amount of work, which, however, more than pays for itself 

 in the quantitative analysis. The qualitative interpretation of such a 

 corrected gradient or isogam map has as its first objective a delineation 

 of the areas which are structurally high and low or which represent oc- 

 currences of heavier and lighter masses. In this preliminary phase it is 

 quite permissible to consider an isogam map as the equivalent of a geologic 

 contour map. Subsequent quantitative analysis will then determine 

 whether the gravity anomalies are due predominantly to one or several 

 geologic features. It is evident that the deviation of the isogam map 

 from the equivalent contour map increases with the number of effective 

 geologic bodies or formations. 



Further determination of the type of geologic body or structure produc- 

 ing a gravitational high or low is possible by estimating its outline, strike, 

 dip, and approximate depth from the anomaly. The outline is given by 

 stations characterized by the longest gradient arrows, by a crowding of 

 the isogams, and by small curvature values (located between stations with 

 different directions of the R lines). The strike of geologic bodies may be 

 expected to be parallel with the trend of the isogams, at right angles to 

 the gradient arrows, and parallel with (or at right angles to) the R lines. 

 The dip of geologic bodies is frequently indicated by the isogam interval, 

 the length of the gradient arrows, and the magnitude of the R lines (com- 

 pare, for instance. Fig. 7-99h with Fig. 7-99f). Regional dip is indicated 

 by uniform magnitude and direction of gradient arrows through consider- 

 able distances. An indication of depth may be obtained from the rapidity 

 of changes in the gradient and curvature values in horizontal direction. 

 The type of change in direction and the magnitude of gradients and 

 curvatures in the direction of strike indicates to what extent geologic 

 bodies may be considered two-dimensional. Fortunately, most forma- 



