252 GRAVITATIONAL METHODS [Chap. 7 



tions and ore deposits are of two-dimensional character; they allow repre- 

 sentation of the field findings by gradient (or curvature) curves, and a 

 somewhat more quantitative interpretation. The following rules apply 

 in semiquantitative interpretation of anomalies due to two-dimensional 

 and some three-dimensional bodies: 



(a) If but one density contrast is effective, the Ag curve is approximately 

 parallel with the outline of the subsurface feature and the isogams represent 

 approximately its surface contours. 



(6) The gradient arrows point toward the highest point of the sub- 

 surface feature. Their magnitude is approximately proportional to the 

 rate of change of subsurface density in horizontal direction. Maxima 

 occur above points or areas of greatest dip; zero points occur above the 

 lines of symmetry of anticlines, vertical dikes, and the like. When a 

 gradient curve has only positive values, a subsurface feature rises. in one 

 direction only. The curve is symmetrical if there occurs a vertical face. 

 Positive and negative values are observed in the gradient curve if the 

 subsurface feature is limited across the strike. If the positive and nega- 

 tive gradient anomalies are symmetrical, the boundaries of the subsurface 

 feature are vertical on both sides or dip equally in opposite directions. If 

 the anomalies are unsymmetrical, the two boundaries dip equally in the 

 same direction (inclined dike) or unequally in opposite directions (anti- 

 cline with unequal flank dips). 



(c) The (curvature) R lines are parallel with the strike of heavier sub- 

 surface features above them, but they are small and perpendicular to the 

 strike beyond them (see, for instance, the curvature anomaly of a fault 

 block in Fig. 7-995). The magnitude of the lines depends in a general 

 way on the curvature in the outline of the heavier masses below the surface ; 

 or, if the outlines are straight for short distances, on the rate of change in 

 their direction. Hence, the curvature values are greatest above the center 

 of a dike or plug-shaped mass, above the crest of an anticline, or above the 

 trough of a syncline. Zero points of curvature values frequently occur 

 above abrupt subsurface changes from light to heavy masses. They 

 coincide sometimes with points of maximum gradient, and vice versa. 



In semiquantitative interpretation, extensive use is made of "type" 

 curves calculated for geologic features most frequently encountered, such 

 as vertical and inclined faults, vertical and inclined dikes and slabs, sym- 

 metrical and asymmetrical anticlines and synclines, and so on. How 

 these are calculated is further explained below\ An extensive file of such 

 curves for a variety of depths, dimensions, and dispositions of geologic 

 bodies is of considerable help in a preliminary analysis of torsion balance 

 data and reduces the work required for a quantitative analysis. 



