CuAP, 2] 



METHODS OF GEOPHYSICAL EXPLORATION 



15 



ing in the direction of maximum change of gravity (see Fig. 2-6) ; curva- 

 ture values are plotted as straight lines through the station, the length of 

 the line being in proportion to the deviation of the equipotential surface 

 from spherical. Torsion balance results may also be plotted in the form 

 of curves along profiles at right angles to the strike (see Fig. 2-6). Rela- 

 tive gravity may be calculated from gradients, and points of equal relative 

 gravity may be connected by "isogams." 



Torsion balance interpretation may be qualitative or quantitative. In 

 the former, gradients are given preference over curvature values. The 



Fig. 2-6. Convergence of equipotential surfaces of gravity caused by subsurface fault 

 and reaction of torsion balance beam of the second kind. 



largest gradients occur above such portions of subsurface geologic features 

 as are characterized by the greatest horizontal variation of density, for 

 example, on flanks of anticlines, synclines, edges of salt domes, igneous 

 intrusions, buried escarpments, and faults. Quantitative interpretation 

 is usually of an indirect nature; geologically plausible assumptions are 

 made about subsurface mass dispositions; their gravity anomalies are 

 calculated and compared with the field findings. Discrepancies between 

 the two are reduced step by step by modifying the assumptions regarding 

 depth, shape, and density of the subsurface bodies. 



