288 



EXPLORATION GEOPHYSICS 



correspondingly small. We may represent by the length of a line the dif- 

 ference in curvature between such sections ; namely, the sections of max- 

 imum and minimum curvature. 



The R-line value, or curvature quantity, as determined by the torsion 

 balance is, specifically, the amount of the difference in curvature of the 

 sections of maximum and of minimum curvature of the gravity equipotential 

 surface through the center of gravity of the instrument at the station of 

 observation. Equipotential surfaces of gravity follow the outline and con- 

 figuration of subsurface heavy masses. 



The general equation for the R-line value is : 



\ P mm. p max. / 



(27) 



R = R-line value; g = gravity; p = radius of curvature of gravity equi- 

 potential surface; min. = minimum; max. = maximum. 



Fig. 147. — Distribution of the R-line value 

 for a traverse across an anticline. Note that 

 R-line is maximum at crest of anticline and lies 

 parallel to its axis, and for stations in the adja- 

 cent syncline the R-line is rotated by 90°. 



Fig. 148. — R-line values from torsion bal- 

 ance measurements over a salt dome: maximum 

 R-line for stations above the edges of the dome; 

 weak erratic curvature quantity values for sta- 

 tions above the dome itself. 



The distribution and character of the curvature quantity or R-line value 

 for torsion balance traverses across a number of typical structural situations 

 are illustrated in Figures 147, 148, and 149. Figure 147 shows the case 

 for an anticline flanked by two adjacent synclines. 



Where stations are above the anticlinal portion of the structure, the 

 R-line value parallels the strike of the subsurface heavy mass. 



Where the station is above the syncline, the R-line lies in a direction at 

 right angles to the strike (of the adjacent anticline). This reversal by 

 90° in the R-line is one means of determining the change from an anticlinal 

 to a synclinal shape of the equipotential surfaces of gravity conformable 

 with the subsurface mass. The maximum value of the curvature quantity 

 is over the crest of the anticline. 



