244 



GRAVITATIONAL METHODS 



[Chap. 7 



called the "sunflower." It consists of a graduated arm rotatable about 

 a horizontal axis, which is shifted until it makes contact with the 

 tunnel wall, thus giving the distance of the wall at any vertical angle 

 from the center of gravity of the torsion balance. The vertical angle 

 of the arm may be read on a dial. If the tunnel is not straight, or if 

 the shape of other subsurface cavities is to be determined, the sunflower 

 may be provided with a vertical axis permitting the variation of section 

 with horizontal azimuth to be obtained. 



E. Graphical Representation of Torsion Balance Data 



For convenience in interpretation, torsion balance results are plotted as 

 vectors or as curves. The corrections discussed in the preceding section 



are applied to the instrument 

 readings algebraically or vec- 

 torially. Geologic interpreta- 

 tion is based on the corrected 

 values represented by vectors 

 or by curves of gradient and 

 curvature components in ge- 

 ologically significant direc- 

 tions (usually at right angles 

 to the strike). For more 

 qualitative interpretation or 

 comparison with gravimeter 

 or pendulum data, relative 

 gravity is calculated from 

 gradients and represented in 

 the form of isogams. 



1. In vector representation, 

 the east gradient (see Fig, 

 7-9 la), Uyz, is laid off to the 

 east if it is positive and the 

 north gradient, f/«, to the 

 north if it also is positive. The resultant vector is the total gradient 

 dg/ds. Its azimuth, a, gives the direction of maximum change of gravity. 

 In plotting curvature values, —Ua is laid ofif toward north and 2Uxv 

 toward east. The resultant vector is the "differential curvature," R, and 

 it makes the angle 2X with north. Therefore, it must be replotted. In 

 doing so, it is customary to shift the R line on itself so that the station 



Fig. 7-90. Tunnel section. 



