14 



METHODS OF GEOPHYSICAL EXPLORATION 



[Chap. 2 



S^cttcn of 



Surface of QravHif 



of (3rai/'/y 



In practice, only a beam of the second type is used. The gradients and 

 curvature values may be resolved into their north and east components. 

 Hence the torsion balance beam is affected by four unknown quantities, 

 to which is added a fifth, the zero or torsionless position of the beam. 

 As the deflection of the beam depends on its azimuth, the action of gravity 

 forces on it may be changed by rotating the entire instrument in a dif- 

 ferent direction. To determine the five unknown quantities, five azimuths 

 are therefore required. To shorten the observation time (20 to 30 minutes 

 in each position), two beams are mounted side by side in antiparallel 



arrangement. The second beam 

 adds its torsionless position as sixth 

 unknown, so that three positions 

 separated by angles of 120° are 

 required to determine all quanti- 

 ties. In present practice, double 

 beam instruments of the second 

 Eotvos type are used exclusively, 

 arranged either for visual observa- 

 tion of the beam deflection or with 

 full automatic recording mecha- 

 nism. Most recent torsion balances 

 carry beams suspended at an an- 

 gle of 45° (see Fig. 2-4c and d). 

 Calculation of gradients and curva- 

 tures proceeds in accordance with 

 formulas or nomographs based on 

 the fundamental theory of the in- 

 strument. 



Corrections on torsion balance 

 results. Torsion balance results 

 must be provided with a number 

 of corrections. Most important is 

 the terrain correction, which is obtained from elevations measured around 

 the instrument in a number of radial directions and along suitably selected 

 concentric circles. In rugged terrain, topographic corrections may become 

 involved and inaccurate, which limits the usefulness of the torsion balance 

 to fairly level country. A second (planetary) correction results from the 

 variation of gravity with latitude. Finally, it is often desirable to correct 

 for regional geologic structure. In torsion balance measurements under- 

 ground, allowance must be made for mass deficiencies due to tunnels, 

 drifts, and so on. 



Interpretation. In plan view, gradients are represented as arrows point- 



FiG. 2-5. Action of torsion-balance beam 

 of first kind in gravity field characterized 

 by cylindrical equipotential surface. 



