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Contrarily to the beam in an ordinary balance, the beam in a torsion balance resolves in a horizontal plane and is reflected from a position corresponding to the torsionless condition of the suspension wire by the unbalance of horizontal forces acting on it.
The beam with two weights attached to its ends at different levels is most frequently used in practice. 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. And the detection of the beam depends on its azimuth, the action of gravity forces on it may be changed by rotating the entire instrument in a different direction. To determine the five unknown quantities, five azimuths are therefore required. To shorten the observations time (20 to 30 minutes in each position), two beams are mounted side by side in antiparallel arrangements. The second beam adds its torsionless position as sixth unknown, so that three positions separated by angles 120° are required to determine all quantities.
Torsion balance interpretation may be qualitative and quantitative. In the former, gradients are given preference over curvature values. The 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, edges of salt domes, igneous intrusions. 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.
(C.A. Heiland. Geophysical Exploration. New York, 1940)
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