16. Read the text, do the exercises. Gravity Method

Variations in the gravitational field may be mapped by the pendulum, gravimeter, and torsion balance. The pendulum and gravimeter measure relative gravity, whereas with the torsion balance, the variations of gravity forces per unit horizontal distance, also known as “gradients” of gravity are determined. Since gravitational effects of geologic bodies are proportional to the contrast in density between them and their surroundings, gravity methods are particularly suitable for location of structures in stratified formations.

Pendulum methods.

Pendulum may be used to determine not only time but gravity as well. Gravity pendulums are kept as contrast as possible in length so that variations in period indicate changes in gravity only. The most common method for securing the necessary accuracy in pendulum observations is the “coincidence” or beat method whereby the gravity is compared with a chronometer or another pendulum of nearly equal period.

Gravimeters.

Pendulum, or “dynamic” methods of measuring gravity have been superseded recently by “static” or “gravimeter” methods in which gravity is compared with an elastic spring force. Mechanically simplest are the Threlfall and Pollock instrument (in which a thin horizontal quartz bar is suspended from a horizontal torsion wire), the Hartley gravimeter (containing a horizontal, hinged beam suspended from two helical springs), the Lindblad-Malmquist and the Askania gravimeters in which the masses are suspended directly from springs, with arrangements for electrical or similar sensitivity may be attained by providing “astatizing” mechanisms which involve the application of a labilizing force nearly equal and opposite to elastic restoring force. For example, the Ising gravimeter, in which a vertical quartz rod is suspended in inverted position from a taut horizontal quartz fiber.

Corrections on gravity values observed with pendulums and gravimeters.

The following corrections must be applied on relative gravity values: (1) a correction for normal variations of gravity (planetary effect); (2) terrain correction; (3) free-air and Bouguer (elevation) correction. The planetary correction is due to the variation of gravity with latitude. The effect of terrain is calculated from elevations along radial lines and concentric circles around the station. Elevation is allowed for by a reduction to sea level (free air correction) to which the influence of the rocks between station and sea level is added (Bouguer reduction).

Torsion balance.

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)