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an instrument for the relative measurement of the acceleration due to gravity. Most gravimeters consist of accurate spring or torsion balances.
Differences in the acceleration due to gravity are measured by means of such gravimeters according to the change in the deformation of a spring or the angle of twist of an elastic thread that are compensating the force of gravity on a small weight. Measurements are made first at a starting point, for which the acceleration due to gravity is known, and then at the point being studied. The main difficulty in constructing a gravimeter is the need to provide precise measurement of small elastic deformations under field conditions. Optical, photoelectric, capacity, and inductive means of recording are used. Gravimeters are used that are based on measurement of the change in frequency of vibration of a string from which a weight is suspended or of the change in rate of precession of gyroscopic instruments because of different forces of gravity at the different points of measurement. The best gravimeters are sensitive to within a few ten-thousandths of a milligal.
There are special gravimeters for measuring the force of gravity at the bottom of shallow bodies of water, in surface and submarine vessels, and in airplanes. For measurements from moving objects, gravimeters are equipped with auxiliary apparatus that records the inclination of the base of the instrument and the acceleration caused by rolling. Some gravimeters make possible the continuous recording of solar-lunar variations in gravity over a period of several months. To calibrate gravimeter readings, measurements are made at points for which the difference in the acceleration due to gravity is known or at a single point with different inclinations of the gravimeter.
Surface and borehole gravimeters provide measurements of the acceleration of gravity that are accurate to 0.01 milligal; sea-bottom gravimeters, to 0.05 milligal; marine shipmounted gravimeters, to 0.5 milligal; and airborne gravimeters, to 5 milligals.
REFERENCESLukavchenko, P. I. Gravimetricheskaia razvedka na neft’ i gaz. Moscow, 1956.
Veselov, K. E., and M. U. Sagitov. Gravimetricheskaia razvedka. Moscow, 1968.
Spravochnik geofizika, vol. 5. Moscow, 1968.
P. I. LUKAVCHENKO and M. U. SAGITOV