Gravimetry

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Gravimetry is the measurement of a gravitational field. Gravity g is usually measured by gravimeters in SI-units of acceleration abbreviated as m/s or in the older unit Gal = cm/s. The value of g refers to the gravity measured at Earth's surface and varies from place to place due to the density of matter. In addition the visco-elastic Earth is continously extensively deformed due to the gravitational forces on moon, sun and planets, variations of air pressure and tides. This deformation provokes variations in the distance between center of mass of Earth and Earth's surface, which can be measured as variations of the gravitational parameter and are called Earth tides. They have to be applied in the analysis of intercontinental baseline measurements based on VLBI and SLR-data. Two different instruments are used in gravimetry: a. the absolute gravimeter determining the absolute value of g by a free falling mass probe used in campaigns, b. the relative gravimeter determining the variations in g as Δg by continous monitoring of the Earth tides.

Gravimetry at TIGO

TIGO operates an absolute gravimeter FG5 since 2006 and the first superconducting (relative) gravity meter in Latin America called SG38 since 2002.

Gravimeter FG5 uses a free falling mass probe. While falling two spacial separated photoelectric barriers determine a time interval which corresponds to a distance interval which is measured by an optical interferometer. Both interval measurements are used to determine the absolute g value.

Superconducting Gravimeter SG38 uses a metal sphere floating in a lossless magnetic field, which is produced by currents in superconducting coils at -268°C. External gravitational forces especially of the moon provoke a change in position of the metal sphere. A control circuitry maintains the position by adding or removing voltage to/from the coils. The achievable sensitivity of the SG is of the order of 1-10nGal, which corresponds to a detectable change in altitude at Earth's surface of 0.03mm. The relative measurements of the SG require from time to time the calibration of its drift with absolute gravimetry.