VUME Upper Mantle of the Earth

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Absolute Gravity

Gravity is continuously changing as a result of a number of phenomena affecting the whole Earth such as, earth tides, the wobble of the Earth on its axis, density variations in the atmosphere, etc. Gravity also varies as a result of very small changes in the elevation of the station and in the density of the material beneath the station.
An absolute gravimeter measures gravity by timing the free fall of a test mass in an evacuated chamber. The air pressure in the cylinder has been reduced to one billionth of an atmosphere to reduce air drag on the mass. Natural vibrations of the floor are compensated for by a "super spring". The measurement is directly referenced to atomic standards of length and time. The laser is stabilized to hyperfine optical absorption peaks in an iodide absorption cell and the time base is locked to hyperfine microwave transitions in a rubidium vapor cell. The instrument takes 2 or 3 hours to set up at a station and another 24 hours to collect enough data for a precise measurement. Data obtained from FG5 instruments confirm both the improved repeatability of the FG5 compared to earlier types of absolute gravimeters and the utility of the instrument at measuring very long period signals like polar motion.

The dynamic range and absence of drift in an absolute gravimeter compared to relative gravimeters with similar precision, permit geophysical studies with no degradation in accuracy in experiments that have durations of many decades, spatial scales greater than 1000 km, or elevation ranges exceeding 3 km.
The current instrumental accuracy estimate for the FG5 gravity meter is 1.1 microGal, or about one part per billion of the gravitational acceleration (g) at the Earth's surface. This is approximately the change in g that would be expected from a 3 mm change in vertical position.