- Gravimeter
A gravimeter or
gravitometer , is an instrument used ingravimetry for measuring the localgravitational field . A gravimeter is a type ofaccelerometer , except that gravimeters are susceptible to allvibration s includingnoise , that cause oscillatory accelerations. This is counteracted by integral vibration isolation andsignal processing . Though the essential principle of design is the same as in accelerometers, gravimeters are typically designed to be much more sensitive than accelerometers in order to measure very tiny changes within theEarth 'sgravity , of 1 "g)". In contrast, accelerometers are often designed to measure 1000 "g" or more, and many perform multi-axial measurements. The constraints ontemporal resolution are usually less for gravimeters, so that resolution can be increased by processing the output with a longer "time constant".Basically, there are two kinds of gravimeters:
relative andabsolute .Most common "relative" gravimeters are spring based. They are used in gravity surveys over large areas for establishing the figure of the
geoid over those areas. A spring-based relative gravimeter is basically a weight on a spring, and by measuring the amount by which the weight stretches the spring, local gravity can be measured. However, the strength of the spring must becalibrate d by placing the instrument in a location with a known gravitational acceleration. Most accurate relative gravimeters aresuperconducting gravimeters, and these are sensitive to one thousandth of one billionth of the Earth surface gravity.Absolute gravimeters, which nowadays are made compact so they too can be used in the field, are based directly on measuring the acceleration of
free fall (for example, of a reflecting prism in avacuum tube ). They are used for establishing the verticalcontrol network . An absolute gravimeter is used to calibrate relative gravimeters, and it operates by letting a mass free fall invacuum and measuring itsrate of acceleration. The mass includes aretroreflector and terminates one arm of aMichelson interferometer . By counting and timing the interference fringes, the velocity of the mass can be measured. [http://www.lacosteromberg.com/fg5Principle.htm] A more recent development is a "rise and fall" version that tosses the mass upward and measures both upward and downward motion. [cite web | title = Miniaturized Gravimeter May Greatly Improve Measurements | author = J. M. Brown, T. M. Niebauer, B. Richter, F. J. Klopping, J. G. Valentine, and W. K. Buxton | work = Eos, Transactions, American Geophysical Union, electronic supplement | date = 1999-08-10 | url = http://www.agu.org/eos_elec/99144e.html] This allows cancellation of somemeasurement error s.A high-grade, calibrated spring gravimeter such as the portable LaCoste-Romberg gravimeter can measure the Earth's gravitational field to within 1 µGal, or (0.1 nm/s²). Measurements of the surface gravity of the Earth are part of geophysical analysis.
References
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