Thursday, March 13, 2008


A gravimeter or gravitometer, is an instrument used in gravimetry for measuring the local gravitational field. A gravimeter is a type of accelerometer, except that gravimeters are susceptible to all vibrations including noise, that cause oscillatory accelerations. This is counteracted by integral vibration isolation and signal 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 the Earth's gravity, of 1 g). In contrast, accelerometers are often designed to measure 1000 g or more, and many perform multi-axial measurements. The constraints on temporal 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 and absolute.

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 be calibrated by placing the instrument in a location with a known gravitational acceleration. Most accurate relative gravimeters are superconducting 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 a vacuum tube). They are used for establishing the vertical control network. An absolute gravimeter is used to calibrate relative gravimeters, and it operates by letting a mass free fall in vacuum and measuring its rate of acceleration. The mass includes a retro reflector and terminates one arm of a Michelson interferometer. By counting and timing the interference fringes, the velocity of the mass can be measured. A more recent development is a "rise and fall" version that tosses the mass upward and measures both upward and downward motion. This allows cancellation of some measurement errors.

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