Everything about Gravimeter totally explained
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
retroreflector and terminates one arm of a
Michelson interferometer. By counting and timing the interference fringes, the velocity of the mass can be measured.
(External Link
) 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.
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.
Further Information
Get more info on 'Gravimeter'.
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