Gradiometry

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Gradiometry is the measurement of a component of a gradient field ; H. of the gradient of the gravitational field or magnetic field . A vector component is registered simultaneously with two sensors that are at a fixed distance. Gradiometry is used in geomagnetics and gravimetry and was theoretically created in the 1920s. Gradient measurement requires precise and fast measurements, which have been possible in magnetics since the development of the fluxgate magnetometer in 1937. Corresponding devices in gravimetry emerged in the 1990s. By measuring differences, there is no need to reduce the measured values ​​in gravimetry ; in magnetics, there is no daily rate correction.

Gradiometer in geomagnetics

There are two arrangements for gradiometers in magnetics.

Gradiometers with a small sensor spacing are used for air measurements . The sensors are mounted at a fixed distance from one another - mostly vertically. Due to the small distance, there are no positional instabilities that could cause measurement errors. However, this also reduces the resolution .

Vertical gradiometers are also used for archaeological prospecting .

Large horizontal sensor distances of over 20 meters are used for sea measurements. The sensors are towed at a great distance from the ship and are connected by a cable. The flexible connection can cause positional instabilities. The resolution increases with the distance between the probes.

When using fluxgate sensors, the measured variable is usually the vertical component, and also the north component of the magnetic field for sea measurements. Proton precession, Overhauser or cesium magnetometers, on the other hand, use the magnetic field amount.

Gradiometer in gravimetry

The gravity gradient measurement has been operated with satellite support for several years . Around 1980, the development of new, gyro-based measurement systems began to be able to automatically record the gravitational field with low-flying artificial earth satellites . Because of the technically highly demanding methodology, however, it was not successful until the end of the 1990s.

One of the most important of these projects is the construction and subsequent operation of the GOCE (Gravity and Steady-State Ocean Circulation Explorer) satellite , which has been developed in cooperation between the space agencies ESA and NASA since 1995 . Its probes consisted of high-precision accelerometers mounted on ultra-stable structures and cantilevers, designed to continuously measure all nine values ​​of the gravity tensor . It was hoped that this would enable the global geoid to be determined with at least centimeter accuracy and a resolution of around 100 kilometers.

In combination with other measurements (especially GPS , satellite altimetry and satellite-to-satellite tracking ), important contributions to oceanography and other geosciences can also be expected. Almost more important than the data on the geoid will be its slow temporal changes, which can be recorded with GOCE for the first time.

The measurand in gravimetry is essentially the vertical component of the gravitational field, since the other components, on the other hand, are negligibly small.

literature

  • Geldart Telford: Applied Geophysics. Cambridge University Press, Cambridge 1990, ISBN 0-521-32693-1