Measurement of gravity in free fall

New devices for the determination of the gravity field in the test

Absolute gravimeter A10 in field use, a mobile measuring device for determining gravity © BKG
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Without gravity there would probably be no life on earth and our everyday life would look completely different. However, because gravity varies regionally, it is continually being redefined not just by satellites but also by ground-based equipment. The Federal Agency for Cartography and Geodesy (BKG) is currently testing a new mobile measuring instrument that is not only small enough for field use, but also achieves a high level of measuring accuracy: with the so-called absolute gravimeter, scientists can even measure changes in the gravitational field caused by fluctuations in the field of gravity Detect groundwater level.

The special thing about gravity is its irregular strength and distribution. Because the attraction is mainly influenced by the variable density differences in the Earth's interior and by the centrifugal force of the Earth's rotation. But even if no-one normally notices that the equator is slightly lighter than the poles, scientists can derive important information from the subtle differences in gravity: geologists use their help to search for mineral resources, and climate researchers determine changes in ocean currents and geodesics show height changes, which are difficult to resolve with other methods.

Satellites supplement terrestrial measurements

"The use of modern satellite missions such as CHAMP, GRACE and GOCE has significantly improved the possibilities of global gravity field determinations and the recording of temporal changes", explains Reinhard Falk from the Federal Agency for Cartography and Geodesy (BKG). Due to the distance of the satellite sensors to the earth, however, only relatively long-wave and thus large-scale components of the gravitational field are imaged with high accuracy. "For small-scale and therefore 'high resolution' gravity field observations, terrestrial measurements are therefore still indispensable, " adds Falk.

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Recently, the BKG has been using so-called field absolute gravimeters. In these devices, the trajectory of a free-falling test mass against a damped suspended reference point is recorded in a vacuum chamber with a laser interferometer. By the respective fall speed the scientists can draw then exact conclusions on the local gravity. "The damping of the reference point is necessary above all because of disturbing environmental influences such as traffic or waves in larger bodies of water. This 'microseismism' would otherwise falsify the measurement results greatly, "Falk explains how the device works. display

Absolute versus relative measurements

The time measurement is highly precise with the help of a rubidium oscillator (atomic clock). As a result, the acceleration of the mass during the free fall can be determined precisely down to a few fractions of nanoseconds. But this high measurement accuracy is only one of many improvements compared to the relative gravimeters used so far. Because these spring systems could determine the strength of gravity only in conjunction with a point of known gravity. In order to do this, the relative gravimeter had to be calibrated by measurements on gravimetric lines in the mountains. The absolute gravimeter, on the other hand, operates autonomously and is thus free of metrological dependencies of earlier measurements.

Absolute gravimeter A10 - flexible in field use BKG

The sensitivity of the device is even high enough that groundwater fluctuations due to precipitation or construction projects in the near vicinity can be perceived, says Falk. Since it does not require reference points to determine the gravitational field, scientists are far more flexible in their choice of measurement points than they used to be. This allows them to take measurements in remote areas such as alpine stations. The new device also makes it possible to simplify and accelerate the measurement setup so that an efficient use of the field is possible . Falk explains the further advantages for the practical work in surveying.

As part of the GOCE GRAND II project from the research and development program "Geotechnologies" of the Federal Ministry of Education and Research (BMBF), the terrestrial gravity field models will now be linked with the data from satellite missions. The researchers want to combine the advantages of both methods and thus achieve an improvement of the previous gravity field model. Before that happens, however, the existing terrestrial data must first be checked using the new absolute gravimeter and examined for any systematic errors.

(Dr. Reinhard Falk (BKG), 13.04.2006 - AHE)