Gravimetry

Airborne gravimetry Jordan Airborne gravimetry Jordan

Gravimetry deals with the gravity of the earth. It belongs to geodesy as well as to geophysics. While geodesy deals with questions of the earth’s figure, especially the determination of the earth’s outer gravity field and the setting-up of a global network of measured points as a reference system, geophysics takes more interest in the density distribution in the earth’s interior. A measured deviation from the ‘normal’ distribution of gravity allows to draw conclusions on density inhomogeneities in the underground.

Adequate mathematical modelling methods allow quantitative statements on geometry and density contrasts of the disturbing bodies. Those can be, for example, magmatic intrusive bodies, lithologic boundaries, sedimentary basins, changes of crust-thickness or inhomogeneities or other causes within the earth’s crust or the upper mantle. In applied geophysics, gravimetry is applied for the exploration of near-surface layers, for example in hydrocarbon exploration, in the exploration of deposits of metallic and non-metallic raw materials, in hydrogeology, in foundation soil and environmental surveys.

Gravity is measured with gravimeters. It can be determined absolutely – for example with the help of pendulums in fall experiments or today with so-called superconducting gravimeters.

Gravimeter measurement

Such measurements, however, are always very time-consuming in order to achieve adequate accuracy. In practice, measurements are normally carried out relatively, i. e. gravity changes between two measuring points are determined. Such gravimeters consist, in principle, of a small sample mass suspended on an extremely sensitive spring system. The measuring accuracy of modern gravimeters is about 10^-8 to 10^-9 of the overall strength of gravity at the earth’s surface. Gravimeters are as sensitive as the mechanical possibilities allow.

The exact knowledge of the coordinates of every measured point, especially its height, is indispensable in gravimetry. Today, the coordinates can be determined easily and with high precision by differential GPS. For a reasonable interpretation of gravity measurements in polar areas, like the ones carried out by BGR, we also need information on the ice cover. Therefore, we measure the ice thickness with the Radio Echo Sounding (RES) method, which can be applied from the air.

The following links are showing examples of gravimetric measurements in the antarctic:

Bouguer Anomaly Victoria Land, Antarctica Source: BGR