Gravimetry

Geophysics

Physical recall

The acceleration of gravity, G , is worth overall on the ground 9.8 m/s ². It varies however according to the place because of the flatness of the terrestrial sphere to the poles (9.83 m/s ² on the latter against 9.78 m/s ² at the equator) and because of the heterogeneous distribution of the various neighbouring masses (assemblies, pits…). These variations are also temporal because of the movements due to the Plate tectonics , with the cast iron of the Glacier S…

Methods

Two complementary methods make it possible to study it: the space Gravimétrie which uses the orbits of the satellites induced by the terrestrial geoid (one goes back thus to scales of 100 km, but for the whole of the sphere), and more local but more precise gravimetry-geodesy.

Measurements of gravity on the surface of the Ground give to the Geophysicist the means of modelling the geological structure basement. Since measurements of the field of Gravité are only proportional to the density of the rocks, the models of the basement will be models of density.

Gravimeters

The used apparatuses are Gravimètre S, which measure relative values of the Pesanteur with an extreme precision (better than 10^-9 near!). If they can be handled only by only one person, they require nevertheless an altimeter setting of precision (better than the centimetre! from where the existence of altimetric terminals of reference). They are used easily in any climatic, geographical situations and of relief i.e. in mountain, forest, plain, downtown, underground galleries, the cold lakes, the glaciers, etc

It is generally about a mass deforming a spring. The more precise apparatuses consist of a metal mass evolving/moving in a superconductive reel : the intensity of the current allowing the levitation of this one is correlated with the acceleration of gravity.

After the acquisition of measurements and their validations, the geophysicist must correct these measurements of altitude and relief as well as effects of Force of tide of the the Moon and Sun. By comparing measurements with theoretical values of gravity at the same point of measurement, one works out an anomaly known as of Bouguer (Pierre Bouguer (1698 - 1758)). This first interpretation can lead to a quantitative analysis by the development of a geometrical model 2D, 2,5D or 3D of the geological basement. One must take care to fix the working hypotheses on recognized elements coming from other methods geophysics or coming directly from mechanical drillings.

Space gravimetry

The trajectory of an Artificial satellite wheel at low altitude is directly influenced by the value of the G . The deviation in altitude thus makes it possible to estimate this one. This technique was employed as of the Eighties.

In 2000, the satellite FIELD is launched. It is followed by the system GRACE ( Gravity recovery and climate experiment ) in 2002. This last is composed of two satellites which are followed closely in low orbit and whose relative positions are given with an extreme precision. The space resolution is then about 400 km. In 2007 should be lance the satellite GOCE ( Gravity field and steady state ocean circulation to explore ), allowing a still better precision.

Applications

The applications are very diverse, in particular to characterize an aquifer, a cavity, a Glacier, a dome of salt, an archaeological site , a metalliferous lodging, a sedimentary Bassin or the structure of an assembly line. Airborne gravimetry makes it possible to study the important wide ones in little time, but with a precision less than in terrestrial gravimetry: this technique gives results applicable to oil and mining exploration. It also makes it possible to evaluate the thickness of the polar icecaps and to thus help with the estimate of climate warming.

References

the gravity observed since space , I Panet, Mr. Diament, For Science, November 2006, p46-53

Metallurgy/Minéralurgie

Gravimetry indicates a whole of techniques of mineral separation, which operates according to the difference of Densité of these minerals.

These techniques are used in particular in minéralurgie to concentrate Minerai S. They are also used in the field of the Recyclage, for example to separate from the plastics of various densities.

As example, one will quote the technique of " liquor dense" , which consists in preparing a liquid phase of intermediate density between those of two minerals to be separated. One of minerals will float, the other will sink, and one will be able to thus collect them separately. One can thus separate an ore from his gangue. The technique is effective only if the difference in density between minerals considered is sufficient. It generally requires a preliminary granulometric preparation (crushing for example).

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