Gravimetry

The term gravimetry (from Latin gravitas, " heaviness " and geometry of Greek τὸ μέτρον - the measure ) refers to the methods by which the local gravitational field of the earth is measured. The determination of this potential field is fundamental for geodesy, geophysics and technology are equally important.

In geodesy are exact severity values ​​for height determination (exact reduction of leveling ) and the geoid determination of importance in geophysics and geodynamics for the exploration of the interior of the earth and its movements. Furthermore, accurate scales and other technical measurement methods need gravimetric data for their calibration.

Using the different strength of the gravitational acceleration at different places statements can be made about the distribution of mass in the Earth's crust and the rock density meet. However, the interpretation is not always clear (inverse problem of potential theory ). Also in the research on other celestial bodies ( Planetary Science, Earth's Moon ) wins the gravity measurement in importance.

Measurement principles

The gravity can be, for example, the period of oscillation of a pendulum or a torsion balance to determine, but the accuracy is limited to a few millionths of gravity. With modern gravimeters, which operate on the principle of the spring balance, contrast, relative uncertainties down to 5:10 -9 are achievable with superconducting gravimeters even up 10-11. In the last 15 years, absolute gravimeters were demonstrated with uncertainties of the order 10-10 with methods of Matter- cold atoms. The measuring principle of " Free fall " ( in a vacuum) can determine the severity and absolute.

Area reduction and interpretation

Before an exact interpretation of the measurement results (the study of the earth's crust or the earth's gravity ) it must be reduced by the influence of the terrain. This computational step is called " road reduction " or " topographic reduction". Were they determined earlier with films on maps, so it is now calculated with digital terrain models ( DTM). The typical distance between the points of a DGM is between 50 m and 500 m.

These reductions and the remaining gravity anomalies can reach over 200 milligal, which means 0.02 % of the average acceleration of gravity. The anomalies provide information on geological irregularities in the underground, which are caused by different density of different rocks, through oil or mineral deposits or different deep mantle. They are often used in the exploration for locating deposits.

Another application of gravimetry is the determination of the level surfaces of the Earth's gravity for geoid determination. The geoid globally deviates from ± 50 m (maximum 110 m) from the Erdellipsoid and can be determined by well-spaced gravity measurements with accuracies of a few centimeters to decimeters.

Gravimetric methods

" Gravimetric " called those

• Methods to determine the course of the geoid, or by measurement of the level of gravity, or
• Methods in Applied Geophysics, what raw materials and rocks of the earth's crust explored by calculation of gravity anomalies.

In flat lands, these methods are particularly economical. Contrast, are difficult to capture the force of gravity in the mountains, the influences of the terrain. Therefore, other methods are there more advantageous, for example, the astrogeodätische geoid determination (measurement of the deflection of the vertical ) or methods of seismics.