A gravity anomaly , also known as a gravitational anomaly , is the local deviation of the gravitational acceleration from the theoretical normal value on a reference surface , which in the case of the earth is usually the reference ellipsoid .
On Earth, gravity anomalies can reach up to ± 200 milligals = ± 0.2 Gal = ± 0.002 m / s², which is 0.02 percent of mean gravity . They provide information about irregularities in the mass distribution in the subsurface , which can have several causes:
- unequal depth of the earth's crust (see also isostasy )
- variable density contrast between the earth's crust and mantle
- different or crooked rocks in the earth's crust
- Deviation of the densities from their average value
- Pore water and storage of raw materials ( ores , hydrocarbons ).
The first two phenomena mentioned cause long-wave (regional) anomalies, while aspects 3–5 mainly have local characteristics.
Gravity measurement goals
Another application is geoid determination - the determination of the level surfaces of the earth's gravity field . The geoid deviates globally by ± 50 meters (maximum 110 m) from the earth's ellipsoid and can be determined by well-distributed gravity measurements with cm to dm accuracy.
For this and also for other planets , satellite geodesy comes into play, in which the orbits of artificial satellites are tracked in which the gravity anomalies are reflected in a weakened manner. The required field continuation downwards is restricted by the inversion problem of potential theory .
In flat countries, the method of exploration of deposits using gravity measurements with gravimeters is particularly economical. In the mountains, however, the effects of the terrain on gravity are difficult to take into account. Seismic methods for detecting underground density variations are therefore more favorable there.
Corrections and reductions of the measured values
- Normal gravity : Calculated from latitude and ellipsoidal height with globally adjusted parameters.
- Rate correction : Correction of the temporally variable influences on gravimetric measurements, especially instrument drift and earth tides .
- Latitude correction : Correction of the deviation in the normal field caused by the geographical latitude . See international formula of gravity .
- Altitude or open air reduction : Correction of the deviations that are caused by different heights above sea level of the measurements.
- Bouguer plate : Correction of the gravity effect of an infinitely extended plate of thickness h.
- Topographical reduction : correction of the local mass attraction of the terrain (mountains or valleys); takes place with a digital terrain model .
- Proportion of density inhomogeneities in the subsurface.
In the literature there are different names for the corrections: instead of the correction, one often speaks of a reduction ( gear reduction, Bouguer reduction / Bouguer's plate reduction ).
Common gravity anomalies
The bougueranomaly is reduced by all parts of the gravitational acceleration. It is caused by density inhomogeneities. They are located in the subsurface below the reference level of the Bouguer plate. A positive bouguer anomaly reflects excess mass.
In the case of the open-air anomaly, the corrections are applied as in the case of the bougueranomaly, except for the bougueranomaly and the topographical correction.
There are regional and local gravity anomalies. The horizontal spread of a gravity anomaly is often referred to as its wavelength.
Local gravity anomalies only extend to small areas, they have short-wave anomalies. Their origins are mostly flat differences in density. Regional gravity anomalies, on the other hand, extend over long distances, they have long-wave anomalies.
These two anomalies can be separated and create a mathematical model of the subsurface. However, these models are never unambiguous and must be supported by other investigations ( drilling , seismic measurements).
- Anomaly-T in the Weddell Sea
- Mascons of the earth moon
- Wilkesland Crater in Antarctica
- Ivrea body on the edge of the Southern Alps
In certain places, the nature of the surrounding area can lead to an optical illusion , which results in the subjective impression that bodies (bottles, cars, etc.) are moving uphill without a drive - that is, against gravity. These perceptual phenomena are sometimes referred to as gravitational or gravity anomalies, although this is not the case.
- The Bouguer anomaly TU Freiberg (pdf, accessed May 29, 2009; 14 kB)
- A Bouguer gravity anomaly map for the Saarland region ( Memento from June 11, 2007 in the Internet Archive ) Uni Karlsruhe
- Natural gravitational anomalies Göde Institute for Gravitational Research (pdf, accessed May 29, 2009; 546 kB)
- Bouguer anomaly map of Switzerland map.geo.admin.ch
- Map of gravity anomalies according to EGM2008 (Earth Gravitational Model 2008)
- Anomalie-T - a gravity anomaly in the Weddell Sea ( Memento from December 23, 2007 in the Internet Archive )
- Christoph Clauser: Introduction to geophysics: global physical fields and processes in the earth . Springer, 2014, ISBN 978-3-662-46883-8 .