Gravity anomaly

src: pubs.usgs.gov

The gravitational anomaly is the difference between the observed free falling acceleration, or gravity, on the surface of the planet, and the corresponding predicted values ​​of the planet's gravitational field model. Usually this model is based on the simplification of assumptions, such that, under its own gravitational motion and rotational motion, the planet assumes an elipsoid figure of revolution. Gravity on the surface of the ellipsoid is then given by a simple formula containing only the latitude, and the reduction of gravity observed in the same location will produce gravity anomalies.

Anomalous values ​​are usually much smaller than those of gravity itself, since the greatest contribution of the total mass of the planet, its rotation and evenness, has been reduced. Thus, gravitational anomalies describe local variations of the gravitational field around the model plane. A location with a positive anomaly shows more gravity than predicted by the model - indicating a positive mass sub-surface anomaly, whereas a negative anomaly shows a lower value than expected - indicating a sub-surface mass deficit. These anomalies have large geophysical and geological interests.

When gravity measurements have been made on topography above sea level, a careful reduction process, also involving local topographic mass effects, should be made to obtain useful geophysical gravity anomalies, of which there are several different types. Clean response to the geology of the local sub-surface is the general goal of applied geophysics.

Video Gravity anomaly

Cause

The lateral variation in gravity anomaly is related to the distribution of anomalous densities within the Earth. The size of gravity helps us understand the internal structure of the planet. Synthetic calculations show that the gravitational anomaly sign of thickened crust (eg, on the orogenic belt produced by continental collisions) is negative and larger in absolute value, relative to cases where thickening affects the entire lithosphere.

Bouguer anomalies are usually negative in the mountain as they involve reducing the mass appeal of the mountain, about 100 milliginal per kilometer from the height of the mountain. In large mountain areas, they are even more negative than this because isostasy: rock density from lower mountain roots, compared to the surrounding earth mantle, causes further gravitational deficits. The typical anomaly in the Central Alps is -150 milligals (-1.5 mm/s²). Somewhat local anomalies are used in applied geophysics: if they are positive, this may indicate metallic ore. On a scale between the entire mountain range and the ore body, Bouguer anomalies can show rock types. For example, the high northeast-southwest trend in central New Jersey (see figure) represents graben of Triassic age mostly filled with dense basalt. The salt dome is usually expressed in the gravity map as the lowest, because the salt has a low density compared to the disturbing dome rocks. Anomalies can help distinguish sedimentary basins whose contents differ in density from surrounding areas - see Gravity Anomaly from the UK and Ireland for example.

Maps Gravity anomaly

Geodesy and geophysics

In geodesy and geophysics, the usual theoretical model is gravity on the surface of a reference ellipsoid such as WGS84.

To understand the nature of gravity anomalies due to subsurface, a number of corrections must be made to the measured gravity value:

1. Theoretical gravity (smoothed normal gravity) should be removed to leave only local effects.
2. The height of the point at which each measured gravity measurement has to be reduced to a reference datum to compare all profiles. This is called Free-Air Correction , and when combined with the theoretical gravity removal leaves the air anomalies free.
3. Normal gravity gradient (rate of gravity change due to altitude change), such as in free air, usually 0.3086 milliginal per meter, or Bouguer gradient of 0.1967 mGal/m (19.67 à , Âμm/(sÃ,²Ã,  · m) considering the mean rock density (2.67 g/cm³³) below the point, this value was found by reducing gravity due to the Bouguer plate, ie 0.1119 mGal/m (11 , 19 Âμm/(² · M)) for this density, Enough, we have to correct the effect of each material between the point where gravimetry is done and geoid.To do this we model the material in between as consisting of an infinite number of thickness sheets t This sheet has no lateral variation in density, but each plate may have a different density than the one above or below it is called Bouguer correction .
4. and (in special cases) the digital terrain model (DTM). Field correction, calculated from the model structure, contributes to the effects of rapid lateral changes in density, eg. the edge of the highlands, cliffs, steep mountains, etc.

For this reduction, different methods are used:

• Gravity changes as we move away from the Earth's surface. For this reason, we must compensate with free air anomalies (or Faye anomalies): application of normal gradient 0.3086 mGal/m, but no terrain models. This anomaly means a downward shift from the point, along with the entire shape of the field. This simple method is ideal for many geodetic applications.
• Bouguer anomaly is simple: downward only with Bouguer gradient (0.1967). This anomaly handles the point as if it were on a flat plain.
• fine (or finished ) Bouguer Anomaly (the usual abbreviation g _B ): DTM is considered as accurate possible, using a standard density of 2.67 g/cm³³ (granite, limestone). Bouguer anomalies are ideal for geophysics because they show the effect of different density rocks beneath the surface.
  • The difference between the two - the differential gravitational effect of the unevenness of the field - is called the field effect. It's always negative (up to 100 milligali).
  • The difference between Faye anomaly and? g _B is called Bouguer Reduction (field appeal).
• specific methods like that of PoincarÃÆ' © -Prey, using an interior gravity gradient of about 0.0848 milligals per meter (848 nm/(sÃ,²Ã,m)). This method applies to gravity in drill holes or for special geoid calculations.

src: www.macpurity.net

Satellite measurement

Large-scale gravity anomalies can be detected from space, as a by-product of satellite gravity missions, for example, GOCE. This satellite mission aims to restore a detailed model of the earth's gravitational field, usually expressed in the expansion of the Earth's gravity-harmonic sphere, but alternative presentations, such as geoid maps of undulations or gravity anomalies, are also produced.

Gravity Recovery and Climate Experiment (GRACE) consists of two satellites that can detect gravitational changes on Earth. Also these changes can be presented as temporal variations of gravity anomalies.

src: advances.sciencemag.org

See also

• Gravimetry
• Physical Geodesy
• Vertical deflection

src: pgc.lyellcollection.org

References

src: www.lpi.usra.edu

Further reading

• Heiskanen, Weikko Aleksanteri; Moritz, Helmut (1967). Physical Geodesy . W.H. Freeman.

Source of the article : Wikipedia