Gravity anomaly

A gravity anomaly is the difference between the observed gravity and a value predicted from a model.

Geodesy and geophysics

In geodesy and geophysics, the usual model is the surface of a global spheroid (ellipsoid of Hayford or WGS84) by rather simple formulae (2 functions of latitude).

The observed value of gravity has to be reduced down to the zero level of the geoid, using

1. the elevation of the point where gravimetry was done. This is called a Free-air Correction.
2. the normal gradient of gravity (rate of change of gravity for change of elevation), as in free air, usually 0.3086 milligals per meter, or the Bouguer gradient of 0.1967 mGal/m (19.67 µm/(s²·m) which considers the mean rock density (2.67 g/cm³) beneath the point; this value is found by subtracting the gravity due to the Bouguer plate, which is 0.1119 mGal/m (11.19 µm/(s²·m)) for this density. Simply, we have to correct for the effects of any material between the point where gravimetry was done and the geoid. To do this we model the material in between as being made up of an infinite number of slabs of thickness t. These slabs have no lateral variation in density, but each slab may have a different density than the one above or below it. This is called the Bouguer Correction.
3. and (in special cases) a terrain model, using a map or a digital terrain model (DTM). A terrain correction, computed from a model structure, accounts for the effects of rapid lateral change in density, eg. edge of plateau, cliffs, steep mountains, etc.

(Bouguer) gravity anomaly map of the state of New Jersey (USGS)

For these reductions, different methods are used:

• free-air anomaly (or Faye's anomaly): application of the normal gradient 0.3086, but no terrain model. This anomaly means a downward shift of the point, together with the whole shape of the terrain. This simple method is ideal for many geodetic applications.
• simple Bouguer anomaly: downward reduction just by the Bouguer gradient (0.1967). This anomaly handles the point as if it is located on a flat plain.
• refined (or complete) Bouguer anomaly (usual abbreviation Δg_B): the DTM is considered as accurate as possible, using a standard density of 2.67 g/cm³ (granite, limestone). Bouguer anomalies are ideal for geophysics because they show the effects of different rock densities in the subsurface.
  • The difference between the two - the differential gravitational effect of the unevenness of the terrain - is called the terrain effect. It is always negative (up to 100 milligals).
  • The difference between Faye anomaly and Δg_B is called Bouguer reduction (attraction of the terrain).
• special methods like that of Poincare-Prey, using an interior gravity gradient of about 0.009 milligal per meter (90 nm/(s²·m)). These methods are valid for the gravity within boreholes or for special geoid computations.

The Bouguer anomalies usually are negative in the mountains because of isostasy: the rock density of their roots is lower, compared with the surrounding earth's mantle. Typical anomalies in the Central Alps are −150 milligals (−1.5 mm/s²). Rather local anomalies are used in applied geophysics: if they are positive, this may indicate metallic ores. At scales between entire mountain ranges and ore bodies, Bouguer anomalies may indicate rock types. For example, the northeast-southwest trending high across central New Jersey (see figure) represents a graben of Triassic age largely filled with dense basalts. Salt domes are typically expressed in gravity maps as lows, because salt has a low density compared to the rocks the dome intrudes.

Astronomy

Any region of space with higher than expected mass density will produce a gravity anomaly. Observations of gravity anomalies on galactic and intergalactic scales, lead to the assumption of dark matter.

See also

• For a very different concept in physics, see gravitational anomaly.
• Physical geodesy
• gravity
• vertical deflection

External links

• Bouguer anomaly theory
• Bouguer anomalies of Belgium. The blue regions are related to deficit masses in the subsurface - e.g. lower rock density beneath the mountains