Gravity anomaly

Gravity anomalies are widely used in geodesy and geophysics. A gravity anomaly is the difference between the observed gravity and its theoretical value, which is calculated at the surface of an global spheroid (ellipsoid of Hayford or WGS84) by rather simple formulas (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
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 milligals/m which considers the mean rock density (2.67 g/cm³) below of the point
3. and (in special cases) a terrain model, using a map or a digital terrain model (DTM).

For these reduction, 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 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 of 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. 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.
Rather local anomalies are used in applied geophysics: if they e.g. are positive, this may indicate metallic ores.

See also: Physical geodesy, gravity, vertical deflection

External links

• Free air anomaly (DOD dictionary)
• 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

See also: Gravity anomaly, Anomaly, Borehole, Bouguer anomaly, Density, Earth's mantle, Elevation, Ellipsoid, Faye, Free-air anomaly