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In geophysics, including gravimetry, the term **gravity anomaly**
indicates the difference between the force of gravity at some
point and that which some model predicts for that point.
The term **gravity** (as opposed to *gravitation*),
in the context of geophysics of Earth (or of any planet or moon),
means the observed/apparent force within its
rotating frame of reference, including the associated
fictitious forces,
i.e., the measured force per unit mass.
Both a body's *gravity* and its *gravity anomalies* are cited
in units of acceleration and consist of the acceleration an object would
experience at that point if free to do so.
A point at ground level can have such an anomaly,
but an anomaly may apply to a specific point above or below ground level:
an anomaly applies to a defined point in relation to the world
and to a specific model of its gravity.

A typical basic gravity model for a rotating
body such as Earth gives a calculated force of gravity
on each point of a surrounding surface close to a smooth-surfaced
sphere, but oblate due to the rotation, i.e., a type of ellipsoid,
and makes simple assumptions about the density distribution of the body's
material, perhaps presuming hydrostatic equilibrium.
Gravity anomalies can result from a point's height relative to the model surface
(for Earth, sea level is commonly used), from the gravity effects of
the terrain, and/or from density differences of the body near that point.
More complex models that include additional factors presumably leave smaller
anomalies on average. The accommodation for some particular
known factor is sometimes referred to as a **correction**.
A gravity anomaly reflects density variation beneath the surface,
but also includes the effects of very clear contributors. Two
"modified" gravity anomalies, anomalies that remain after removing these
clear effects are:

- free-air anomaly - the anomaly remaining after removing the effects of the altitude difference, i.e., for being further from the body's center of mass.
- Bouguer anomaly - the anomaly remaining after removing the effects of both the altitude difference and of the presumed mass of the landform that raised the altitude (or the presumed missing mass due to the valley that lowered the altitude.)

A **positive gravity anomaly** is a gravity anomaly that is a positive
number, indicating more gravity at the site, i.e., the resulting
acceleration is higher. The converse for a **negative gravity anomaly**.

http://en.wikipedia.org/wiki/Gravity_anomaly

http://en.wikipedia.org/wiki/Gravimetry

http://www.geology.cwu.edu/facstaff/tim/TEACHING/Geophysics/gravity_geoid.pdf

admittance

Bouguer anomaly

free-air anomaly

GRAIL

Trace Gas Orbiter (TGO)