gravity anomaly

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 spheroid, 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, the model surface often is sea level or an idealization of it), 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. Even with such a correction, there may remain a gravity anomaly and some cited types of anomalies refer to such an anomaly after such a correction or corrections:

• 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.