DEFINITION OF GRAVITY ANOMALY

The difference between the observed

value of gravity at a point and the theoretically calculated

value of gravity at that point, based on a simple gravity

model. The value of gravity at a point reflects the distribution

of mass and rock units at depth, as well as topography. The

average gravitational attraction on the surface is 32 feet per

seconds squared (9.8 m/s2), with one gravity unit (g.u.) being

equivalent to one ten-millionth of this value. Another older

unit of measure, the milligal, is equivalent to 10 gravity units.

The range in gravity on the Earth’s surface at sea level is

about 50,000 g.u., 32.09–32.15 feet per seconds squared

(9.78–9.83 m/s2). A person would weigh slightly more at the

equator than at the poles because the Earth has a slightly

larger radius at the equator than at the poles.

Geologically significant variations in gravity are typically

only a few tenths of a gravity unit, so instruments to measure

gravity anomalies must be very sensitive. Some gravity surveys

are done using closely to widely spaced gravity meters

on the surface, whereas others are done using observations of

the perturbations of orbits of satellites.

The determination of gravity anomalies involves subtracting

the effects of the overall gravity field of the Earth,

accomplished by removing the gravity field at sea level

(geoid), leaving an elevation-dependent gravity measurement.

This measurement reflects a lower gravitational attraction

with height and distance from the center of the Earth, as well

as an increase in gravity caused by the gravitational pull of

the material between the point and sea level. The free-air

gravity anomaly is a correction to the measured gravity calculated

using only the elevation of the point and the radius and

mass of the Earth. A second correction depends on the shape

and density of rock masses at depth and is known as the

Bouguer gravity anomaly. Sometimes a third correction is

applied to gravity measurements, known as the isostatic correction.

This applies when a load such as a mountain, sedimentary

basin, or other mass is supported by mass

deficiencies at depth, much like an iceberg floating lower in

the water. However, there are several different mechanisms of

possible isostatic compensation, and it is often difficult to

know which mechanisms are important on different scales.

Therefore, this correction is often not applied.

Different geological bodies are typically associated with

different magnitudes and types of gravity anomalies. Belts of

oceanic crust thrust on continents (ophiolites) represent

unusually dense material and are associated with positive

gravity anomalies of up to several thousand g.u.. Likewise,

dense massive sulfide metallic ore bodies are unusually dense

and are also associated with positive gravity anomalies. Salt

domes, oceanic trenches, and mountain ranges all represent

an increase in the amount of low-density material in the

crustal column and are therefore associated with increasingly

negative gravity anomalies, with negative values of up to

6,000 g.u. associated with the highest mountains on Earth,

the Himalayan chain.

See also GEOID; ISOSTACY.

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