From Wikipedia, the free encyclopedia
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
- the elevation of the
point where gravimetry
was done. This is called a Free-air Correction.
- 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.
- 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 ΔgB):
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
ΔgB 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
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