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.
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 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 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
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 (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
External links