Isostatic correction
The long wavelengths of the Bouguer gravity field correlate inversely with the long
wavelengths of topography. Masses or "roots" at the base of the crust supporting the
topography cause this correlation according to the theory of isostasy. These regionalscale anomalies are especially prominent in mountainous areas and near the edges of
continents (i.e. continental shelves) and often obscure anomalies caused by upper
crustal structures.
Polynomial fitting or wavelength-filtering techniques have been routinely used for the
removal of topography-induced regional and the enhancement of gravity anomalies
related to shallow geologic features. However, these techniques suffer from the fact
that they eliminate all wavelengths longer than some threshold, whether of not they
are related to topographic features In fact, long-wavelength anomalies due entirely to
lateral variations in crustal density will be eliminated by these techniques. The
isostatic correction is preferable because the isostatic regional accounts for the effect
of the topographic "roots", thereby removing the observed correction between
Bouguer values and topography. Isostatic residual gravity maps reveal more clearly
than most gravity maps the density distributions within the upper crust that are of
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interest in most kinds of geologic and tectonic analyses.
(A) Analogue of Airy isostasy: taller wood blocks float higher in water.
(B) Analogue of Pratt isostasy: less dense wood blocks (lighter shading) float
higher in water than denser wood blocks (darker shading). All blocks have
the same mass and have even bases. (C) A topographically high region is
underlain by a lower-density crustal root. Every column of material weighs
the same, indicating isostatic equilibrium.
Airy model

In most cases a better approximation to earth structure

applies well to mountain ranges, variable thickness of continental crust
Pratt model

applies to mid-ocean ridges: ridges elevated compared to rest of ocean floor.
 Reason: crust+uppermost mantle hot under ridges and therefore less dense than
surroundings
The principle of isostasy states that mass excesses, represented by topographic loads
at the surface, are compensated by mass deficiencies at depth which are referred to as
isostatic roots. The effect of these mass deficiencies are not accounted for in the
Bouguer reduction and there exists an inverse correlation between broad Bouguer
anomaly lows and positive topography. The isostatic correction removes the gravity
effect of the isostatic roots.
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