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The Isostatic Residual extension calculates a depth to the Moho (the “root”) using the topographic grid, terrain density, Moho density contrast and depth of sea level compensation.
The Isostatic Residual extension to Oasis montaj uses a modified version of the USGS algorithm to calculate the Airy isostatic regional and residual gravity from a topographic grid.
The Isostatic Residual extension calculates a depth to the Moho (the "root") using the topographic grid, terrain density, Moho density contrast and depth of sea level compensation. It then calculates the 3-D gravity response of that root, at sea level, out to 166.7 km. The output must be combined with a solution beyond 166.7 km to make a complete Airy regional.
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 regional-scale 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. Simpson et al., (1986) have shown this correlation analytically for the Conterminous U.S.; a qualitative comparison of the Bouguer anomaly and topography maps for other continents show this same correlation.
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 interest in most kinds of geologic and tectonic analyses.
The isostatic approach applies a consistent, 3-dimensional technique at virtually all scales for an entire continent including the continental margins. Additional geophysical data such as seismic refraction can be incorporated into the determination of the isostatic residual. The isostatic anomaly is based on an accepted geologic concept rather than a mathematical filter. The alternate methods can still be applied to the isostatic residual to remove regional trends not related to topographic loads.