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The gravity field of Sumatra and the surrounding marine areas is shown in Figure 3.1. Contours in the onshore area of Bouguer gravity, but offshore are of free-air gravity. Terrain corrections have not been applied. Although marine gravity measurements have been made in the forearc basin and elsewhere on a number of research cruises (e.g. Kieckhefer et al. 1981), the data from these generally widely spaced lines have not been used in preparing the maps because free-air gravity values obtained from inversion of satellite radar altimetry provide more systematic coverage and can resolve anomalies with widths of as little as 7 km (Sandwell & Smith 1997). The onshore and satellite-derived offshore data were matched at coastlines without undue difficulty, as should be the case because both free-air and Bouguer corrections are zero at sea level. However, gradients tend to be steep at the coasts in the forearc region, partly because of the change from free-air gravity, which is strongly correlated with local bathymetry, to Bouguer gravity, which is corrected for local topography.
Figure 3.2 shows the locations of the onshore stations used in preparing Figure 3.1, but not of the offshore estimates, distributed on a regular 2 minute grid. Onshore data were obtained from a variety of sources, but unfortunately the results of the many detailed gravity surveys carried out by oil companies remain confidential. The largest single available data set was assembled as part of the collaboration between the British Geological Survey (BGS) and the Geological Research and Development
