BATHYMETRIC PREDICTION FROM DENSE SATELLITE ALTIMETRY AND SPARSE SHIPBOARD BATHYMETRY

The southern oceans (south of 30 degrees S) are densely covered with s atellite-derived gravity data (track spacing 2-4 km) and sparsely cove red with shipboard depth soundings (hundreds of kilometers between tra cks in some areas). Flexural isostatic compensation theory suggests th at bathymetry and downward continued gravity data may show linear corr elation in a band of wavelengths 15-160 km, if sediment cover is thin and seafloor relief is moderate. At shorter wavelengths, the gravity f ield is insensitive to seafloor topography because of upward continuat ion from the seafloor to the sea surface; at longer wavelengths, isost atic compensation cancels out most of the gravity field due to the sea floor topography. We combine this theory with Wiener optimization theo ry and empirical evidence for gravity noise-to-signal ratios to design low-pass and band-pass filters to use in predicting bathymetry from g ravity. The prediction combines long wavelengths (> 160 km) from low-p ass-filtered soundings with an intermediate-wavelength solution obtain ed from multiplying downward continued, band-pass filtered (15-160 km) gravity data by a scaling factor S. S is empirically determined from the correlation between gravity data and existing soundings in the 15- 160 km band by robust regression and varies at long wavelengths. We fi nd that areas with less than 200 m of sediment cover show correlation between gravity and bathymetry significant at the 99% level, and S may be related to the density of seafloor materials in these areas. The p rediction has a horizontal resolution limit of 5-10 km in position and is within 100 m of actual soundings at 50% of grid points and within 240 m at 80% of these. In areas of very rugged topography the predicti on underestimates the peak amplitudes of seafloor features. Images of the prediction reveal many tectonic features not seen on any existing bathymetric charts. Because the prediction relies on the gravity field at wavelengths < 160 km, it is insensitive to errors in the navigatio n of sounding lines but also cannot completely reproduce them. Therefo re it may be used to locate tectonic features but should not be used t o assess hazards to navigation. The prediction is available from the N ational Geophysical Data Center in both digital and printed form.