Abstract for Fall 1996 AGU Meeting, San Francisco, CA
Global Seafloor Topography from Satellite Altimetry
Walter H F Smith (NOAA Geosciences Lab, code N/OES-12, 1305 East-West Highway, Silver Spring, MD 20910-3281; 301-713-2860)
David T Sandwell (Scripps Institution of Oceanography, La Jolla, CA 92093-0225)
We have combined shipboard depth soundings with gravity data derived from satellite altimetry to produce a gridded estimate of sea floor topography for all ice-free ocean areas within +/-72[ring] latitude. The depth data were obtained by screening 6905 surveys from the NGDC (Trkdas CD version 3.2), the Scripps and Lamont data banks, and other data, using quality control procedures based on those of Smith [J. Geophys. Res. 98, #B6, 9591-9603, 1993]. The satellite gravity field combines all data from the ERS-1 and Geosat satellites including the data declassified last year; it has an RMS accuracy of 3-5 mGal and a resolution of 20 km wavelength.
Our method is similar to one we developed earlier for data south of 30[ring]S which were declassified in 1992 [Smith and Sandwell, J. Geophys. Res. 99, #B11, 21,803-21,824, 1994]. Shipboard depth surveys are used to calibrate the transfer function from gravity to topography, and to supply the long-wavelength part of the solution. (Isostatic compensation of topography limits the gravity-based estimation to wavelengths shorter than 160 km.) Correlation of estimated and measured depths is high at wavelengths greater than 20 km, and the difference between them is usually less than 100 m. Differences can be much larger in rugged areas, and the minimum depths at seamount tops are not very accurate. As a final step in the processing, we perturb the prediction in a smooth manner so that it matches the ship soundings as well as high resolution shoreline data from GMT.
The topography map we present here is visually quite different from the satellite gravity field. Isostatically compensated topography stands out in full relief, and edges have been sharpened by downward-continuation of the gravity field as well as by the ship sounding constraints. These effects reveal dramatic scarps such as at the eastern edge of the Manihiki Plateau. Gravity anomalies associated with sub-seafloor structures have disappeared, so that some fracture zones cannot be traced under thick sediments in this map, whereas they can in the gravity map. The correlation and admittance between gravity and topography can be used to estimate sediment thicknesses in such areas.