Publications

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2014
Sandwell, DT, Smith WHF.  2014.  Slope correction for ocean radar altimetry. Journal of Geodesy. 88:765-771.   10.1007/s00190-014-0720-1   AbstractWebsite

We develop a slope correction model to improve the accuracy of mean sea surface topography models as well as marine gravity models. The correction is greatest above ocean trenches and large seamounts where the slope of the geoid exceeds 100 rad. In extreme cases, the correction to the mean sea surface height is 40 mm and the correction to the along-track altimeter slope is 1-2 rad which maps into a 1-2 mGal gravity error. Both corrections are easily applied using existing grids of sea surface slope from satellite altimetry.

1999
Yale, MM, Sandwell DT.  1999.  Stacked global satellite gravity profiles. Geophysics. 64:1748-1755.   10.1190/1.1444680   AbstractWebsite

Gravity field recovery from satellite altimetry provides global marine coverage but lacks the accuracy and resolution needed for many exploration geophysics studies. The repeating ground tracks of the ERS-1/2, Geosat, and Topex/Poseidon altimeters offer the possibility of improving the accuracy and resolution of gravity anomalies along widely spaced (similar to 40-km spacing) tracks. However, complete ocean coverage is usually needed to convert the sea-surface height (br along-track slope) measurements into gravity anomalies. Here we develop and test a method for constructing stacked gravity profiles by using a published global gravity grid (Sandwell and Smith, 1997), V7.2, as a reference model for the slope-to-gravity anomaly conversion. The method is applied to stacks (averages) of Geosat/ERM (up to 62 cycles), ERS-1/2 (up to 43 cycles), and Topex (up to 142 cycles) satellite altimeter profiles. We assess the accuracies of the ERS-1/2 profiles through a comparison with a gravity model of the northern Gulf of Mexico (profiles provided by EDCON Inc.). The 40 ERS profiles evaluated have a mean rms difference of 3.77 mGal and full wavelength resolution (0.5 coherence) of 24 km. Our processing retains wavelengths as short as 10 km so smaller, large-amplitude features can be resolved, especially in shallow ocean areas (<1000 m deep). We provide an example of combining these higher resolution profiles with lower resolution gravity data in the Caspian Sea.

1995
Levitt, DA, Sandwell DT.  1995.  Lithospheric Bending at Subduction Zones Based on Depth Soundings and Satellite Gravity. Journal of Geophysical Research-Solid Earth. 100:379-400.   10.1029/94jb02468   AbstractWebsite

A global study of trench flexure was performed by simultaneously modeling 117 bathymetric profiles (original depth soundings) and satellite-derived gravity profiles. A thin, elastic plate flexure model was fit to each bathymetry/gravity profile by minimization of the L(1) norm. The six model parameters were regional depth, regional gravity, trench axis location, flexural wavelength, flexural amplitude, and lithospheric density. A regional tilt parameter was not required after correcting for age-related trend using a new high-resolution age map. Estimates of the density parameter confirm that most outer rises are uncompensated. We find that flexural wavelength is not an accurate estimate of plate thickness because of the high curvatures observed at a majority of trenches. As in previous studies, we find that the gravity data favor a longer-wavelength flexure than the bathymetry data. A joint topography-gravity modeling scheme and fit criteria are used to limit acceptable parameter values to models for which topography and gravity yield consistent results. Even after the elastic thicknesses are converted to mechanical thicknesses using the yield strength envelope model, residual scatter obscures the systematic increase of mechanical thickness with age; perhaps this reflects the combination of uncertainties inherent in estimating flexural wavelength, such as extreme inelastic bending and accumulated thermoelastic stress. The bending moment needed to support the trench and outer rise topography increases by a factor of 10 as lithospheric age increases from 20 to 150 Ma; this reflects the increase in saturation bending moment that the lithosphere can maintain. Using a stiff, dry-olivine theology, we find that the lithosphere of the GDH1 thermal model (Stein and Stein, 1992) is too hot and thin to maintain the observed bending moments. Moreover, the regional depth seaward of the oldest trenches (similar to 150 Ma) exceeds the GDH1 model depths by about 400 m.

Yale, MM, Sandwell DT, Smith WHF.  1995.  Comparison of Along-Track Resolution of Stacked Geosat, Ers-1, and Topex Satellite Altimeters. Journal of Geophysical Research-Solid Earth. 100:15117-15127.   10.1029/95jb01308   AbstractWebsite

Cross-spectral analysis of repeat satellite altimeter profiles was performed to compare the along-track resolution capabilities of Geosat, ERS 1 and TOPEX data. Geophysical Data Records were edited, differentiated, low-pass-filtered, and resampled at 5 Hz. All available data were then loaded into three-dimensional files where repeat cycles were aligned along-track (62 cycles of Geosat/Exact Repeat Mission; 16 cycles of ERS 1, 35-day orbit; 73 cycles of TOPEX). The coherence versus wave number between pairs of repeat profiles was used to estimate along-track resolution for individual cycles, eight-cycle-average profiles, and 31-cycle-average profiles (Geosat and TOPEX only). Coherence, which depends on signal to noise ratio, reflects factors such as seafloor gravity amplitude, regional seafloor depth, instrument noise, oceanographic noise, and the number of cycles available for stacking (averaging). Detailed resolution analyses are presented for two areas: the equatorial Atlantic, a region with high tectonic signal and low oceanographic noise; and the South Pacific, a region with low tectonic signal and high oceanographic variability. For all three altimeters, along-track resolution is better in the equatorial Atlantic than in the South Pacific. Global maps of along-track resolution show considerable geographic variation. On average globally, the along-track resolution (0.5 coherence) of eight-cycle stacks are approximately the same, 28, 29, and 30 km for TOPEX, Geosat, and ERS 1, respectively. TOPEX 31-cycle stacks (22 km) resolve slightly shorter wavelengths than Geosat 31-cycle stacks (24 km). The stacked data, which are publicly available, will be used in future global gravity grids, and for detailed studies of mid-ocean ridge axes, fracture zones, sea mounts, and seafloor roughness.

1994
Smith, WHF, Sandwell DT.  1994.  Bathymetric Prediction from Dense Satellite Altimetry and Sparse Shipboard Bathymetry. Journal of Geophysical Research-Solid Earth. 99:21803-21824.   10.1029/94jb00988   AbstractWebsite

The southern oceans (south of 30 degrees S) are densely covered with satellite-derived gravity data (track spacing 2-4 km) and sparsely covered with shipboard depth soundings (hundreds of kilometers between tracks in some areas). Flexural isostatic compensation theory suggests that bathymetry and downward continued gravity data may show linear correlation in a band of wavelengths 15-160 km, if sediment cover is thin and seafloor relief is moderate. At shorter wavelengths, the gravity field is insensitive to seafloor topography because of upward continuation from the seafloor to the sea surface; at longer wavelengths, isostatic compensation cancels out most of the gravity field due to the seafloor topography. We combine this theory with Wiener optimization theory and empirical evidence for gravity noise-to-signal ratios to design low-pass and band-pass filters to use in predicting bathymetry from gravity. The prediction combines long wavelengths (> 160 km) from low-pass-filtered soundings with an intermediate-wavelength solution obtained 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 find 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 prediction 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 prediction 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 navigation of sounding lines but also cannot completely reproduce them. Therefore it may be used to locate tectonic features but should not be used to assess hazards to navigation. The prediction is available from the National Geophysical Data Center in both digital and printed form.