Publications

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2017
Zhang, SJ, Sandwell DT, Jin TY, Li DW.  2017.  Inversion of marine gravity anomalies over southeastern China seas from multi-satellite altimeter vertical deflections. Journal of Applied Geophysics. 137:128-137.   10.1016/j.jappgeo.2016.12.014   AbstractWebsite

The accuracy and resolution of marine gravity field derived from satellite altimetry mainly depends on the range precision and dense spatial distribution. This paper aims at modeling a regional marine gravity field with improved accuracy and higher resolution (1' x V') over Southeastern China Seas using additional data from CryoSat-2 as well as new data from AltiKa. Three approaches are used to enhance the precision level of satellite-derived gravity anomalies. Firstly we evaluate a suite of published retracking algorithms and find the two-step retracker is optimal for open ocean waveforms. Secondly, we evaluate the filtering and resampling procedure used to reduce the full 20 or 40 Hz data to a lower rate having lower noise. We adopt a uniform low-pass filter for all altimeter missions and resample at 5 Hz and then perform a second editing based on sea surface slope estimates from previous models. Thirdly, we selected WHU12 model to update the corrections provided in geophysical data record. We finally calculated the 1' x 1' marine gravity field model by using EGM2008 model as reference field during the remove/restore procedure. The root mean squares of the discrepancies between the new result and DTU10, DTU13, V23.1, EGM2008 are within the range of 1.8-3.9 mGal, while the verification with respect to shipboard gravity data shows that the accuracy of the new result reached a comparable level with DTU13 and was slightly superior to V23.1, DTU10 and EGM2008 models. Moreover, the new result has a 2 mGal better accuracy over open seas than coastal areas with shallow water depth. (C) 2016 Elsevier B.V. All rights reserved.

Zhang, SJ, Sandwell DT.  2017.  Retracking of SARAL/AltiKa Radar Altimetry Waveforms for Optimal Gravity Field Recovery. Marine Geodesy. 40:40-56.   10.1080/01490419.2016.1265032   AbstractWebsite

The accuracy of the marine gravity field derived from satellite altimetry depends on dense track spacing as well as high range precision. Here, we investigate the range precision that can be achieved using a new shorter wavelength Ka-band altimeter AltiKa aboard the SARAL spacecraft. We agree with a previous study that found that the range precision given in the SARAL/AltiKa Geophysical Data Records is more precise than that of Ku-band altimeter by a factor of two. Moreover, we show that two-pass retracking can further improve the range precision by a factor of 1.7 with respect to the 40 Hz-retracked data (item of range_40 hz) provided in the Geophysical Data Records. The important conclusion is that a dedicated Ka-band altimeter-mapping mission could substantially improve the global accuracy of the marine gravity field with complete coverage and a track spacing of <6 km achievable in similar to 1.3 years. This would reveal thousands of uncharted seamounts on the ocean floor as well as important tectonic features such as microplates and abyssal hill fabric.

2014
Garcia, ES, Sandwell DT, Smith WHF.  2014.  Retracking CryoSat-2, Envisat and Jason-1 radar altimetry waveforms for improved gravity field recovery. Geophysical Journal International. 196:1402-1422.   10.1093/gji/ggt469   AbstractWebsite

Improving the accuracy of the marine gravity field requires both improved altimeter range precision and dense track coverage. After a hiatus of more than 15 yr, a wealth of suitable data is now available from the CryoSat-2, Envisat and Jason-1 satellites. The range precision of these data is significantly improved with respect to the conventional techniques used in operational oceanography by retracking the altimeter waveforms using an algorithm that is optimized for the recovery of the short-wavelength geodetic signal. We caution that this new approach, which provides optimal range precision, may introduce large-scale errors that would be unacceptable for other applications. In addition, CryoSat-2 has a new synthetic aperture radar (SAR) mode that should result in higher range precision. For this new mode we derived a simple, but approximate, analytic model for the shape of the SAR waveform that could be used in an iterative least-squares algorithm for estimating range. For the conventional waveforms, we demonstrate that a two-step retracking algorithm that was originally designed for data from prior missions (ERS-1 and Geosat) also improves precision on all three of the new satellites by about a factor of 1.5. The improved range precision and dense coverage from CryoSat-2, Envisat and Jason-1 should lead to a significant increase in the accuracy of the marine gravity field.

2010
Marks, KM, Smith WHF, Sandwell DT.  2010.  Evolution of errors in the altimetric bathymetry model used by Google Earth and GEBCO. Marine Geophysical Research. 31:223-238.   10.1007/s11001-010-9102-0   AbstractWebsite

We analyze errors in the global bathymetry models of Smith and Sandwell that combine satellite altimetry with acoustic soundings and shorelines to estimate depths. Versions of these models have been incorporated into Google Earth and the General Bathymetric Chart of the Oceans (GEBCO). We use Japan Agency for Marine-Earth Science and Technology (JAMSTEC) multibeam surveys not previously incorporated into the models as "ground truth" to compare against model versions 7.2 through 12.1, defining vertical differences as "errors." Overall error statistics improve over time: 50th percentile errors declined from 57 to 55 to 49 m, and 90th percentile errors declined from 257 to 235 to 219 m, in versions 8.2, 11.1 and 12.1. This improvement is partly due to an increasing number of soundings incorporated into successive models, and partly to improvements in the satellite gravity model. Inspection of specific sites reveals that changes in the algorithms used to interpolate across survey gaps with altimetry have affected some errors. Versions 9.1 through 11.1 show a bias in the scaling from gravity in milliGals to topography in meters that affected the 15-160 km wavelength band. Regionally averaged (> 160 km wavelength) depths have accumulated error over successive versions 9 through 11. These problems have been mitigated in version 12.1, which shows no systematic variation of errors with depth. Even so, version 12.1 is in some respects not as good as version 8.2, which employed a different algorithm.

2009
Becker, JJ, Sandwell DT, Smith WHF, Braud J, Binder B, Depner J, Fabre D, Factor J, Ingalls S, Kim SH, Ladner R, Marks K, Nelson S, Pharaoh A, Trimmer R, Von Rosenberg J, Wallace G, Weatherall P.  2009.  Global Bathymetry and Elevation Data at 30 Arc Seconds Resolution: SRTM30_PLUS. Marine Geodesy. 32:355-371.   10.1080/01490410903297766   AbstractWebsite

A new 30-arc second resolution global topography/bathymetry grid (SRTM30_PLUS) has been developed from a wide variety of data sources. Land and ice topography comes from the SRTM30 and ICESat topography, respectively. Ocean bathymetry is based on a new satellite-gravity model where the gravity-to-topography ratio is calibrated using 298 million edited soundings. The main contribution of this study is the compilation and editing of the raw soundings, which come from NOAA, individual scientists, SIO, NGA, JAMSTEC, IFREMER, GEBCO, and NAVOCEANO. The gridded bathymetry is available for ftp download in the same format as the 33 tiles of SRTM30 topography. There are 33 matching tiles of source identification number to convey the provenance of every grid cell. The raw sounding data, converted to a simple common format, are also available for ftp download.

2006
Sandwell, DT, Smith WHF, Gille S, Kappel E, Jayne S, Soofi K, Coakley B, Geli L.  2006.  Bathymetry from space: Rationale and requirements for a new, high-resolution altimetric mission. Comptes Rendus Geoscience. 338:1049-1062.   10.1016/j.crte.2006.05.014   AbstractWebsite

Bathymetry is foundational data, providing basic infrastructure for scientific, economic, educational, managerial, and political work. Applications as diverse as tsunami hazard assessment, communications cable and pipeline route planning, resource exploration, habitat management, and territorial claims under the Law of the Sea all require reliable bathymetric maps to be available on demand. Fundamental Earth science questions, such as what controls seafloor shape and how seafloor shape influences global climate, also cannot be answered without bathymetric maps having globally uniform detail. Current bathymetric, charts are inadequate for many of these applications because only a small fraction of the seafloor has been surveyed. Modern multibeam echosounders provide the best resolution, but it would take more than 200 ship-years and billions of dollars to complete the job. The seafloor topography can be charted globally, in five years, and at a cost under $100M. A radar altimeter mounted on an orbiting spacecraft can measure slight variations in ocean surface height, which reflect variations in the pull of gravity caused by seafloor topography. A new satellite altimeter mission, optimized to map the deep ocean bathymetry and gravity field, will provide a global map of the world's deep oceans at a resolution of 6-9 kin. This resolution threshold is critical for a large number of basic science and practical applications, including: determining the effects of bathymetry and seafloor roughness on ocean circulation, mixing, climate, and biological communities, habitats, and mobility; understanding the geologic processes responsible for ocean floor features unexplained by simple plate tectonics, such as abyssal hills, seamounts, microplates, and propagating rifts;. improving tsunami hazard forecast accuracy by mapping the deep-ocean topography that steers tsunami wave energy; mapping the marine gravity field to improve inertial navigation and provide homogeneous coverage of continental margins; providing bathymetric maps for numerous other practical applications, including reconnaissance for submarine cable and pipeline routes, improving tide models, and assessing potential territorial claims to the seabed under the United Nations Convention on the Law of the Sea. Because ocean bathymetry is a fundamental measurement of our planet, there is a broad spectrum of interest from government, the research community, industry, and the general public. Mission requirements. The resolution of the altimetry technique is limited by physical law, not instrument capability. Everything that can be mapped from space can be achieved now, and there is no gain in waiting for technological advances. Mission requirements for Bathymetry from Space are much less stringent and less costly than typical physical oceanography missions. Long-term sea-surface height accuracy is not needed; the fundamental measurement is the slope of the ocean surface to an accuracy of similar to 1 prad (1 mm km(-1)). The main mission requirements are: improved range precision (a factor of two or more improvement in altimeter range precision with respect to current altimeters is needed to reduce the noise due to ocean waves); - fine cross-track spacing and long mission duration (a ground track spacing of 6 km or less is required. A six-year mission would reduce the error by another factor of two); moderate inclination (existing satellite altimeters have relatively high orbital inclinations, thus their resolution of east-west components of ocean slope is poor at low latitudes. The new mission should have an orbital inclination close to 60 degrees or 120 degrees so as to resolve north-south and east-west components almost equally while still covering nearly all the world's ocean area); near-shore tracking (for applications near coastlines, the ability of the instrument to track the ocean surface close to shore, and acquire the surface soon after leaving land, is desirable).

2005
Sandwell, DT, Smith WHF.  2005.  Retracking ERS-1 altimeter waveforms for optimal gravity field recovery. Geophysical Journal International. 163:79-89.   10.1111/j.1365-246X.2005.02724.x   AbstractWebsite

We have reprocessed ERS-1 radar altimeter waveforms using an algorithm designed to minimize sea surface slope error and decouple it from significant wave height (SWH) error. Standard waveform retracking estimates three parameters-arrival time, SWH and amplitude. We show that errors in retracked estimates of arrival time and SWH are inherently correlated because of the noise characteristics of the returned waveform. This suggests that some of what is called 'sea state bias' in the literature may be caused by correlated errors rather than true electromagnetic or skewness bias. We have developed a retracking algorithm that reduces this error correlation and makes the resolution of sea surface slope signals independent of sea state. The main assumption is that the SWH varies smoothly along the satellite track over wavelengths of 90 km. This approach reduces the rms error in sea surface slope to only 62 per cent of that of standard retracking methods. While our method is optimized for gravity field recovery, it may also improve the resolution of sea surface height signals of interest to physical oceanographers.

2000
Gille, ST, Yale MM, Sandwell DT.  2000.  Global correlation of mesoscale ocean variability with seafloor roughness from satellite altimetry. Geophysical Research Letters. 27:1251-1254.   10.1029/1999gl007003   AbstractWebsite

Both seafloor bathymetry and eddy kinetic energy at the ocean surface can be estimated by making use of satellite altimeters. Comparing the two quantities shows that in regions of the ocean deeper than about 4800 m, surface eddy kinetic energy is greater over smooth abyssal plains than over rough bathymetry, while the opposite is true in shallower waters. Thus in the deep ocean, bottom roughness may dissipate eddy kinetic energy. A simple model indicates that the dissipation rate increases as root-mean-squared bottom roughness increases from 0 to 250 m and decreases to negative values (implying eddy generation) for higher roughness.

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.

1992
Sandwell, DT.  1992.  Antarctic Marine Gravity-Field from High-Density Satellite Altimetry. Geophysical Journal International. 109:437-448.   10.1111/j.1365-246X.1992.tb00106.x   AbstractWebsite

Closely spaced satellite altimeter profiles (< 5 km) collected during the Geosat Geodetic Mission (Geosat/GM), and those planned for the extended ERS-1 mission, are easily converted to grids of vertical gravity gradient and gravity anomaly. As profile spacing decreases, it becomes increasingly difficult to perform a crossover adjustment on the original geoid height profiles without introducing large cross-track gradients. If one is only interested in the horizontal and vertical derivatives of the gravitational potential, however, adjustment of the profile is unnecessary. The long-wavelength radial orbit error is suppressed well below the noise level of the altimeter by simply taking the along-track derivative of each profile. Ascending and descending slope profiles are then interpolated onto separate uniform grids. These two grids are summed and differenced to form comparable grids of east and north vertical deflection. Using Laplace's equation, the vertical gravity gradient is calculated directly from the vertical deflection grids. Fourier analysis is required to construct gravity anomalies from the two vertical deflection grids. These techniques are applied to high-density (approximately 2 km profile spacing) Geosat/GM profiles in Antarctic waters (60-degrees-S to 72-degrees-S). Gridding and interpolation are performed using the method of projection onto convex sets where the smoothness criteria corresponds to upward continuation through 4 km of ocean. The resultant gravity grids have resolution and accuracy comparable to shipboard gravity profiles. After adjustment of a DC shift in the shipboard gravity profiles (approximately 5 mGal) the rms difference between the ship and satellite gravity is 5.5 mGal. Many interesting and previously uncharted features are apparent in these new gravity maps including a propagating rift wake and a large 'leaky transform' along the Pacific-Antarctic Rise.

Sandwell, DT, Ruiz MB.  1992.  Along-Track Gravity-Anomalies from Geostat and Seasat Altimetry - Gebco Overlays. Marine Geophysical Researches. 14:165-205.   10.1007/bf01270629   AbstractWebsite

To provide easy access to the large number of Seastat and Geosat altimeter observations collected over the last decade, we have plotted these satellite altimeter profiles as overlays to the General Bathymetric Chart of the Oceans (GEBCO). Each of the 32 overlays displays along-track gravity anomalies for either ascending (southeast to northwest) or descending (northeast to southwest) altimeter passes. Where Seasat and Geosat profiles coincide, only the more accurate Geosat profiles were plotted. In poorly charted southern ocean areas, satellite altimeter profiles reveal many previously undetected features of the seafloor.

1990
Royer, J-Y, Gahagan LM, Lawver LA, Mayes CL, Nuernberg D, Sandwell DT, Scotese CR.  1990.  A tectonic chart for the Southern Ocean derived from Geosat altimetry data. AAPG Studies in Geology. 31( St. John B, Ed.).:89-99., Tulsa, OK, United States (USA): American Association of Petroleum Geologists, Tulsa, OK AbstractWebsite

Presented is a new tectonic fabric map of the southern ocean south of 45S, derived from Geosat altimeter profiles and published bathymetric charts and magnetic anomaly picks. The interpretation of the Geosat data is based on an analysis of the first derivative of the geoid profiles (i.e., vertical deflection profiles). To improve the accuracy and resolution of the vertical deflection profiles, 22 repeat cycles from the first year of the Geosat/Exact Repeat Mission (Geosat/ERM) were averaged. At wavelengths less than about 200 km, the vertical deflection is highly correlated with sea-floor topography and thus reveals major features in areas that were previously unsurveyed. The density of the Geosat data is greatest in the high latitudes where lineated bathymetric features such as fracture zones, spreading ridges, trenches, and rifted margins stand out. To construct the tectonic fabric chart, the Geosat data are analyzed in combination with available shipboard bathymetric data and magnetic anomaly identifications. (Auth.)