Publications

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Book Chapter
Phiilips, RJ, Johnson CL, Mackwell SJ, Morgan P, Sandwell DT, Zuber MT.  1997.  Lithospheric Mechanics and Dynamics of Venus. Venus II--geology, geophysics, atmosphere, and solar wind environment. ( Bougher SW, Hunten DM, Phillips RJ, Eds.)., Tucson, Ariz.: University of Arizona Press Abstract
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Journal Article
Keating, B, Cherkis NZ, Fell PW, Handschmacher D, Hey RN, Lazarewicz A, Naar DF, Perry RK, Sandwell D, Schwank DC, Vogt P, Zondek B.  1984.  Field-Tests of Seasat Bathymetric Detections. Marine Geophysical Researches. 7:69-71.   10.1007/bf00305411   AbstractWebsite

Knowledge of the locations and sizes of seamounts is of great importance in applications such as inertial navigation and ocean mining. The quality and density of bathymetry data in the equatorial regions and the southern hemisphere are, unifortunately, highly variable. Our present knowledge of bathymetry, and in particular of seamount locations and characteristics, is based upon ship surveys, which are both time-consuming and expensive. It is likely that a significant number of uncharted seamounts exist throughout the oceans, and remote-sensing techniques may be the most effective means of locating them.

Sandwell, DT, Zhang B.  1989.  Global Mesoscale Variability from the Geosat Exact Repeat Mission - Correlation with Ocean Depth. Journal of Geophysical Research-Oceans. 94:17971-17984.   10.1029/JC094iC12p17971   AbstractWebsite

We have developed a new technique for extracting global mesoscale variability from satellite altimeter profiles having large radial orbit error (∼3 m). Long-wavelength radial orbit error, as well as other long-wavelength errors (e.g., tides, ionospheric-atmospheric delay, and electromagnetic bias), are suppressed by taking the derivative (slope) of each altimeter profile. A low-pass filter is used to suppress the short-wavelength altimeter noise (λ<100 km). Twenty-two repeat slope profiles are then averaged to produce a mean sea surface slope profile having a precision of about 0.1 μrad. Variations in sea surface slope, which are proportional to changes in current velocity, are obtained by differencing individual profiles from the average profile. Slopes due to mesoscale dynamic topography are typically 1 μrad (i.e., a 0.1-m change in topography over a 100-km distance). Root-mean-square (rms) slope variability as low as 0.2 μrad are found in the southeast Pacific, and maximum slope variations up to 6–8 μrad are found in major western boundary currents (e.g., Gulf Stream, Kuroshio, Falkland, and Agulhas) and Antarctic Circum-polar Current (ACC) systems. The global rms variability map shows previously unknown spatial details that are highly correlated with seafloor topography. Over most areas, the rms slope variability is less than 1 μrad. However at mid-latitudes, areas of higher variability occur in deep water (>3 km) adjacent to continental shelves, spreading ridges, and oceanic plateaus. Variability is low in shallower areas (<3 km). Along the ACC, the meso-scale variability appears to be organized by the many shallow areas in its path. We do not see convincing evidence that variability is higher downstream from topographic protrusions. Instead, the areas of highest variability occur in the deep basins (>4km).

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.

Shum, CK, Werner RA, Sandwell DT, Zhang BH, Nerem RS, Tapley BD.  1990.  Variations of Global Mesoscale Eddy Energy Observed from GEOSAT. Journal of Geophysical Research-Oceans. 95:17865-&.   10.1029/JC095iC10p17865   AbstractWebsite

The global distribution of eddy kinetic energy has been synoptically observed from analysis of the Geosat Exact Repeat Mission (ERM) altimeter data collected for a 2-year period from November 1986 through November 1988. Using a technique developed by Sandwell and Zhang (1989), altimeter data from forty-four 17-day repeat cycles (2 years) were processed into sea surface slopes along the satellite ground track, averaged, and filtered to produce a mean sea surface slope profile having an estimated accuracy of 0.2 μrad (2 cm sea level change over 100 km distance). A series of global eddy kinetic energy maps, each averaged over 3 months, and their mean were then generated. The maximum mean eddy kinetic energy per unit mass exceeds 2000 cm^2/s^2 for most of the western boundary currents; however, it only reaches approximately 500 cm^2/s^2 for the Antarctic Circumpolar Current (ACC). More than 65% of the world ocean has relatively low variability with an eddy kinetic energy of less than 300 cm^2/s^2. Results obtained from this study are in general agreement with other Geosat ocean variability studies (e.g., Zlotnicki et al., 1989). However, significantly higher variability is found when compared with either Seasat or ship drift data. Significant seasonal variations were found in the Gulf Stream and Kuroshio currents. The ACC system exhibits no apparent seasonal variation.