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Small, C, Sandwell DT.  1989.  An Abrupt Change in Ridge Axis Gravity with Spreading Rate. Journal of Geophysical Research-Solid Earth and Planets. 94:17383-17392.   10.1029/JB094iB12p17383   AbstractWebsite

The global mid-ocean ridge system shows a marked change in morphology and isostatic compensation as a function of spreading rate. Fast spreading ridges have axial highs with little bathymetric relief and low-amplitude gravity signatures indicating that they are nearly in local isostatic equilibrium. Slow spreading ridges have large axial valleys bounded by rugged topography (Macdonald, 1982) and large axial gravity troughs indicating that they are dynamically maintained. While this variation in ridge axis morphology with spreading rate has been observed, it has not been analyzed in a comprehensive manner. Moreover, it is not known whether the transition from axial valley to axial high is a continuous function of spreading rate or whether it occurs abruptly at a particular rate. Such observations would provide important constraints on models of ridge axis dynamics. Vertical deflection profiles collected by the Geosat radar altimeter have sufficient accuracy and resolution to reveal the change in ridge axis gravity with spreading rate. In this study, we have analyzed 44 Geosat profiles over ridges with spreading rates ranging from 14 to 155 mm/yr. In agreement with previous studies, we find that slow spreading ridges (<60 mm/yr) usually have high amplitude gravity troughs (40–100 μrad = 40–100 mGal), while fast spreading ridges (>70 mm/yr) are characterized by low-amplitude ridge axis highs (∼15 μrad). Unexpectedly, we find that the transition from axial trough to axial high occurs abruptly at a spreading rate of 60–70 mm/yr. Ridge axis gravity signatures are highly variable for rates less than 65 mm/yr and very uniform at higher rates. The transition of the gravity signature appears to be more abrupt than the transition of the topographic signature, suggesting an abrupt change in the style of isostatic compensation with spreading rate. Published models of ridge axis dynamics do not explain this sharp transition.

Sandwell, DT, Myer D, Mellors R, Shimada M, Brooks B, Foster J.  2008.  Accuracy and Resolution of ALOS Interferometry: Vector Deformation Maps of the Father's Day Intrusion at Kilauea. Ieee Transactions on Geoscience and Remote Sensing. 46:3524-3534.   10.1109/tgrs.2008.2000634   AbstractWebsite

We assess the spatial resolution and phase noise of interferograms made from L-band Advanced Land Observing Satellite (ALOS) synthetic-aperture-radar (SAR) data and compare these results with corresponding C-band measurements from European Space Agency Remote Sensing Satellite (ERS). Based on cross-spectral analysis of phase gradients, we find that the spatial resolution of ALOS interferograms is 1.3x better than ERS interferograms. The phase noise of ALOS (i.e., line-of-sight precision in the 100-5000-m wavelength band) is 1.6x worse than ERS (3.3 mm versus 2.1 mm). In both cases, the largest source of error is tropospheric phase delay. Vector deformation maps associated with the June 17, 2007 (Father's day) intrusion along the east rift zone of the Kilauea Volcano were recovered using just four ALOS SAR images from two look directions. Comparisons with deformation vectors from 19 continuous GPS sites show rms line-of-site precision of 14 mm and rms azimuth precision (flight direction) of 71 mm. This azimuth precision is at least 4x better than the corresponding measurements made at C-band. Phase coherence is high even in heavily vegetated areas in agreement with previous results. This improved coherence combined with similar or better accuracy and resolution suggests that L-band ALOS will outperform C-band ERS in the recovery of slow crustal deformation.

Smith, B, Sandwell D.  2003.  Accuracy and resolution of shuttle radar topography mission data. Geophysical Research Letters. 30   10.1029/2002gl016643   AbstractWebsite

[1] We assess the accuracy and resolution of topography data provided by the Shuttle Radar Topography Mission (SRTM) through spectral comparisons with the National Elevation Dataset (NED) and a high-resolution laser data set of the 1999 Hector Mine earthquake rupture. We find that SRTM and the NED are coherent for wavelengths greater than 200 m, however the spatial resolution of the NED data is superior to the SRTM data for wavelengths shorter than 350 m, likely due to the application of a boxcar filter applied during final SRTM processing stages. From these results, a low-pass filter/decimation algorithm can be designed in order to expedite large-area SRTM applications.

Sandwell, DT.  1984.  Along-track deflection of the vertical from Seasat : GEBCO overlays. , Rockville, Md.: U.S. Dept. of Commerce, National Oceanic and Atmospheric Administration, National Ocean Service, National Geodetic Survey, Charting and Geodetic Services : For sale by the National Geodetic Information Center Abstract
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.

Marks, KM, Sandwell DT.  1991.  Analysis of Geoid Height Versus Topography for Oceanic Plateaus and Swells Using Nonbiased Linear-Regression. Journal of Geophysical Research-Solid Earth and Planets. 96:8045-8055.   10.1029/91jb00240   AbstractWebsite

We have investigated the relationship between geoid height and topography for 53 oceanic plateaus and swells to determine the mode of compensation. The ratio of geoid height to topography was obtained from the slope of a best line fit by functional analysis (i.e. nonbiased linear regression), a method that minimizes both geoid height and topography residuals. This method is more appropriate than traditional least squares analysis that minimizes only geoid height residuals, because uncertainties are present in both data types. We find that approximately half of the oceanic and continental plateaus analyzed have low ratios that are consistent with Airy-compensated crustal thickening. The remaining plateaus, however, have higher geoid/topography ratios than predicted by the simple Airy model, and the seismically determined Moho depths beneath some of these features are too shallow for crustal thickening alone. A two-layer Airy compensation model, composed of thickened crust underlain by an anomalously low density "mantle root", is used to explain these observations. The Walvis Ridge, and the Agulhas, Crozet, and north Kerguelen plateaus have geoid/topography ratios and Moho depths that are consistent with the two-layer Airy model. The proximity of the Agulhas Plateau to a RRR triple junction during its early development, and the excessive volcanism at active spreading ridges that created the Crozet and north Kerguelen plateaus and the Walvis Ridge, may have produced regions of enhanced depletion and hence the low-density mantle anomalies. If this explanation is correct, then the low-density mantle anomaly persists over time and remains embedded in the lithosphere beneath the oceanic feature.

Small, C, Sandwell DT.  1992.  An Analysis of Ridge Axis Gravity Roughness and Spreading Rate. Journal of Geophysical Research-Solid Earth. 97:3235-3245.   10.1029/91jb02465   AbstractWebsite

Fast and slow spreading ridges have radically different morphologic and gravimetric characteristics. In this study, altimeter measurements from the Geosat Exact Repeat Mission (Geosat ERM) are used to investigate spreading rate dependence of the ridge axis gravity field. Gravity roughness provides an estimate of the amplitude of the gravity anomaly and is robust to small errors in the location of the ridge axis. We compute gravity roughness as a weighted root mean square (RMS) of the vertical deflection at 438 ridge crossings on the mid-ocean ridge system. Ridge axis gravity anomalies show a decrease in amplitude with increasing spreading rate up to an intermediate rate of approximately 60-80 mm/yr and almost no change at higher rates; overall the roughness decreases by a factor of 10 between the lowest and highest rates. In addition to the amplitude decrease, the range of roughness values observed at a given spreading rate shows a similar order of magnitude decrease with transition between 60 and 80 mm/yr. The transition of ridge axis gravity is most apparent at three relatively unexplored locations on the Southeast Indian Ridge and the Pacific-Antarctic Rise; on these intermediate rate ridges the transition occurs abruptly across transform faults.

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, Lawver LA, Dalziel IWD, Smith WHF, Wiederspahn M.  1992.  ANTARCTICA Gravity Anomaly and Infrared Satellite Image, USGS MAP 1-2284. : U.S. Geol. Survey Abstract
Cheney, RE, Douglas BC, Sandwell DT, Marsh JG, Martin TV.  1984.  Applications of Satellite Altimetry to Oceanography and Geophysics. Marine Geophysical Researches. 7:17-32.   10.1007/bf00305408   AbstractWebsite

Satellite-borne altimeters have had a profound impact on geodesy, geophysics, and physical oceanography. To first order approximation, profiles of sea surface height are equivalent to the geoid and are highly correlated with seafloor topography for wavelengths less than 1000 km. Using all available Geos-3 and Seasat altimeter data, mean sea surfaces and geoid gradient maps have been computed for the Bering Sea and the South Pacific. When enhanced using hill-shading techniques, these images reveal in graphic detail the surface expression of seamounts, ridges, trenches, and fracture zones. Such maps are invaluable in oceanic regions where bathymetric data are sparse. Superimposed on the static geoid topography is dynamic topography due to ocean circulation. Temporal variability of dynamic height due to oceanic eddies can be determined from time series of repeated altimeter profiles. Maps of sea height variability and eddy kinetic energy derived from Geos-3 and Seasat altimetry in some cases represent improvements over those derived from standard oceanographic observations. Measurement of absolute dynamic height imposes stringent requirements on geoid and orbit accuracies, although existing models and data have been used to derive surprisingly realistic global circulation solutions. Further improvement will only be made when advances are made in geoid modeling and precision orbit determination. In contrast, it appears that use of altimeter data to correct satellite orbits will enable observation of basin-scale sea level variations of the type associated with climatic phenomena.