Publications

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1990
Sandwell, DT, McAdoo DC.  1990.  High-Accuracy, High-Resolution Gravity Profiles from 2 Years of the Geosat Exact Repeat Mission. Journal of Geophysical Research-Oceans. 95:3049-3060.   10.1029/JC095iC03p03049   AbstractWebsite

Satellite altimeter data from the first 44 repeat cycles (2 years) of the Geosat Exact Repeat Mission (Geosat ERM) were averaged to improve accuracy, resolution and coverage of the marine gravity field. Individual 17-day repeat cycles (two points per second) were first edited and differentiated resulting in alongtrack vertical deflection (i.e., alongtrack gravity disturbance). To increase the signal to noise ratio, 44 of these cycles were then averaged to form a single, highly accurate vertical deflection profile. The largest contributions to the vertical deflection error is short-wavelength altimeter noise and longer-wavelength oceanographic variability; the combined noise level is typically 6 μrad. Both types of noise are reduced by averaging many repeat cycles. Over most ocean areas the uncertainly of the average profile is less than 1 μrad (0.206 arcsec) which corresponds to 1 mgal of alongtrack gravity disturbance. However, in areas of seasonal ice coverage, its uncertainty can exceed 5 μrad. To assess the resolution of individual and average Geosat gravity profiles, the cross-spectral analysis technique was applied to repeat profiles. Individual Geosat repeat cycles are coherent (>0.5) for wavelengths greater than about 30 km and become increasingly incoherent at shorter wavelengths. This Emit of resolution is governed by the signal-to-noise ratio. Thus when many Geosat repeat profiles are averaged together, the resolution limit typically improves to about 20 km. Except in shallow water areas, further improvements in resolution will be increasingly difficult to achieve because the short-wavelength components are attenuated by upward continuation from the seafloor to the sea surface. These results suggest that the marine gravity field can be completely mapped to an accuracy of 2 mgal and a half-wavelength resolution of 12 km by a 4.5-year satellite altimeter mapping mission.

Mayes, CL, Lawver LA, Sandwell DT.  1990.  Tectonic History and New Isochron Chart of the South-Pacific. Journal of Geophysical Research-Solid Earth and Planets. 95:8543-8567.   10.1029/JB095iB06p08543   AbstractWebsite

We have developed an internally consistent isochron chart and a tectonic history of the South Pacific using a combination of new satellite altimeter data and shipboard magnetic and bathymetric data. Highly accurate, vertical deflection profiles (1–2 μrad), derived from 22 repeat cycles of Geosat altimetry, reveal subtle lineations in the gravity field associated with the South Pacific fracture zones. These fracture zone lineations are correlated with sparse shipboard bathymetric identifications of fracture zones and thus can be used to determine paleospreading directions in uncharted areas. The high density of Geosat altimeter profiles reveals previously unknown details in paleospreading directions on all of the major plates. Magnetic anomaly identifications and magnetic lineation interpretations from published sources were combined with these fracture zone lineations to produce a tectonic fabric map. The tectonic fabric was then used to derive new poles of rotation for 12 selected times in the Late Cretaceous and Cenozoic. From our reconstructions, we estimated the former location of the spreading centers in order to derive a new set of isochrons (interpreted unes of equal age on the ocean floor). We believe that the use of new Geosat altimeter data in combination with a multi-plate reconstruction has led to an improvement in our understanding of South Pacific tectonics.

Koeberl, C, Sharpton VL, Harrison MT, Sandwell D, Murali AV, Burke K.  1990.  The Kara/Ust-Kara twin impact structure; a large-scale impact event in the Late Cretaceous. Special Paper - Geological Society of America. 247( Sharpton VL, Ward PD, Eds.).:233-238., Boulder, CO, United States (USA): Geological Society of America (GSA), Boulder, CO AbstractWebsite
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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.)

1989
Sandwell, DT, Mackenzie KR.  1989.  Geoid Height Versus Topography for Oceanic Plateaus and Swells. Journal of Geophysical Research-Solid Earth and Planets. 94:7403-7418.   10.1029/JB094iB06p07403   AbstractWebsite

Oceanic plateaus and swells are a major component of the seafloor topography, yet they remain among the most poorly understood features. This is especially true of the oceanic plateaus which show large variations in crustal thickness. To determine the depth and mode of compensation for 53 of the largest plateaus and swells, we analyzed the relationship between geoid height and topography in polygonal areas containing each feature. Both geoid height and topography were first band-pass filtered (400 km < l < 4000 km) to isolate the signal associated with local compensation from flexural and deep mantle signals. The ratio of geoid height to topography was then determined by fitting a straight line to the data. Except for nine of the smaller features there is a high correlation between geoid height and topography that is positive in accordance with Airy and thermal compensation models. Eighteen features have high geoid/topography ratios that cannot be explained by the Airy compensation model of crustal thickening. These features (thermal swells) are partially supported by thermal buoyancy forces in the lower half of the lithosphere. The ratios are highest for active hot spot swells and decay, with the thermal age of the swell, to values consistent with Airy compensation of the enduring volcanic edifice. The remaining features (plateaus) have lower geoid/topography ratios in agreement with the Airy compensation model. Those plateaus with average height greater than 4 km are thought to be continental fragments; the shorter plateaus tend to be volcanic features. Modified continental plateaus, presumably small fragments of extended and intruded continental margin crust, cluster around heights of ∼3 km, overlapping the range associated with oceanic plateaus. Since the origin of many plateaus is poorly understood, this global geoid/topography analysis provides a new technique for comparing the deep structure of oceanic plateaus and swells.

McAdoo, DC, Sandwell DT.  1989.  On the Source of Cross-Grain Lineations in the Central Pacific Gravity-Field. Journal of Geophysical Research-Solid Earth and Planets. 94:9341-9352.   10.1029/JB094iB07p09341   AbstractWebsite

Subtle lineations in the marine gravity field of the central Pacific derived from Seasat altimeter data were observed by Haxby and Weissel (1986). They suggested that these “cross-grain” lineations were evidence of small-scale convection beneath the Pacific plate. We have examined these features by comparing multiple, collinear gravity and bathymetry profiles in the Fourier transform domain. Our nine gravity profiles were each obtained by stacking (averaging) three or more individual, repeat Geosat/ERM altimeter passes. Prior to stacking, the individual Geosat passes were fit to a cubic spline and then differentiated along track to produce along-track deflections of the vertical (or horizontal gravity). Corresponding bathymetric profiles were produced by projecting, onto Geosat ground tracks, bathymetric observations from six R/V Thomas Washington legs and three R/V Conrad legs that virtually coincide with these Geosat tracks. After Fourier transforming the resulting gravity and bathymetry profiles, we estimate admittances of gravity to bathymetry. These admittances are generally low; they also tend to be negative at very short wavelengths (λ<50 km). They are consistent with models of flexural isostatic compensation by a very thin lithosphere (approximately 2 km). They are not consistent with models of dynamic compensation. We suggest, therefore, that either (1) these cross-grain lineations began to form very near the East Pacific Rise or (2) they formed on older, anomalously weak lithosphere. We also suggest that the gravity lineations result primarily from loads beneath the seafloor in combination with, secondarily, loads on the seafloor. Depths of these subseafloor loads appear not to exceed significantly typical Moho depths.

1988
Sandwell, DT, McAdoo DC.  1988.  Marine Gravity of the Southern-Ocean and Antarctic Margin from Geosat. Journal of Geophysical Research-Solid Earth and Planets. 93:10389-&.   10.1029/JB093iB09p10389   AbstractWebsite

In November of 1986 the U.S. Navy satellite Geosat began collecting unclassified (gravity) altimeter data as part of its exact repeat mission (ERM). For national security reasons the Geosat orbit was arranged so that it closely follows the Seasat satellite altimeter ground track. However, there are two advantages of the Geosat data over the Seasat data. First, because of improvements in altimeter design, Geosat profiles are about 3 times more precise than Seasat profiles. This corresponds to an accuracy of 2–3 μrad (i.e., 2–3 mGal) for wavelengths greater than 20 km. Second, the Geosat altimeter data were collected when the Antarctic ice coverage was minimal (February 1987 to March 1987), while Seasat was only active during an Antarctic winter (June 1978 to September 1978). These new data reveal many previously uncharted seamounts and fracture zones in the extreme southern ocean areas adjacent to Antarctica. Seven large age-offset fracture zones, apparent in the Geosat data, record the early breakup of Gondwana. Finally, the new data reveal the detailed gravity signatures of the passive and active continental margins of Antarctica. These data are an important reconnaissance tool for future studies of these remote ocean areas.

Gahagan, LM, Scotese CR, Royer JY, Sandwell DT, Winn JK, Tomlins RL, Ross MI, Newman JS, Muller RD, Mayes CL, Lawver LA, Heubeck CE.  1988.  Tectonic Fabric Map of the Ocean Basins from Satellite Altimetry Data. Tectonophysics. 155:1-&.   10.1016/0040-1951(88)90258-2   AbstractWebsite

Satellite altimetry data provide a new source of information on the bathymetry of the ocean floor. The tectonic fabric of the oceans (i.e., the arrangement of fracture zones, ridges, volcanic plateaus and trenches) is revealed by changes in the horizontal gravity gradient as recorded by satellite altimetry measurements. SEASAT and GEOSAT altimetry data have been analyzed and a global map of the horizontal gravity gradient has been produced that can be used to identify a variety of marine tectonic features. The uniformity of the satellite coverage provides greater resolution and continuity than maps based solely on ship-track data. This map is also the first global map to incorporate the results of the GEOSAT mission, and as a result, new tectonic features are revealed at high southerly latitudes.This map permits the extension of many tectonic features well beyond what was previously known. For instance, various fracture zones, such as the Ascension, Tasman, and Udintsev fracture zones, can be extended much closer to adjacent coninental margins. The tectonic fabric map also reveals many features that have not been previously mapped. These features include extinct ridges, minor fracture zone lineations and seamounts. In several areas, especially across aseismic plateaus or along the margins of the continents, the map displays broad gravity anomalies whose origin may be related to basement structures.

1986
Mammerickx, J, Sandwell D.  1986.  Rifting of Old Oceanic Lithosphere. Journal of Geophysical Research-Solid Earth and Planets. 91:1975-1988.   10.1029/JB091iB02p01975   AbstractWebsite

Geophysical data from five regions in the Pacific and Indian oceans reveal that long distance (>400 km) spreading center jumps have occurred in the past. The present-day seafloor morphology is used to develop a scenario for a spreading center jump. The major events are (1) thinning and weakening of the lithosphere at the future rifting site, (2) rifting of the weakened lithosphere (during rifting, the crack is filled from above by normal faulting and wedge subsidence; viscous upwelling fills the crack from below), (3) spreading at the rift site results in a ridge bounded by two troughs (spreading ceases at the dying spreading center, resulting in a deep central graben surrounded by flexural ridges; periods of slow spreading at both spreading centers produce rough topography), (4) ageing and cooling that produce a general deepening of the abandoned spreading ridge and also reduce the thermal contrast across the fossil rifting site. The new spreading center develops into a normal spreading rift. The major topographic expressions apparent in the seafloor today are the deep trough of the abandoned spreading center and the proximal and distal troughs which formed when the emerging spreading center bisected the fossil rifting site. The proximal trough (nearer the new spreading ridge) and the distal trough (farther from the new ridge) are first-order topographic features, 100–1000 km long and 300 km wide, resembling fracture zones with which they are often confused. They share with fracture zones the characteristic of bringing together fragments of lithosphere of different ages, but unlike fracture zones they are generally parallel to magnetic lineations.

Sandwell, DT, Milbert DG, Douglas BC.  1986.  Global Nondynamic Orbit Improvement for Altimetric Satellites. Journal of Geophysical Research-Solid Earth and Planets. 91:9447-9451.   10.1029/JB091iB09p09447   AbstractWebsite

The largest source of error in satellite altimetry is in the radial position of the satellite. Radial orbit errors of more than a few decimeters prohibit basin-scale studies of sea surface height variability. We explore nondynamic techniques for reducing this error. Sea surface height differences at intersections of satellite altimeter profiles (crossover data) provide a strong constraint on radial orbit error but do not uniquely define it. The portion of orbit error that is a function of latitude and longitude only produces no crossover differences and therefore cannot be recovered with crossover data. Using mathematics (inclination functions) originally developed for satellite dynamics, we determine the entire class of orbit error functions not recoverable with crossover data. These functions are mappings of surface spherical harmonics into the orbit plane. For example, the l = 1, m = 0 surface harmonic maps into sinusoidal orbit error with a frequency of once per orbit. Nonzonal harmonics map into linear combinations of three or more frequencies that are linked by the inclination functions. Between frequencies of 0 and 2.2 cycles per orbit there are nine orbit error components that cannot be recovered using crossover data. These components are uniquely defined, however, by nine globally distributed radial tracking points. Fewer tracking points are sufficient if a smoothness criteria is applied to the orbit correction curve. Our findings suggest that radial orbit error can be significantly reduced by including a few globally distributed radar reflectors (or transponders) in the tracking network.

Cheney, RE, Douglas BC, McAdoo DC, Sandwell DT.  1986.  Geodetic and oceanographic applications of satellite altimetry. Space geodesy and geodynamics. ( Anderson A, Cazenave A, Eds.)., London, United Kingdom (GBR): Academic Press, London AbstractWebsite
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1985
McAdoo, DC, Sandwell DT.  1985.  Folding of Oceanic Lithosphere. Journal of Geophysical Research-Solid Earth and Planets. 90:8563-8569.   10.1029/JB090iB10p08563   AbstractWebsite

Folding of the lithosphere just south of the Bay of Bengal appears as (1) undulations in acoustic basement topography and (2) as linear geoid undulations in the Seasat altimeter data. From the Seasat data we find that the east-west trending folds have wavelengths ranging from 130 to 250 km and clustering about 190 km. The horizontal gravity disturbances due to the folds range in amplitude from 15 to 50 mGal. Elastic models of oceanic lithosphere have, in the past, been used to demonstrate the implausibility of lithosphere buckling, or folding, in response to compression. These elastic models typically predict that compressive stresses of about 5 GPa are required to buckle oceanic lithosphere with an age comparable to that of the northeastern Indian Ocean (40–70 Ma). These stresses exceed the strength of lithospheric rock. We use an elastic-plastic model to show that oceanic lithosphere of this age should have a net compressive strength equal to about 12% of the elastic buckling stress. We further demonstrate that loads approaching the net compressive strength cause the lithosphere to fold with a wavelength about 200 km, i.e., the wavelength observed from Seasat. Our results reinforce earlier speculation that this folding may be related to the Himalayan orogeny.

1984
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.