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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.

Sandwell, DT, Agreen RW.  1985.  Seasonal-Variation in Wind-Speed and Sea State from Global Satellite Measurements - Reply. Journal of Geophysical Research-Oceans. 90:5009-5010.   10.1029/JC090iC03p05009   AbstractWebsite
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
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.

Sandwell, DT.  1984.  A Detailed View of the South-Pacific Geoid from Satellite Altimetry. Journal of Geophysical Research. 89:1089-1104.   10.1029/JB089iB02p01089   AbstractWebsite

Images of sea surface undulations in the South Pacific have been constructed from GEOS 3 and SEASAT altimeter data. Height discrepancies at crossover points, associated with long-wavelength radial orbit error, were suppressed by taking along-track derivatives of the ascending and descending profiles. These geoid slopes were then rotated and scaled to produce the north and east components of the deflection of the vertical. Finally, the results are displayed by using the hill shading technique, where gray-tone images represent the innner product of the deflection vector with an assigned sun vector. Less apparent sea surface undulations can be enhanced by varying the sun's zenith and azimuth. Shorterwavelength sea surface undulations reflect seafloor topography. For instance, fracture zones (FZ's) appear as elongated sharp steps in the sea surface, while seamounts produce circular bumps. Since large areas of the South Pacific are unsurveyed, many previously undetected features appear on the images. Comparisons with bathymetric charts reveal 72 uncharted seamounts having geoid expressions greater than or equal to Easter Island's expression. The dominant features in the images, however, are the large age-offset FZ's such as the Eltanin and Udintsev FZ's. The images reveal that the Eltanin FZ is connected to the Louisville Ridge; combined they produce a continuous geoid signature across most of the South Pacific. This supports the hypothesis of Hayes and Ewing (1968) that the Louisville Ridge is the northwest extension of the Eltanin FZ.

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.

Wagner, CA, Sandwell DT.  1984.  The Gravsat Signal over Tectonic Features. Journal of Geophysical Research. 89:4419-4426.   10.1029/JB089iB06p04419   AbstractWebsite

The range rate between two close gravitational satellites (GRAVSAT) in low earth orbit has been evaluated over model tectonic features such as mountains and ranges, fracture zones, and trenches. Models are locally compensated and consist of both point mass dipoles and sheet mass dipoles. Masses and depths of compensation are chosen to approximate known gravity signatures. The results show that for two satellites at 160 km altitude with 3° separation, significant signal power (>1 μm/s) remains for most extended features at wavelengths less than 200 km. Furthermore, there is strong sensitivity in the signal from these features to lateral and vertical changes of the order of 1 km and less. In addition, the signal of hidden geologic structures such as dikes, salt domes, and ore bodies may also stand above 1 μm/s for this low orbiting pair. Thus, it may prove to be efficient to model the high-frequency GRAVSAT signal directly in terms of the parameters of tectonic-topographic features and their compensation.

Douglas, BC, Agreen RW, Sandwell DT.  1984.  Observing Global Ocean Circulation with Seasat Altimeter Data. Marine Geodesy. 8:67-83. AbstractWebsite
Sandwell, DT, Agreen RW.  1984.  Seasonal-Variation in Wind-Speed and Sea State from Global Satellite Measurements. Journal of Geophysical Research-Oceans. 89:2041-2051.   10.1029/JC089iC02p02041   AbstractWebsite

The GEOS 3 altimeter, which collected data intermittently for nearly 4 years, has measured significant wave heights and surface wind speeds over most of the world's oceans. Using these data, we have constructed contour maps of spatial variations in sea state and wind speed for winter and summer. To obtain reliable averages in the southern oceans, we low-pass filtered the data using a two-dimensional Gaussian filter with a half width of 600 km. The wind speed maps show that the zonal surface wind patterns, such as the westerlies, the horse latitudes, the trade winds, and the doldrums, shift south by about 10° between winter and summer. As expected, the highest wind speeds and sea states occur during the winter months in the mid-latitudes, 30°–60°. The most striking feature of the maps, however, is the large asymmetry in the summer to winter variation between the two hemispheres. The largest seasonal variations in sea state and wind speed occur in the northern hemisphere oceans and especially in the North Atlantic, where there is almost a factor of 2 variation. In contrast, the summer to winter variation in wind speed and sea state in the southern hemisphere oceans is relatively small. For example, the summer to winter increase in wind speed at 50°S is less than 10%, while at 50°N it is more than 50%. This differing variability can be attributed to the asymmetric distribution of continental area between the two hemispheres and the low effective heat capacity of the continents relative to the oceans.

Sandwell, DT.  1984.  Thermomechanical Evolution of Oceanic Fracture-Zones. Journal of Geophysical Research. 89:1401-1413.   10.1029/JB089iB13p11401   AbstractWebsite

A fracture zone (FZ) model is constructed from existing models of the thermal and mechanical evolution of the oceanic lithosphere. As the lithosphere cools by conduction, thermal and mechanical boundary layers develop and increase in thickness as (age)1/2. Surface expressions of this development, such as topography and deflection of the vertical (i.e., gravity field), are most apparent along major FZ's because of the sharp age contrast. A simple model, including the effects of lateral heat transport but with no elastic layer, predicts that variations in seafloor depth and deflection of the vertical will become increasingly smooth and ultimately disappear as the FZ ages. Observations, however, show that both the FZ topography and the deflection of the vertical remain sharp as the FZ evolves. These two observations, as well as the observed asymmetry in deflection of the vertical profiles across the Udintsev, Romanche, and Mendocino FZ's, are explained by including a continuous elastic layer in the model. The asymmetry in deflection of the vertical is a consequence of elastic thickness variations across the FZ. Modeling also shows that the evolution of the FZ topography is extremely sensitive to the initial thermal structure near the ridge-transform intersection. Model geoid steps and their development with age are used to access techniques for measuring geoid offsets across FZ's. Reasonable step estimation techniques will underestimate the overall step amplitude by up to 50%. This implies that abnormally thin thermal boundary layers, derived from studies of geoid height versus age, are not required by the data.

Sandwell, DT, Schubert G.  1982.  Geoid Height-Age Relation from Seasat Altimeter Profiles across the Mendocino Fracture-Zone. Journal of Geophysical Research. 87:3949-3958.   10.1029/JB087iB05p03949   AbstractWebsite

Twenty-eight SEASAT altimeter profiles crossing the Mendocino Fracture Zone are used together with seafloor ages determined from magnetic lineations to estimate the change in oceanic geoid height with age, between ages of 15 and 135 m.y. An unbiased estimate of the overall geoid offset along each profile is determined from a least-squares fit of the along-track derivative of the geoid to the geoid slope predicted from a simple two-layer gravitational edge effect model. Uncertainties based upon the statistical properties of each profile are also determined. A geoid slope-age relation is constructed by normalizing the geoid offsets and uncertainties by the age offsets. The results are in agreement with geoid slope-age relations determined from symmetrically spreading ridges (Sandwell and Schubert, 1980). However, the fracture zone estimates have smaller uncertainties and show less scatter. A comparison of these results with the geoid slope-age prediction of the boundary layer cooling model shows that the thermal structure begins to deviate from this model at an early age (20–40 m.y.). A plate cooling model with a thickness of 125 km is most compatible with the geoid slope-age estimates, although significant deviations occur; these may indicate that the lithospheric thermal structure is not entirely age dependent.

Sandwell, D, Schubert G.  1982.  Lithospheric Flexure at Fracture-Zones. Journal of Geophysical Research. 87:4657-4667.   10.1029/JB087iB06p04657   AbstractWebsite

Bathymetric profiles across six major fracture zones (FZ's) in the North Pacific are used to demonstrate the absence of vertical slip on the fossil fault planes. The scarp heights on these FZ's are constant with age and equal to the initial vertical offsets at the ridge-transform fault-FZ intersections. Because of the frozen-in scarp and the differential subsidence of lithosphere far from the FZ, the lithosphere bends in the vicinity of the FZ. This flexure results in a characteristic ridge-trough topographic FZ signature. The flexural amplitude, which is the difference between the scarp height and the overall change in depth across the FZ, increases with age. Good fits to the bathymetric profiles across the Mendocino and Pioneer FZ's are obtained by modelling the topography as the flexure of a thin elastic plate with an age-dependent effective elastic thickness. Results of the modelling indicate that the base of the elastic lithosphere is approximately defined by the 450°C isotherm. Maximum bending stresses at FZ's are on the order of 100 MPa, substantially less than the stresses encountered at subduction zones. Because the Mendocino and Pioneer FZ's are separated by less than a flexural wavelength, they are elastically coupled.

Liu, CS, Sandwell DT, Curray JR.  1982.  The Negative Gravity-Field Over the 85-Degrees-E Ridge. Journal of Geophysical Research. 87:7673-7686.   10.1029/JB087iB09p07673   AbstractWebsite

An isopach map made from seismic reflection and refraction data in the Bay of Bengal shows two prominent N-S trending features in the basement topography. One is the northernmost portion of the Ninetyeast Ridge which is totally buried by sediments north of 10°N. The other buried ridge trends roughly N-S for 1400 km at 85°E to the latitude of Sri Lanka and then curves toward the west. It has basement relief up to 6 km. Two free-air gravity anomaly profiles across the region show a strong gravity low (∼−60 mGal) over the 85°E Ridge, while the Ninetyeast Ridge shows a gravity high. To understand the negative free-air gravity anomaly over the 85°E Ridge, we model the lithosphere as a thin elastic plate and calculate its flexural and gravitational response to an uneven sediment load. A plausible formation history for a buried ridge consists of at least two major episodes. The first is the formation of the ridge on a lithosphere with a flexural rigidity of D1. At some later time the ridge is buried by an influx of sediments, the lithosphere is cooler, and the flexural rigidity has increased to D2. The character of the gravity field depends primarily upon the initial and final values of flexural rigidity. These D1 and D2 values are varied to obtain good agreement between the model and observed gravity anomalies. Best fitting models have a 180 times increase in flexural rigidity between ridge formation and sediment burial. An approximate relationship between flexural rigidity and crustal age shows that the 85°E Ridge was formed on relatively young lithosphere, 5–15 m.y. old and that it was buried when the lithosphere was 40–80 m.y. old.

Sandwell, DT.  1982.  Thermal isostasy; response of a moving lithosphere to a distributed heat source. Journal of Geophysical Research. 87:1001-1014., Washington, DC, United States (USA): American Geophysical Union, Washington, DC   10.1029/JB087iB02p01001   AbstractWebsite

Spreading ridges and hot spot swells are identified by their high surface heat flow, shallow seafloor, and high geopotential. To understand these and other thermotectonic features, the oceanic lithosphere is modeled as a thermomechanical boundary layer moving through a three-dimensional, time-independent heat source. The heat source mimics the heat advection associated with a spreading ridge or hot spot without introducing the nonlinearities of these flow processes. The Fourier transforms of three Green's functions (response functions), which relate the three observable fields to their common heat source, are determined analytically. Each of these reponse functions is highly anisotropic because the lithosphere is moving with respect to the source. However, the ratio of the gravity response function to the topography response function (i.e., gravity/topography transfer function) is nearly isotropic and has a maximum lying between the flexural wavelength and 2pi times the thickness of the thermal boundary layer. The response functions are most useful for determining the surface heat flow, seafloor topography, and geopotential for complex lithospheric thermal structures. In practice, these three observables are calculated by multiplying the Fourier transform of the heat source by the appropriate response function and inverse transforming the products. Almost any time-independent thermotectonic feature can be modeled using this technique. Included in this report are examples of spreading ridges and thermal swells, although more complex geometries such as ridges offset by transform faults and RRR-type triple junctions can also be modeled. Because forward modeling is both linear and computationally simple, the inverse of this technique could be used to infer some basic characteristics of the heat source directly from the observed fields.

Sandwell, DT.  1981.  Spreading Ridges, Fractures Zones, and Thermal Swells. Ph. D.:214., Los Angeles: University of California, Los Angeles Abstract
Sandwell, DT, Poehls KA.  1980.  A Compensation Mechanism for the Central Pacific. Journal of Geophysical Research. 85:3751-3758.   10.1029/JB085iB07p03751   AbstractWebsite

Geos 3 derived geoid heights and sea floor topography were averaged into 256 square areas (203 km on a side) for a region in the central Pacific containing a large portion of the Hawaiian Island chain. The whole region is about 500 m shallower than normal sea floor of the same age. The major portion of the depth anomaly is the Hawaiian swell. Data were analyzed using a two-dimensional fast Fourier transform. A transfer function was computed to determine the part of the observed geoid height that is coherent and in phase with the topography. A number of compensation models were tested against this function. Of these models no single physically reasonable model was found to have an acceptable fit. Accordingly, two models were introduced, one compensating short-wavelength topography at a shallow depth (14 km) and the other compensating the longer wavelengths by a deep mechanism. Acceptable deep compensation models include Airy-Heiskanen type compensation at depths between 40 and 80 km. Using the transition wavelength between the two models (1100 km), an estimate is made of the amplitude and shape of the heat anomaly needed to uplift the Hawaiian swell. The peak of the anomaly has an amplitude of 530 mW m−2 and is located 275 km east of Hawaii.

Sandwell, D, Schubert G.  1980.  Geoid Height Versus Age for Symmetric Spreading Ridges. Journal of Geophysical Research. 85:7235-7241.   10.1029/JB085iB12p07235   AbstractWebsite

Geoid height-age relations have been extracted from Geos 3 altimeter data for large areas in the North Atlantic, South Atlantic, southeast Indian, and southeast Pacific oceans. Except for the southeast Pacific area, geoid height decreases approximately linearly with the age of the ocean floor for ages less than about 80 m.y. in agreement with the prediction of an isostatically compensated thermal boundary layer model (Haxby and Turcotte, 1978). The geoid-age data for 0 to 80 m.y. are consistent with constant slopes of −0.094±0.025, −0.131±0.041, and −0.149±0.028 m/m.y. for the South Atlantic, southeast Indian, and North Atlantic regions, respectively. For ages greater than 80 m.y. the geoid-age relation for the North Atlantic is nearly flat, indicating a reduction in the rate of boundary layer thickening with age. The uncertainties in the geoid slope-age estimates are positively correlated with spreading velocity.