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At least 1,027 records · Page 57
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Structure of the St. Francois Mountains and surrounding lead belt, S. E. Missouri: Inferences from thermal IR and other data sets

Day-IR, day-visible, and night-IR image data sets were analyzed. All three images were contrast enhanced, using a linear stretch. The night-IR image was destriped using a box-car filtering approach. Apparent thermal inertia images were generated from the data. The enhanced data and the apparent thermal inertia image were also registered and overlayed onto shaded relief images depicting topography and onto a colored version of the Missouri geologic map. The combination of the apparent thermal inertial image and the shaded relief map proved to have the greatest discriminability in terms of portraying linear features. The reason seems to be that the vegetation canopy on the Ozark Plateau is different topography, tend to emphasize subtle topographic effects.

Arvidson, R. E.↗
NASA NTRS1981

Observations and models of inertial waves in the deep ocean

A study of the structure of the inertial peak in deep ocean kinetic energy is presented, based on records taken from Polymode arrays deployed in the western North Atlantic Ocean. Results are interpreted in terms of both local sources and turning point effects on internal waves generated at lower latitudes, and it is found that three classes of environment and their corresponding spectra emerge from peak height variations: (1) the 1500-m level near the Mid-Atlantic Ridge, with the greatest peak height of 18 dB; (2) the upper and deep ocean over rough topography and the deep ocean underneath the Gulf Stream, with the intermediate peak height of 11.5 dB; and (3) the deep ocean over smooth topography, with the lowest peak height of 7.5 dB. Using the globally valid wave functions obtained by Munk and Phillips (1968), frequency spectra near f are calculated numerically. The model is latitudinally dependent, with the frequency shift and bandwidth of the inertial peak decreasing with latitude.

Fu, L.-L.↗
NASA NTRS1982

The Seasat altimeter mean sea surface model

Gridding techniques are used to combine an 18-day set of Seasat altimeter data and two precisely-computed Seasat ephemerides, in order to arrive at global contour maps of the mean sea surface topography. The altimeter data have an rms agreement of 111 cm with the SS3 mean sea surface computed by means of the PGS-S3 ephemerides, and of 70 cm with the SS4 mean surface derived from the PGS-S4 ephemerides. While comparisons with the GEM 10B 1 x 1 deg gravimetric geoid have yielded rms differences of 2.8 m, those with a global mean sea surface derived from GEOS 3 altimeter data show rms differences of 1.3 m and 1.1 m for the cases of the SS3 and SS4 surfaces, respectively. An SS4 mean sea surface topograph is featured among the study findings presented. Further improvements in the representation of mean sea surface topography are expected with the development of more accurate gravity models for orbit computation.

Marsh, J. G.↗
NASA NTRS1982

Support of topographic and other loads on the moon and on the terrestrial planets

The existence of mascons on the moon indicates that the lunar elastic lithosphere can support substantial loads for about three billion years. Lunar topography also appears to be uncompensated. Observations of gravity on Mars show that the Tharsis uplift is only partially compensated. A number of authors have attributed this support to lithospheric flexure. In this paper it is shown that membrane stresses play an important role in the support of loads on the moon, Mars, and Mercury. For loads that have been expressed in terms of spherical harmonics analytical expressions for the degree of compensation are obtained as a function of the degree of the load. The results are compared with the observed dependence of the ratio of gravitational potential to topography on degree. It is also concluded that membrane stresses can support a significant ellipticity for a tidally despun planetary body.

Willemann, R. J.↗
NASA NTRS1982

The evolution of impact basins - Viscous relaxation of topographic relief

A topographic profile of the young large lunar basin, Orientale, is presented in order to examine the effects of viscous relaxation on basin topography. Analytical models for viscous flow are considered, showing a wavelength-dependence of time constants for viscous decay on the decrease in viscosity with depth and on the extent of the isostatic compensation of the initial topography. Lunar rheological models which are developed include a half-space model for uniform Newtonian viscosity, density, and gravitational acceleration, a layer over inviscid half space model with material inviscid over geological time scales, and a layer with isostatic compensation where a uniformly viscous layer overlies an inviscid half space of higher density. Greater roughness is concluded, and has been observed, on the moon's dark side due to continued lower temperatures since the time of heavy bombardment.

Solomon, S. C.↗
NASA NTRS1981

Bathymetric imaging

Digital topography has, for some years, been formatted and processed into shaded relief images for specific studies involving land use and thermal properties. Application to bathymetry is a new and seemingly fruitful extension of these techniques. Digital terrain models of the earth - combining subaerial topography with an extensive collection of bathymetric soundings - have been processed to yield shaded relief images. These images provide new and exciting insights into submarine geomorphology and portray many aspects of plate tectonic physiography in a manner not previously possible.

Paluzzi, P. R.↗
NASA NTRS1983

Surface finish measurement studies

The performance of stylus instruments for measuring the topography of National Transonic Facility (NTF) model surfaces both for monitoring during fabrication and as an absolute measurement of topography was evaluated. It was found that the shop-grade instruments can damage the surface of models and that their use for monitoring fabrication procedures can lead to surface finishes that are substantially out of range in critical areas of the leading edges. The development of a prototype light-scattering instrument which would allow for rapid assessment of the surface finish of a model is also discussed.

Teague, E. C.↗
NASA NTRS1983

Elastohydrodynamic contacts. Effects of dents and grooves on traction and local film thickness

Traction and film thickness were simultaneously measured under sliding elastohydrodynamic (EHD) conditions. The influence of surface topography was investigated by using simulated surfaces produced by depressing dents and grooves in highly polished steel balls. Significant changes in traction occurred depending on the orientation of the surface defects and their location with respect to the contact region. The results can be explained in terms of changes in overall film thickness and redistribution of pressure within the contact region due to micro-EHD effects. It can be concluded that the traction capability of mechanical components operating in thin film lubrication can be enhanced particularly by surface topographical orientation perpendicular to the surface motion. Associated with the higher traction are increases in local shear stress and normal stress as well as an increase in temperature at asperity sites. It is postulated that the local surface topography can become involved in run in or failure initiation even without actual asperity contact.

Wedeven, L. D.↗
NASA NTRS1983

An investigation to model radar backscatter from ridged first year sea ice for the period ending December 31, 1981

The response of the microwave scatterometer to meter scale roughness of first year sea ice was investigated. An empirical, one dimensional model was developed to predict the back scattered power from ridged first year sea ice of known surface topography, by interpreting the surface as a series of angular facets and summing the proportional power from each facet. The model is evaluated by comparing the predicted values to scatterometer measurements obtained during NASA's Sea Ice Radar Experiment (SIRE). The input to the model is surface slope, which for SIRE was calculated from the ice topography, as measured by an airborne laser profiler. The correlation between the predicted and the measured power is .62 to .68. It is concluded that the model adequately describes the backscatter from regions of first year sea ice whose dimensions are of a few tens of meters.

Jackson, B. L. W.↗
NASA NTRS1981

Drumlin fields and glaciated mountains - A contrast in geomorphic perception from Seasat radar images

Digitally correlated Seasat synthetic-aperture radar (SAR) images of the Alaska Range, Alaska, and the drumlin-drift belt in Ireland are analyzed for the perception and identification of geomorphic features. The two terrains display strongly contrasted types of glacial topography whose identification in each case is related to the geometry of the Seasat imaging radar. Identification of terrain shape and form is important within the caveats imposed by the intrinsic distortions on the radar images. Image texture serves coarsely to distinguish topography. Image tones are scene-dependent and do not uniquely identify specific targets. Extensive alignments of linear and curvilinear features provide some of the more important image information from which to make geologic interpretations in each case.

Ford, J. P.↗
NASA NTRS1982

Effects of differential rotation on the gravitational figures of Jupiter and Saturn

It is assumed that observed zonal currents in the atmospheres of Jupiter and Saturn correspond to a state of permanent rotation, and that the angular velocity is constant on cylindrical surfaces parallel to the rotation axis. The equation of hydrostatic equilibrium for a rotating planet is solved under these restrictive assumptions, and the effect of the hypothesized rotation state on the planet's gravity harmonics and external shape is investigated. Spacecraft data on zonal currents are used to derive nearly model-independent corrections to the first four zonal gravity harmonic coefficients, which can be used to correct observed gravity harmonics to values appropriate for solid-body rotation. If the assumed rotation state is applicable, then zonal currents lead to measurable topography of isopycnic surfaces with respect to the reference figure defined by the magnetospheric rotation period and the gravity harmonics. The amplitude of the topography is on the order of 5 km for Jupiter and 60 km for Saturn.

Hubbard, W. B.↗
NASA NTRS1983

Geodynamics Branch research report, 1982

The research program of the Geodynamics Branch is summarized. The research activities cover a broad spectrum of geoscience disciplines including space geodesy, geopotential field modeling, tectonophysics, and dynamic oceanography. The NASA programs which are supported by the work described include the Geodynamics and Ocean Programs, the Crustal Dynamics Project, the proposed Ocean Topography Experiment (TOPEX) and Geopotential Research Mission. The individual papers are grouped into chapters on Crustal Movements, Global Earth Dynamics, Gravity Field Model Development, Sea Surface Topography, and Advanced Studies.

Kahn, W. D.↗
NASA NTRS1984

Somali current studied from SEASAT altimetry

Mesoscale variability has been obtained for the world ocean from satellite altimetry by using the repetitive tracks data of SEASAT. No significant results were obtained for the Somali current area for two main reasons: the repetitive tracks are too sparse to cover the expected eddy pattern and these data were obtained in late September and early October when the current is strongly decaying. The non-repetitive period of SEASAT offers the possibility to study a dozen of tracks parallel to the eddy axis or crossing it. These are used here to deduce the dynamic topography of the Somali current. Data error reduction and tide and orbit corrections are addressed. A local geoid was built using a collocation inverse method to combine surface gravity data and altimetry: the repetitive tracks show no variability (which confirms that the current is quasi-inexistent at that time) and can be used as data for the local geoid. This should provide a measure of the absolute dynamic topography of the Somali current.

Perigaud, C.↗
NASA NTRS1984

Research activities of the Geodynamics Branch

A broad spectrum of geoscience disciplines including space geodesy, geopotential field modeling, tectonophysics, and dynamic oceanography are discussed. The NASA programs, include the Geodynamics and Ocean Programs, the Crustal Dynamics Project, the proposed Ocean Topography Experiment (TOPEX), and the Geopotential Research Mission (GRM). The papers are grouped into chapters on Crustal Movements, Global Earth Dynamics, Gravity Field Model Development, Sea Surface Topography, and Advanced Studies.

Kahn, W. D.↗
NASA NTRS1984

Investigation of mesoscale pre-convective forcing mechanisms

The installation and testing of a numerical mesoscale boundary layer model is discussed. A data analysis is first made using the rawinsonde network which shows a strong baroclinic zone developing in the region where convection was initiated. The observed wind field shows an easterly component developing in opposition to the synoptic flow producing convergence. The numerical boundary layer model is used to diagnose the physical development of the thermal and flow fields. Two physical mechanisms are examined; topography and cloud shading. The results shows that while heated sloping topography alone does create a baroclinic zone and convergence values, cloud shading is a stronger mechanism for producing a thermal field representative of observed conditions

Mcnider, R. T.↗
NASA NTRS1983

Barotropic oscillations of the Mediterranean and Adriatic Seas

Calculations of the periods and structures of several of the lowest barotropic free modes of oscillation of the combined Mediterranean-Adriatic Basin are presented which take into account basin morphometry, bottom topography and the earth's rotation. The numerical calculations, based on a Galerkin procedure developed by Rao and Schwab (1976), were first carried out to find the normal modes of the combined Mediterranean-Adriatic system at a resolution of 1 deg on a Mercator projection, and used to determine the mouth of the Adriatic, which was then examined on a finer grid without rotation. Comparison of the periods of the lowest gravitational modes of the Mediterranean Sea under different conditions show the most significant effect to be due to variable basin topography. Periods of 38.5, 11.4, 8.4 and 7.4 h are computed for the lowest modes of the Mediterranean Sea, while periods of 21.9, 10.7 and 6.7 h are computed for the Adriatic, in agreement with observed periods.

Schwab, D. J.↗
NASA NTRS1983

On determining the large-scale ocean circulation from satellite altimetry

It is contended that a spherical harmonic expansion of the difference between the altimeter-derived mean sea surface and the geoid estimate should reveal the large-scale circulation of the ocean surface layer when the low-degree terms are examined. Methods based on this principle are proposed and partially demonstrated over the Pacific Ocean with the aid of the mean sea surface derived from the Seasat altimeter and the Goddard Earth Model 9 earth gravity model. The preliminary results reveal a well-defined clockwise gyre in the North Pacific and a much less well defined counterclockwise gyre in the South Pacific. When the dynamic topography thus obtained is compared with Wyrtki's (1975) dynamic topography derived from hydrographic data, the agreement is found to be within the limit of geoid uncertainties and satellite orbital errors.

Tai, C.-K.↗
NASA NTRS1984

Tectonic contrasts between Venus and the earth

The long-wave features of the gravity field of Venus differ from those of the earth's field not only in their strong positive correlation with topography, but also in their gentler spectral slope. These properties are inconsistent with generation of the gravity field by plate tectonics or by processes at great depths; they are consistent with generation by a mantle convective system supporting the broad features in topography with an effective compensation depth of about 450 km.

Kaula, W. M.↗