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At least 217 records · Page 12

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NASA NTRS1985

Analysis of a method for precisely relating a seafloor point to a distant point on land

A study of the environmental constraints and engineering aspects of the acoustic portion of a system for making geodetic ties between undersea reference points and others on land is described. Important areas in which to make such observations initially would be from the California mainland out to oceanic points seaward of the San Andreas fault, and across the Aleutian Trench. The overall approach would be to operate a GPS receiver in a relative positioning (interferometric) mode to provide the long range element of the baseline determination (10 to 1,000 km) and an array of precision sea floor acoustic transponders to link the locally moving sea surface GPS antenna location to a fixed sea floor point. Analyses of various environmental constrants (tides, waves, currents, sound velocity variations) lead to the conclusion that, if one uses a properly designed transponder having a remotely controllable precise retransmission time delay, and is careful with regard to methods for installing these on the sea floor, one should, in many ocean locations, be able to achieve sub-decimeter overall system accuracy. Achievements of cm accuracy or better will require additional understanding of time and space scales of variation of sound velocity structure in the ocean at relevant locations.

Spiess, F. N.↗

NASA NTRS1985

Samples at the Apollo 15 landing site: Types and distribution

Samples, consisting of more than 350 individually numbered samples of rock and regolith, were collected during the three EVAs on the Apollo 15 mission. Samples consisted of rock specimens, and scooped, trenched, and cored regolith samples. Petrographic and chemical analysis are discussed.

Ryder, G.↗

NASA NTRS1986

Age constraints on the evolution of the Quetico belt, Superior Province, Ontario

Much attention has been focused on the nature of Archean tectonic processes and the extent to which they were different from modern rigid-plate tectonics. The Archean Superior Province has linear metavolcanic and metasediment-dominated subprovinces of similar scale to cenozoic island arc-trench systems of the western Pacific, suggesting an origin by accreting arcs. Models of the evolution of metavolcanic belts in parts of the Superior Province suggest an arc setting but the tectonic environment and evolution of the intervening metasedimentary belts are poorly understood. In addition to explaining the setting giving rise to a linear sedimentary basin, models must account for subsequent shortening and high-temperature, low-pressure metamorphism. Correlation of rock units and events in adjacent metavolcanic and metasedimentary belts is a first step toward understanding large-scale crustal interactions. To this end, zircon geochronology has been applied to metavolcanic belts of the western Superior Province; new age data for the Quetico metasedimentary belt is reported, permitting correlation with the adjacent Wabigoon and Wawa metavolcanic subprovinces.

Percival, J. A.↗

NASA NTRS1985

The Martian dust storm of Sol 1742

After nearly five earth years on Mars, the Mutch Memorial Station (Viking Lander 1) finally witnessed a local dust storm that eroded trenches, conical piles, and other disturbed surfaces in the sample field and near the Lander. The event, called the Dust Storm of Sol 1742, occurred late in the third winter of Lander observations between Sols 1728 and 1757. Analyses of tiny new wind tails and movement of materials indicate that the eroding winds were variable but northeasterly than those that had previously shaped the surface. Pebbly residues and movement of 4-5 mm clods suggest drag velocities or friction speeds of the winds were about 2.2-4.0 m/s. Wind speeds at the height of the meteorology boom (1.6 m) were probably about 40-50 m/s. Much of the observed erosion could have occurred in a few to several tens of seconds, but somewhat longer times are suggested by analogy with the erosion of terrestrial soils. Most of the erosion occurred where preexisting equilibrium conditions of surface configurations and surface material properties had been altered by the Lander during landing and during surface-sampler activities, but thin layers of bright fine-grained dust were also removed and redistributed. Surfaces where preexisting equilibrium conditions were unaltered appeared to be uneroded.

Moore, H. J.↗

NASA NTRS1985

Earthquake deformation cycle in the Andean Back Arc, western Argentina

It is pointed out that the 1977 Caucete earthquake was an east-west compressive event which occurred at a depth of about 17 km in the Andean back arc approximately 450 km east of the Chile trench. The present study is primarily concerned with the mechanics of fault behavior prior to and after the 1977 earthquake, as evidenced by preseismic and postseismic vertical deformation. Simple fault models for each phase of deformation are presented. The models of preseismic slip, coseismic slip, and postseismic slip consist of uniform dislocations in an elastic half-space following the method of Chinnery (1961). The basically elastic models presented demonstrate that deformations in the vicinity of the 1977 Caucete, Argentina, earthquake are consistent with activity on some portion of the coseismic fault. This is evidence that these deformations are directly related to the processes of strain accumulation and release associated with the event.

Reilinger, R.↗

NASA NTRS1986

Spectral mixture modeling - A new analysis of rock and soil types at the Viking Lander 1 site

A Viking Lander 1 image was modeled as mixtures of reflectance spectra of palagonite dust, gray andesitelike rock, and a coarse rocklike soil. The rocks are covered to varying degrees by dust but otherwise appear unweathered. Rocklike soil occurs as lag deposits in deflation zones around stones and on top of a drift and as a layer in a trench dug by the lander. This soil probably is derived from the rocks by wind abrasion and/or spallation. Dust is the major component of the soil and covers most of the surface. The dust is unrelated spectrally to the rock but is equivalent to the global-scale dust observed telescopically. A new method was developed to model a multispectral image as mixtures of end-member spectra and to compare image spectra directly with laboratory reference spectra. The method for the first time uses shade and secondary illumination effects as spectral end-members; thus the effects of topography and illumination on all scales can be isolated or removed. The image was calibrated absolutely from the laboratory spectra, in close agreement with direct calibrations. The method has broad applications to interpreting multispectral images, including satellite images.

Adams, J. B.↗

NASA NTRS1986

Spherical harmonic expansions of the Earth's gravitational potential to degree 360 using 30' mean anomalies

Two potential coefficient fields that are complete to degree and order 360 have been computed. One field (OSU86E) excludes geophysically predicted anomalies while the other (OSU86F) includes such anomalies. These fields were computed using a set of 30' mean gravity anomalies derived from satellite altimetry in the ocean areas and from land measurements in North America, Europe, Australia, Japan and a few other areas. Where no 30' data existed, 1 deg x 1 deg mean anomaly estimates were used if available. No rigorous combination of satellite and terrestrial data was carried out. Instead advantage was taken of the adjusted anomalies and potential coefficients from a rigorous combination of the GEML2' potential coefficient set and 1 deg x 1 deg mean gravity anomalies. The two new fields were computed using a quadrature procedure with de-smoothing factors. The spectra of the new fields agree well with the spectra of the fields with 1 deg x 1 deg data out to degree 180. Above degree 180 the new fields have more power. The fields have been tested through comparison of Doppler station geoid undulations with undulations from various geopotential models. The agreement between the two types of undulations is approximately + or - 1.6 m. The use of a 360 field over a 180 field does not significantly improve the comparison. Instead it allows the comparison to be done at some stations where high frequency effects are important. In addition maps made in areas of high frequency information (such as trench areas) clearly reveal the signal in the new fields from degree 181 to 360.

Rapp, Richard H.↗

NASA NTRS1986

Evaluation of the airborne imaging spectrometer for remote sensing of forest stand conditions

Five pairs of plots were established in forest stands with one of each pair trenched and covered to prevent precipitation from reaching the tree roots. High winds and falling limbs destroyed the covers on three of the plots. The two remaining plots were in a red pine plantation and in a natural stand of sugar maple. Trees in both plots developed levels of moisture stress more than nine bars higher than control trees on the dates of overflights with the Airborne Imaging Spectrometer (AIS) and the Collins' Airborne Spectroradiometer (CAS). Hemispherical reflectance from stressed and control trees was measured with a Beckman DK2A spectrophotometer. On the day of the AIS overflight, stressed maple foliage was less reflective than the control from 1000 to 1300 nm, but more reflective at wavelengths longer than 1300 nm. Pine foliage was less reflective than the control from 1000 to 1600 nm, but the difference was small at wavelengths longer than 1350 nm. AIS data collected showed brightness values for both maple and pine to be lower than for the controls from 1000 to 1300 nm. CAS data were used to determine the gain in species identification accuracy obtainable with high spectral resolution data.

Olson, Charles E., Jr.↗

NASA NTRS1987

Venusian tectonics: Convective coupling to the lithosphere?

The relationship between the dominant global heat loss mechanism and planetary size has motivated the search for tectonic style on Venus. Prior to the American and Soviet mapping missions of the past eight years, it was thought that terrestrial style plate tectonics was operative on Venus because this planet is approximately the size of the Earth and is conjectured to have about the same heat source content per unit mass. However, surface topography mapped by the altimeter of the Pioneer Venus spacecraft did not show any physiographic expression of terrestrial style spreading ridges, trenches, volcanic arcs or transform faults, although the horizontal resolution was questionable for detection of at least some of these features. The Venera 15 and 16 radar missions mapped the northern latitudes of Venus at 1 to 2 km resolution and showed that there are significant geographic areas of deformation seemingly created by large horizontal stresses. These same high resolution images show no evidence for plate tectonic features. Thus a fundamental problem for venusian tectonics is the origin of large horizontal stresses near the surface in the apparent absence of plate tectonics.

Phillips, R. J.↗

NASA NTRS1986

The Nazca-South American convergence rate and the recurrence of the great 1960 Chilean earthquake

The seismic slip rate along the Chile Trench estimated from the slip in the great 1960 earthquake and the recurrence history of major earthquakes has been interpreted as consistent with the subduction rate of the Nazca plate beneath South America. The convergence rate, estimated from global relative plate motion models, depends significantly on closure of the Nazca - Antarctica - South America circuit. NUVEL-1, a new plate motion model which incorporates recently determined spreading rates on the Chile Rise, shows that the average convergence rate over the last three million years is slower than previously estimated. If this time-averaged convergence rate provides an appropriate upper bound for the seismic slip rate, either the characteristic Chilean subduction earthquake is smaller than the 1960 event, the average recurrence interval is greater than observed in the last 400 years, or both. These observations bear out the nonuniformity of plate motions on various time scales, the variability in characteristic subduction zone earthquake size, and the limitations of recurrence time estimates.

Stein, S.↗

NASA NTRS1987

The effects of gas mixtures on ion engine erosion and performance

Erosion measurements were performed on a modified J-series 30 cm ion engine operating on xenon propellant. Erosion data was obtained by measuring the trench depth etched into masked polished metal samples for test durations of up to 24 hours. The data indicates that erosion is greatest at the cathode side of the baffle, with tantalum being the material with the least erosion of all materials tested. There is a clear indication of a significant reduction in erosion of all materials tested when nitrogen is added to the propellant. The technique used in these experiments requires test samples which are extremely smooth and flat.

Garner, Charles E.↗

NASA NTRS1988

Calculation of flow about posts and powerhead model

A large number of computational fluid mechanics (CFD) problems were investigated. The primary studies include: the analysis of the turnaround duct/hot gas manifold/transfer tubes (fuel side) of the Space Shuttle Main Engine (SSME); the analysis of the LOX-T manifold (oxidizer side) of the SSME; the analysis of hydrogen accumulation in the Vandeburg flame trench; and modification of the Intel/VT241 systems to accommodate the EADS and PLOT 3D. Some of the analyses were exploratory in nature, using the CONTINUSYS code to provide preliminary information to enhance understanding of the problem, while in other the primary thrust was to acquire design information. In all cases the ability to predict information rapidly in these very complex analyses is seen to be an important demonstration of the power and utility of this mature predictive capability.

Source record↗

NASA NTRS1988

A physical model for earthquakes. I - Fluctuations and interactions. II - Application to southern California

The idea that earthquakes represent a fluctuation about the long-term motion of plates is expressed mathematically through the fluctuation hypothesis, under which all physical quantities which pertain to the occurance of earthquakes are required to depend on the difference between the present state of slip on the fault and its long-term average. It is shown that under certain circumstances the model fault dynamics undergo a sudden transition from a spatially ordered, temporally disordered state to a spatially disordered, temporally ordered state, and that the latter stages are stable for long intervals of time. For long enough faults, the dynamics are evidently chaotic. The methods developed are then used to construct a detailed model for earthquake dynamics in southern California. The result is a set of slip-time histories for all the major faults, which are similar to data obtained by geological trenching studies. Although there is an element of periodicity to the events, the patterns shift, change and evolve with time. Time scales for pattern evolution seem to be of the order of a thousand years for average recurring intervals of about a hundred years.

Rundle, John B.↗

NASA NTRS1988

Intraplate deformation and closure of the Australia-Antarctica-Africa plate circuit

Plate motion data along the Southeast, Southwest, and Central Indian ridges have been reduced to 67 spreading rates, 38 transform fault azimuths, and 135 earthquake slip vectors in order to study the current motion between the Australian, Antarctic, and African plates and to investigate whether this plate circuit obeys closure. Magnetic profiles are modeled to determine rates consistently over a 3-m.y. time-averaging interval, and the new rates are shown to differ from published rates by as much as 5 mm/yr. The results indicate that Indian Ocean plate circuit nonclosure and the deformation that it suggests are much smaller than previously supposed, and support a model in which the significant deformation occurs in a diffuse plate boundary along the equatorial Indian Ocean between the Central Indian Ridge and the Sumatra Trench.

Demets, Charles↗

NASA NTRS1989

Tectonism on Venus: A review

Venus is more similar to Earth than to any other planet. It has elevated regions associated with marginal fold and thrust belts, fracture zones that extend tens of thousands of kilometers, crustal swells and shields that are hundreds of kilometers in diameter and 1 to 2 km high, and sublinear accumulations of volcanic cones and domes that stretch for thousands of kilometers across the plains. The Venusian surface is, however, distinctly different from Earth's in that: (1) its elevated terrains cannot be distinguished from its low plains on a hypsometric curve; (2) trenches have not been found plainsward of the marginal belts; (3) fracture zones bear no resemblance to mid-oceanic ridges; and (4) some features, such as the ridge-belt zone near 210 deg E, seem to have no terrestrial analog. Various theories about tectonism on Venus and Earth of other authors are reviewed.

Kozak, Richard C.↗

NASA NTRS1989

Methods for local gravity field approximation

The most widely known modern method for estimating gravity field values from observed data is least-squares collocation. Its advantages are that it can make estimates at arbitrary locations based on irregularly spaced observations, and that it makes use of statistical information about errors in the input data while providing corresponding information about the quality of the output estimates. Disadvantages of collocation include the necessity of inverting square matrices of dimension equal to the number of data values and the need to assume covariance models for the gravity field and the data errors. Fourier methods are an important alternative to collocation; having the advantage of greater computational efficiency, but requiring data estimates to be on a regular grid and not using or providing statistical accuracy information. The GEOFAST algorithm is an implementation of collocation that achieves high computational efficiency by transforming the estimation equations into the frequency domain where an accurate approximation may be made to reduce the workload. The forward and inverse Fast Fourier Transforms (FFTs) are utilized. The accuracy and computational efficiency of the GEOFAST algorithm is demonstrated using two sets of synthetic gravity data: marine gravity for an ocean trench region including wavelengths longer than 200 km; and local land gravity containing wavelengths as short as 5 km. These results are discussed along with issues such as the advantages of first removing reference field models before carrying out the estimation algorithm.

Sailor, R. V.↗

NASA NTRS1988

Energy Efficient Engine: High-pressure compressor test hardware detailed design report

The objective of the NASA Energy Efficient Engine program is to identify and verify the technology required to achieve significant reductions in fuel consumption and operating cost for future commercial gas turbine engines. The design and analysis is documented of the high pressure compressor which was tested as part of the Pratt and Whitney effort under the Energy Efficient Engine program. This compressor was designed to produce a 14:1 pressure ratio in ten stages with an adiabatic efficiency of 88.2 percent in the flight propulsion system. The corresponding expected efficiency for the compressor component test rig is 86.5 percent. Other performance goals are a surge margin of 20 percent, a corrected flow rate of 35.2 kg/sec (77.5 lb/sec), and a life of 20,000 missions and 30,000 hours. Low loss, highly loaded airfoils are used to increase efficiency while reducing the parts count. Active clearance control and case trenches in abradable strips over the blade tips are included in the compressor component design to further increase the efficiency potential. The test rig incorporates variable geometry stator vanes in all stages to permit maximum flexibility in developing stage-to-stage matching. This provision precluded active clearance control on the rear case of the test rig. Both the component and rig designs meet or exceed design requirements with the exception of life goals, which will be achievable with planned advances in materials technology.

Howe, David C.↗

NASA NTRS1989

Tectonics and volcanism in central Mexico - A Landsat Thematic Mapper perspective

Digitally enhanced Landsat Thematic Mapper (TM) images were used to map neotectonic deformation in central Mexico. This region has been studied for decades using a variety of geological and geophysical techniques, but synoptic mapping of neotectonic activity and major fault zones there, and an evaluation of their regional relationship to the character and location of volcanism were not previously possible until the application of synoptic, high resolution satellite imagery. Interpretation of the TM images shows that the tectonic deformation is closely linked in time and space to the dominantly calc-alkaline volcanics of the Mexican Volcanic Belt (MVB). The eruptive style and distribution of the volcanics is clearly related to the deformation resulting from relative motions of three large crustal blocks south of the MVB. Therefore, zones of weakness within the crust of central Mexico, which may be inherited from earlier episodes of deformation, are a principal factor controlling the oblique orientation of the MVB relative to the Acapulco Trench.

Johnson, C. A.↗