Characterization of the Structure of River-Bed Gravels Using Two-Dimensional Fractal Analysis   总被引：7，自引：0，他引：7  

Justin B. Butler  Stuart N. Lane  Jim H. Chandler 《Mathematical Geology》2001,33(3):301-330

This paper is concerned with the application of fractal analysis to understand the structure of water-worked gravel-bed river surfaces. High resolution digital elevation models, acquired using digital photogrammetric methods, allowed the application of two-dimensional fractal methods. Previous gravel-bed river studies have been based upon sampled profiles and hence one-dimensional fractal characterisation. After basic testing that bed elevation increments are Gaussian, the paper uses two-dimensional variogram surfaces to derive directionally dependent estimates of fractal dimension. The results identify mixed fractal behavior with two characteristic fractal bands, one associated with the subgrain scale and one associated with the grain scale. The subgrain scale characteristics were isotropic and sensitive to decisions made during the data collection process. Thus, it was difficult to differentiate whether these characteristics were real facets of the surfaces studied. The second band was anisotropic and not sensitive to data collection issues. Fractal dimensions were greater in the downstream direction than in other directions suggesting that the effects of water working are to alter the level of surface organisation, by increasing surface irregularity and hence roughness. This is an important observation as it means that water-worked surfaces may have a distinct anisotropic signal, revealed when using a fractal type analysis.  相似文献   

Thermal convection in a cylindrical annulus with a non-Newtonian outer surface     

Stuart A. Weinstein 《Pure and Applied Geophysics》1996,146(3-4):551-572

This study presents the results of numerical simulations of a model for lithospheremantle coupling in a terrestrial type planet. To first order, a geologically active terrestrial type planet may consist of a metallic core, silicate mantle and lithosphere, with the lithosphere being rheologically different from the mantle. Therefore we have developed a numerical model consisting of a thin non-Newtonian fluid hoop that is dynamically coupled to a thick Newtonian fluid cylindrical annulus. Thus the rheological dichotomy between mantle and lithosphere is built into the model. Time-dependent calculations show the existence of at least two regimes of behaviors. In one regime, the behavior of the hoop switches between periods characterized by low or high speeds, in response to changes in convective vigor and planform. This regime may apply to the planet Venus where the available evidence indicates that prior to 500 myr ago, the planet was resurfaced on a time scale of <100 myr. Since that time, large-scale tectonic activity on Venus has been sharply curtailed. In the other regime, which is more like plate tectonics on Earth, the hoop speeds rise and fall on short time scales.  相似文献   

The impact of the hexos programme     

Stuart D. Smith  Kristina B. Katsaros  Wiebe A. Oost  Patrice G. Mestayer 《Boundary-Layer Meteorology》1996,78(1-2):121-141

In HEXOS, a programme of coordinated laboratory, field and model studies, an international group of participants has extended the range of measurements of evaporation from the sea and has investigated the role of droplets in the transfer of water to the atmosphere. Predictions of a rapid rise in the evaporation coefficient at wind speeds above 15 m s^-1 have not been substantiated. Wind stress measurements showed a relationship with wave age. New methods were developed for coping with flow distortion.  相似文献   

Agricultural Chemical News     

Stuart Cohen 《Ground Water Monitoring & Remediation》1996,16(1):56-57

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Spatial and temporal variability in the lower food web of the tidal freshwater Hudson River     

Stuart Findlay  Michael Pace  David Fischer 《Estuaries and Coasts》1996,19(4):866-873

The mid Hudson River is a heterotrophic system where allochthonous inputs apparently fuel the largest proportion of secondary production and ecosystem metabolism. We have analyzed a 6-yr dataset collected quarterly at six stations spanning a 150-km reach to assess variability at inter- and intra-annual time scales and regional spatial scales. The major components of the lower food web: bacterial biomass, detrital particulate organic carbon (POC), and dissolved organic carbon (DOC), show surprisingly discordant patterns in temporal and spatial variability. Bacterial abundance shows significant variability at all three scales, but the interannual variability is by far the greatest. DOC concentrations showed greatest variability among years, with intra-annual and spatial variability roughly equal. Freshwater flow is commonly considered a major driving force in river-estuarine variability but simple discharge was not a strong predictor of any component of suspended matter or DOC. For organisms in the Hudson River food web, these multiple scales of variability indicate highly unpredictable food resources in time and space, and these fluctuations may contribute to the variability in higher trophic levels.  相似文献   

Agricultural Chemical News     

Stuart Cohen 《Ground Water Monitoring & Remediation》1993,13(4):85-86

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Tektites: A post-Apollo view     

Stuart Ross Taylor 《Earth》1973,9(2):101-123

Prior to the receipt of the lunar samples, it was the scientific consensus that tektites were melted and splashed material formed during large cometary or meteorite impact events. Whether the impact took place on the Earth or the Moon was the topic of a long-standing scientific debate, which raged with particular intensity during the decade previous to the lunar landings.Four definite and separate tektite-strewn fields are known: bediasites (North America, 34 m.y.); moldavites (Czechoslovakia, 14 m.y.); Ivory Coast (1.3 m.y.); and Southeast Asian and Australian fields (0.7 m.y.). A fifth possible occurrence, of high-Na australites, possibly 3–4 m.y. old, remains to be substantiated. The age of infall of the australites is not agreed upon. Radiometric and fission track dates agree with the magnetic stratigraphy for deep-sea core microtektite occurrences at about 0.7 m.y. Terrestrial stratigraphic evidence favours a recent (30,000 years) date.The chemistry of tektites appears to reflect that of the parent material, and losses during fusion appear to be restricted to elements and compounds more volatile than cesium. Terrestrial impact glasses provide small-scale analogues of tektite-forming events, and indicate that only the most volatile components are lost during fusion.The Apollo lunar missions provide critical evidence which refutes the hypothesis of lunar origin of tektites. Tektite chemistry is totally distinct from that observed in lunar maria basalts. These possess Cr contents which are two orders of magnitude higher than tektites, distinctive REE patterns with large Eu depletions, high Fe and low SiO₂ contents, low K/U ratios and many other diagnostic features, none of which are observed in the chemistry of tektites. The lunar uplands compositions, as shown by Apollo 14, 15 and 16 samples and the μ-ray and XRF orbiter data, are high-Al, low-SiO₂ compositions totally dissimilar to those of tektites. The composition of lunar rock 12013 shows typical lunar features and is distinct from that of tektites. The small amounts of lunar K-rich granitic material found in the soils have K/Mg and K/Na ratios 10–50 times those of tektites.The ages of the lunar maria (3.2–3.8 aeons) and uplands (> 4.0 aeons) are an order of magnitude older than the parent material of the Southeast Asian and Australian tektites, which yield Rb-Sr isochrons indicating ages of the order of 100–300 m.y. The lunar lead isotopic compositions are highly radiogenic whereas tektites have terrestrial Pb isotopic ratios. Lunar δ¹⁸ O values are low (< 7 per mil) compared with values of +9.6 to +11.5 per mil for tektites. In summary, a lunar impact origin for tektites is not compatible with the chemistry, age or isotopic composition of the lunar samples. A lunar volcanic origin, recently revived by O'Keefe (1970) encounters most of the same problems. Recent lunar volcanism (< 50 m.y.), if the source of tektites, should contribute tektite glass to the upper layers of the regolith. None has been found. The presence of meteoritic components in tektites, and the high pressure phase coesite, are more readily interpreted as evidence of impact.The element abundances and inter-element variations in tektites do not resemble those in terrestrial igneous rocks, but show a close similarity to terrestrial sandstones. The composition of the Southeast Asian tektites, australites and moldavites resembles that of micaceous sandstones or subgreywackes, the Ivory Coast tektite composition is similar to that of greywacke, and the bediasite chemistry is analogous to that of arkose.No suitable terrestrial impact site has been identified for the bediasites, Southeast Asian tektites and australites. It is suggested that a search for the source of these latter strewnfields be made using satellite photographs to look for wide shallow craters produced by super-Tunguska type events on areas of Mesozoic sandstones. The moldavites were possibly formed during the Ries Crater event but, if so, the precise source of the material remains to be identified. The Ivory Coast tektites are linked by chemistry, isotope and age evidence to the Bosumtwi Crater, Ghana. The overall evidence now supports the origin of tektites by cometary (or meteorite) impact on terrestrial sedimentary rocks.  相似文献   

TRANSACTIONAL AND TRANSFORMATIONAL THEORIES IN BEHAVIORAL GEOGRAPHY     

Stuart C. Aitken  Elaine M. Bjorklund 《The Professional geographer》1988,40(1):54-64

Inadequate theoretical conceptualization of person/environment relations has characterized behavioral geography as an eclectic and incoherent subfield. Past work has failed to appreciate the person/environment unity as an entity of change. A transactional/transformational perspective is adopted in this paper as a basis for understanding the relationships between the dynamics of human behavior and the dynamics of the environment. Two conceptual models are developed which provide bases for understanding (i) different modes of person/environment transformation, and (ii) human behavior as it mediates change. The potential relevance of a transactional perspective for empirical research in behavioral geography is noted.  相似文献   

Bathymetry and Canyons of the western Solomon Sea     

H. L. Davies  J. B. Keene  K. Hashimoto  M. Joshima  J. E. Stuart  D. L. Tiffin 《Geo-Marine Letters》1987,6(4):181-191

The floor of the western Solomon Sea (for new bathymetric map see inside back cover of this issue) is dominated by the arched and ridged basement of the Solomon Sea Basin, the partly-sediment-filled New Britain Trench, and a more completely filled trench, the Trobriand Trough. There is a deep basin where the trenches join (149° Embayment), and a silled basin west of the New Britain Trench (Finsch Deep). Submarine canyons descend from the west and south to the 149° Embayment. Abyssal fans and plains are structurally defined and locally disturbed by young faults. Probable submerged pinnacle reefs stand in water depths as great as 1,200 m.  相似文献   

Further aspects of weak shock theory applied to the solar chromosphere     

Stuart D. Jordan 《Solar physics》1973,30(2):327-334

The low chromosphere now seems definitely to require mechanical heating, and dissipation of initially acoustic waves by shocking is one of the most promising possibilities. Results of recent calculations indicate that the weak shock theory may be applicable here, but discrepancies exist among various applications of this theory, and the explanations offered to date are not completely satisfactory. It is shown here that the different approximations used by different authors to evaluate the mechanical flux integral play an important role in producing these discrepancies, in addition to the already well known effects of the density scale heights and the wave periods. Arguments are presented favoring Ulmschneider's method for evaluation of this flux integral.  相似文献