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排序方式: 共有175条查询结果,搜索用时 15 毫秒
71.
The region of the 8200 Å Band of H2O was studied in spectra of Venus obtained with an echelle grating spectrograph operated at an altitude of 14.6 km in the NASA Learjet research aircraft. Taking advantage of low foreground absorption, observing at a time of velocity quadrature, differential spectroscopy with respect to lunar spectra, and spectrum averaging, we establish a value of H2O of 3 ± 20 μ for the total path over the entire disk. This value differs from earlier studies of the integrated disk but supports the low values recently derived from infrared bands and by very high spectral resolution groundbased studies. 相似文献
72.
Eight-Day Swamp is known to be contaminated with heavy metals, especially mercury. Sediment cores were collected to approximately 32 cm at 17 sites on four transects and analyzed at 1 or 2 cm intervals for seven metals and organic matter. Very high metal levels were found throughout the site. Long and Morgan's "effects range-median" (ER-M) was exceeded in more than 50% of slices for all elements except As. Hg had the highest concentration relative to ER-M; median Hg concentration was 72 times its ER-M. On the marsh plain, all metals showed enrichment at 14-20 cm depth of 10-40X over surficial sediments. 137Cs analysis showed sedimentation rates ranging from 0.33 to 0.50 cm yr(-1) over the last approximately 40 yr. These rates indicate that metal contamination peaks occurred in sediments deposited in the early 1960s. Thus, newer, less contaminated sediments are burying older, more contaminated layers and peak levels of contaminants are becoming less available to benthos. 相似文献
73.
This study investigates regions on cognitive maps and methods used to identify their boundaries. Subjects either drew regional boundaries on a map or decided in which region sample points were located. These methods produced similar results with differences associated more with regional boundaries than cores. The amount of time needed to make point decisions demonstrated the subjects' cognitive process and corroborated boundaries and cores for the cognitive regions. Differences among subjects' maps were greater when boundaries were drawn. 相似文献
74.
A procedure has been developed for the long-term distribution of the short-term extreme values of ship wave bending moments. The procedure has been applied in a systematic manner to investigate the effect of the principal ship characteristics on the design wave bending monent. The results have been compared to Classification Societies' Rules and demonstrate a rational way to improve them. The same procedure can also be applied to other ship responses or the responses of any structure excited by seaways. 相似文献
75.
Abstract. Three-weekly transects were done over a 14-month period at Sundays River Beach, a high energy beach in Algoa Bay, South Africa, to determine the life histories, breeding patterns, and fecundities of three intertidal cirolanid isopods. Eurydice longicornis. Pontogeloides latipes , and Excirolana naialensis. E. longicornis exhibits an annual, multivoltine life history with a more extended breeding period than the other two species. P. latipes and E. natalensis both have biennial. univoltine life histories with lower fecundities than E. longicornis. 相似文献
76.
Debra A. Meese Anthony J. Gow Paul A. Mayewski Walter Ficklin Theodore C. Loder 《Estuarine, Coastal and Shelf Science》1987,24(6)
The purpose of this study was to provide general information on the chemical, physical and structural properties of estuarine ice and show how it compares with sea ice found at higher latitudes in order to determine whether the ice in Great Bay can be used as an analog in the study of arctic sea ice.Ice cores and water samples were collected during the 1983–1984 winter season at Adams Point in Great Bay, New Hampshire. Concentrations of chloride, nitrogen (as nitrate and nitrite), bromide, phosphate, sulfate and silicate were determined for samples chosen on the basis of identifiable stratigraphic layers (i.e. bubble size and shape, sediment layers, etc.).Similarities between ice formation in Great Bay and those in the arctic regions include the nature of the freezing process and the ice types produced. In addition, the distribution and concentration of chemical constituents were found to be similar to those observed in arctic sea ice. Factors affecting the chemistry of the ice in Great Bay include rainfall during the freezing season, the presence of sediment layers in the ice cores, the nature of incorporation of brine into the crystal structure of the ice and the drainage of brine. 相似文献
77.
The influence of the Evros River on the recent sedimentation of the inner shelf of the NE Aegean Sea
Theodore D. Kanellopoulos Vasilios Kapsimalis Serafim E. Poulos Michael O. Angelidis Aristomenis P. Karageorgis Kosmas Pavlopoulos 《Environmental Geology》2008,53(7):1455-1464
The transboundary Evros River discharges into the Alexandroupolis Gulf, located in the inner shelf of the northeastern Aegean
Sea, where it has formed an extended delta. Grain-size and mineralogical analyses of five sediment cores, collected in the
subaqueous delta, provide the following information about recent sedimentation processes in the northeastern part of the Aegean
shelf: (a) river mouth deposits, consisting of coarse-grained sediments, are mainly deposited in front of the active mouth,
whilst some sandy material is expected to be transported alongshore by nearshore currents; (b) delta front deposits are characterised
by fine-grained sediments that include evidence of human activities which have taken place, in a more intense way, since the
1950s; and (c) prodelta deposits are represented by almost uniform riverine mud that cover the pre-existed relict sands of
the shelf, indicating also the limit (some 15 km to the SW) of the influence of riverine sedimentation on the seabed of the
inner shelf of the Alexandroupolis Gulf. 相似文献
78.
Mark W. Fabian Theodore A. Endreny Andrea Bottacin‐Busolin Laura K. Lautz 《水文研究》2011,25(10):1630-1646
A characterization of hyporheic exchange for dry and wet season baseflow, as well as partially dewatered discharge, was done in Prieta Creek, a first‐order cascade in northern Honduras. The cascade had discharges from 1 to 15 l s?1, had average slopes of 12%, pool spacing of 3 m, and shallow substrate of sand and gravel. Tracer tests were conducted in a 15‐m sub‐reach, a length considered to be adequate for the experiment based on the DaI test, a ratio of exchange and transport processes. In the three tests, between 9 and 18% of tracer was not recovered, possibly due to entrainment in flowpaths passing beneath the downstream monitoring location. Tracer data were analysed by the one‐dimensional transport with inflow and storage (OTIS) transient storage model (TSM) to derive standard exchange parameters, and by the solute transport in rivers (STIR) model to examine hyporheic residence time distributions (RTDs). The best fit of the observed tracer breakthrough curves was obtained by using the STIR model with a combination of two exponential RTDs to represent hyporheic retention. With increasing discharge, the OTIS model predicted increasing storage exchange fluxes and exchange coefficients and decreasing storage zone areas and transient storage times, which are trends supported by riparian and streambed piezometric head data. Riparian water levels rose during the transition from the dry to wet season, which could constrict the hyporheic storage zone. Thirteen of the 19 streambed piezometers recorded seasonal changes in hydraulic gradients and flux direction, with fewer yet stronger upwelling zones during higher discharges. The MODFLOW model missed the observed seasonal changes, possibly due to subtle changes in the seasonal change in water surface profiles. We conclude that partially dewatered dry season exchange, compared to wet season exchange, was initiated and terminated with smaller pressure gradients and, in different streambed locations, was smaller in volume, had longer residence times, and may connect with deeper and longer flow paths. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
79.
Material properties are essential in the design and evaluation of pavements. In this paper, the potential of support vector regression (SVR) algorithm is explored to predict the resilient modulus (MR), which is an essential property in designing and evaluating pavement materials, particularly hot mix asphalt typically used in Oklahoma. SVR is a statistical learning algorithm that is applied to regression problems; in our study, SVR was shown to be superior to the least squares (LS). Compared with the widely used LS method, the results of this study show that SVR significantly reduces the mean‐squared error and improves the correlation coefficient. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
80.
Harry Y. McSween Jr. Theodore C. Labotka Christina E. Viviano‐Beck 《Meteoritics & planetary science》2015,50(4):590-603
Compositions of basaltic and ultramafic rocks analyzed by Mars rovers and occurring as Martian meteorites allow predictions of metamorphic mineral assemblages that would form under various thermophysical conditions. Key minerals identified by remote sensing roughly constrain temperatures and pressures in the Martian crust. We use a traditional metamorphic approach (phase diagrams) to assess low‐grade/hydrothermal equilibrium assemblages. Basaltic rocks should produce chlorite + actinolite + albite + silica, accompanied by laumontite, pumpellyite, prehnite, or serpentine/talc. Only prehnite‐bearing assemblages have been spectrally identified on Mars, although laumontite and pumpellyite have spectra similar to other uncharacterized zeolites and phyllosilicates. Ultramafic rocks are predicted to produce serpentine, talc, and magnesite, all of which have been detected spectrally on Mars. Mineral assemblages in both basaltic and ultramafic rocks constrain fluid compositions to be H2O‐rich and CO2‐poor. We confirm the hypothesis that low‐grade/hydrothermal metamorphism affected the Noachian crust on Mars, which has been excavated in large craters. We estimate the geothermal gradient (>20 °C km?1) required to produce the observed assemblages. This gradient is higher than that estimated from radiogenic heat‐producing elements in the crust, suggesting extra heating by regional hydrothermal activity. 相似文献