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排序方式: 共有102条查询结果,搜索用时 250 毫秒
1.
Josh T. Kohut Hugh J. Roarty Scott M. Glenn 《Oceanic Engineering, IEEE Journal of》2006,31(4):876-884
A network of high-frequency (HF) radars is deployed along the New Jersey coast providing synoptic current maps across the entire shelf. These data serve a variety of user groups from scientific research to Coast Guard search and rescue. In addition, model forecasts have been shown to improve with surface current assimilation. In all applications, there is a need for better definitions and assessment of the measurement uncertainty. During a summer coastal predictive skill experiment in 2001, an array of in situ current profilers was deployed near two HF radar sites, one long-range and one standard-range system. Comparison statistics were calculated between different vertical bins on the same current profiler, between different current profilers, and between the current profilers and the different HF radars. The velocity difference in the vertical and horizontal directions were then characterized using the observed root-mean-square (rms) differences. We further focused on two cases, one with relatively high vertical variability, and the second with relatively low vertical variability. Observed differences between the top bin of the current profiler and the HF radar were influenced by both system accuracy and the environment. Using the in situ current profilers, the environmental variability over scales based on the HF radar sampling was quantified. HF radar comparisons with the current profilers were on the same order as the observed environmental difference over the same scales, indicating that the environment has a significant influence on the observed differences. Velocity variability in the vertical and horizontal directions both contribute to these differences. When the potential effects of the vertical variability could be minimized, the remaining difference between the current profiler and the HF radar was similar to the measured horizontal velocity difference (~2.5 cm/s) and below the resolution of the raw radial data at the time of the deployment 相似文献
2.
R. H. Rapp 《Journal of Geodesy》1997,71(5):282-289
This paper suggests that potential coefficient models of the Earth's gravitational potential be used to calculate height
anomalies which are then reduced to geoid undulations where such quantities are needed for orthometric height determination
and vertical datum definition through a potential coefficient realization of the geoid. The process of the conversion of the
height anomaly into a geoid undulation is represented by a height anomaly gradient term and the usual N–ζ term that is dependent on elevation and the Bouguer anomaly. Using a degree 360 expansion of 30′ elevations and the OSU91A
potential coefficient model, a degree 360 representation of the correction terms was computed. The magnitude of N–ζ reached –3.4 m in the Himalaya Mountains with smaller, but still significant, magnitudes in other mountainous regions.
Received: 6 May 1996; Accepted: 30 October 1996 相似文献
3.
Richard L. Reynolds Joseph G. Rosenbaum Josh Rapp Michael W. Kerwin J. Platt Bradbury Steven Colman David Adam 《Journal of Paleolimnology》2004,31(2):217-233
Petrological and textural properties of lacustrine sediments from Upper Klamath Lake, Oregon, reflect changing input volumes of glacial flour and thus reveal a detailed glacial history for the southern Cascade Range between about 37 and 15 ka. Magnetic properties vary as a result of mixing different amounts of the highly magnetic, glacially generated detritus with less magnetic, more weathered detritus derived from unglaciated parts of the large catchment. Evidence that the magnetic properties record glacial flour input is based mainly on the strong correlation between bulk sediment particle size and parameters that measure the magnetite content and magnetic mineral freshness. High magnetization corresponds to relatively fine particle size and lower magnetization to coarser particle size. This relation is not found in the Buck Lake core in a nearby, unglaciated catchment. Angular silt-sized volcanic rock fragments containing unaltered magnetite dominate the magnetic fraction in the late Pleistocene sediments but are absent in younger, low magnetization sediments. The finer grained, highly magnetic sediments contain high proportions of planktic diatoms indicative of cold, oligotrophic limnic conditions. Sediment with lower magnetite content contains populations of diatoms indicative of warmer, eutrophic limnic conditions. During the latter part of oxygen isotope stage 3 (about 37–25 ka), the magnetic properties record millennial-scale variations in glacial-flour content. The input of glacial flour was uniformly high during the Last Glacial Maximum, between about 21 and 16 ka. At about 16 ka, magnetite input, both absolute and relative to hematite, decreased abruptly, reflecting a rapid decline in glacially derived detritus. The decrease in magnetite transport into the lake preceded declines in pollen from both grass and sagebrush. A more gradual decrease in heavy mineral content over this interval records sediment starvation with the growth of marshes at the margins of the lake and dilution of detrital material by biogenic silica and other organic matter. 相似文献
4.
Uraniferous Holocene sediments occur in the Carson Range of Nevada and California, U.S.A., between Lake Tahoe and Carson Valley. The hosts for the uranium include peat and interbedded organic-rich sand, silt, and mud that underly valley floors, fens, and marshes along stream valleys between the crest of the range and the edge of Lake Tahoe. The known uranium accumulations extend along the Carson Range from the area just southeast of South Lake Tahoe northward to the area just east of Carson City; however, they almost certainly continue beyond the study area to the north, west, and south. Due to the young age of the accumulations, uranium in them is in gross disequilibrium with its highly radioactive daughter products. These accumulations have thus escaped discovery with radiation detection equipment in the past. The uranium content of these sediments approaches 0.6 percent; however, the average is in the range of 300–500 ppm. Waters associated with these sediments locally contain as much as 177 ppb uranium. Modest levels of helium and radon also occur in these waters.Uraniferous waters are clearly entering the private and public water supply systems in some parts of the study area; however, it is not known how much uranium is reaching users of these water supplies. Many of the waters sampled in the study area exceed the published health effects guidance level of the Environmental Protection Agency. Regulatory standards for uranium in waters have not been published, however.Much uranium is stored in the sediments along these stream valleys. Estimates for a marsh and a fen along one drainage are 24,000 and 15,000 kg, respectively. The potential effects of man-induced environmental changes on the uranium are uncertain. Laboratory studies of uraniferous sediment rich in organic matter may allow us to evaluate the potential of liberating uranium from such sediments and creating transient increases in the level of uranium moving in water in the natural environment. 相似文献
5.
Anders Rapp 《Climatic change》1986,9(1-2):19-31
Two efforts by UN organizations to diagnose and map the distribution and trends of soil degradation in drylands are briefly described and compared.Soil degradation by water is greatly accelerated under poor vegetation cover in the Tropics and Subtropics, probably for two major reasons: high rates of decomposition of organic matter and high intensities of rainfall. The consequences of soil losses and high water runoff can reach far downstream in the river basin, causing damage by sedimentation on floodplains and in reservoirs.Damages by wind erosion and deposition are also much accelerated by overexploitation of the meagre vegetation cover of drylands. Overgrazing, overcultivation, firewood collection are the consequences of human and animal pressure, leading to more or less long-lasting desertification. The finer material, clay, silt, and organic matter, is blown by dust storms over long distances and can be deposited as dry or wet fallout in oceans or on land under the dust trajectories.Salinization often affects highly productive soils under poor irrigation practices. It is therefore a form of dryland degradation which causes particularly high losses of potential crops. 相似文献
6.
7.
8.
Undulation and anomaly estimation using Geos-3 altimeter data without precise satellite orbits 总被引:1,自引:0,他引:1
The paper describes results obtained from the processing of 53 Geos-3 arcs of altimeter data obtained during the first weeks
after the launch of the satellite in April, 1975. The measurement from the satellite to the ocean surface was used to obtain
an approximate geoid undulation which was contaminated by long wavelength errors caused primarily by altimeter bias and orbit
error. This long wavelength error was reduced by fitting with a low degree polynomial the raw undulation data to the undulations
implied by the GEM 7 potential coefficients, in an adjustment process that included conditions on tracks that cross. The root
mean square crossover discrepancy before this adjustment was ±12.4 meters while after the adjustment it was ±0.9 m. These
adjusted undulations were used to construct a geoid map in the Geos-3 calibration area using a least squares filter to remove
remaining noise in the undulations. Comparing these undulations to ones computed from potential coefficients and terrestrial
gravity data indicates a mean difference of 0.25 m and a root mean square difference of ±1.92 m.
The adjusted undulations were also used to estimate several 5o, 2o, and 1o anomalies using the method of least squares collocation. The resulting predictions agreed well with known values although
the 1o x 1o anomalies could not be considered as reliably determined. 相似文献
9.
There is variability in the Mg isotopic composition that is a reflection of the widespread heterogeneity in the isotopic composition of the elements in the solar system at approximately 100 ppm. Measurements on a single calcium‐aluminum‐rich inclusion (CAI) gave a good correlation of 26Mg/24Mg with 27Al/24Mg, yielding an isochron corresponding to an initial (26Al/27Al)o = (5.27 ± 0.18) × 10?5 and an initial (26Mg/24Mg)o = ?0.127 ± 0.032‰ relative to the standard. This isochron is parallel to that obtained by Jacobsen et al. (2008) , but is distinctively offset. This demonstrates that there are different initial Mg isotopic compositions in different samples with the same 26Al/27Al. No inference about uniformity/heterogeneity of 26Al/27Al on a macro scale can be based on the initial (26Mg/24Mg)o values. Different values of 26Al/27Al for samples representing the same point in time would prove heterogeneity of 26Al/27Al. The important issue is whether the bulk solar inventory of 26Al/27Al was approximately 5 × 10?5 at some point in the early solar system. We discuss ultra refractory phases of solar type oxygen isotope composition with 26Al/27Al from approximately 5 × 10?5 to below 0.2 × 10?5. We argue that the real issues are: intrinsic heterogeneity in the parent cloud; mechanism and timing for the later production of 16O‐poor material; and the relationship to earlier formed 16O‐rich material in the disk. 26Al‐free refractories can be produced at a later time by late infall, if there is an adequate heat source, or from original heterogeneities in the placental molecular cloud from which the solar system formed. 相似文献
10.
Aleksandr?S.?StepanovEmail author Ian?Campbell Robert?P.?Rapp Jessica?Lowczak Andrey?V.?Korsakov 《Contributions to Mineralogy and Petrology》2017,172(4):19
Wang et al. (Contrib Mineral Petrol 171:62, 2016a) present data on composition of xenolith from Southern Tibet and conclude that ulrapotassic melts from the region formed by melting mantle, and complex interaction with a crustal component. In this discussion we demonstrate that numerous observations presented by Wang et al. (2016a) can be explained by partial melting of crust followed by interaction between that melt and the mantle. We show that this model can explain the variability of magmas in such suits without evoking occurrence of coincidental, unrelated events. Moreover we demonstrate that our model of a crustal origin of the proto-shoshonite melts is now supported by independent lines of evidence such as geochemistry of restites after high- and ultrahigh- pressure melting and melt inclusion studies. 相似文献