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排序方式: 共有327条查询结果,搜索用时 15 毫秒
111.
T. Rogers R. McEntaffer T. Schultz J. McCoy D. Miles J. Tutt 《Experimental Astronomy》2017,43(2):201-210
Gaseous Electron Multiplier detectors, or GEMs, show promise for use on space-based X-ray missions. Operating pressure strongly affects the gain of the detector and must be optimized for best performance. We have measured the gain characteristics of a GEM detector at various pressures below atmosphere using a mixture of Ar:CO2 with the goal of maximizing gain to push GEM capabilities to the lowest energies possible. This paper discusses our tests, results, and their implications for choosing a detector pressure. We found that at any operating pressure the detector voltage can be adjusted to achieve roughly the same maximum gain prior to the onset of electrical discharges. We also find that the gain varies substantially by spatial location across the detector, but this variation is insensitive to changes in pressure allowing it to be calibrated and corrected if necessary. The detector pressure can therefore be optimized in the interest of other performance parameters such as leak rate, window stress, power requirements, or quantum efficiency without concern for negatively affecting the gain. These results can inform the choice of operating pressure and voltage for GEMs used onboard future space missions. 相似文献
112.
Intertidal bars are common in mesotidal/macrotidal low-to-moderate energy coastal environments and an understanding of their morphodynamics is important from the perspective of both coastal scientists and managers. However, previous studies have typically been limited by considering bar systems two-dimensionally, or with very limited alongshore resolution. This article presents the first multi-annual study of intertidal alongshore bars and troughs in a macrotidal environment using airborne LiDAR (light detection and ranging) data to extract three-dimensional (3D) bar morphology at high resolution. Bar and trough positions are mapped along a 17.5 km stretch of coastline in the northwest of England on the eastern Irish Sea, using eight complete, and one partial, LiDAR surveys spanning 17 years. Typically, 3–4 bars are present, with significant obliquity identified in their orientation. This orientation mirrors the alignment of waves from the dominant south-westerly direction of wave approach, undergoing refraction as they approach the shoreline. Bars also become narrower and steeper as they migrate onshore, in a pattern reminiscent of wave shoaling. This suggests that the configuration of the bars is being influenced by overlying wave activity. Net onshore migration is present for the entire coastline, though rates vary alongshore, and periods of offshore migration may occur locally, with greatest variability between northern and southern regions of the coastline. This work highlights the need to consider intertidal bar systems as 3D, particularly on coastlines with complex configurations and bathymetry, as localized studies of bar migration can overlook 3D behaviour. Furthermore, the wider potential of LiDAR data in enabling high-resolution morphodynamic studies is clear, both within the coastal domain and beyond. © 2019 John Wiley & Sons, Ltd. 相似文献
113.
Abigail McQuatters-Gollop Alison J. Gilbert Laurence D. Mee Jan E. Vermaat Yuri Artioli Christoph Humborg Fred Wulff 《Estuarine, Coastal and Shelf Science》2009,82(4):583-596
Anthropogenic eutrophication affects the Mediterranean, Black, North and Baltic Seas to various extents. Responses to nutrient loading and methods of monitoring relevant indicators vary regionally, hindering interpretation of ecosystem state changes and preventing a straightforward pan-European assessment of eutrophication symptoms. Here we summarize responses to nutrient enrichment in Europe's seas, comparing existing time-series of selected pelagic (phytoplankton biomass and community composition, turbidity, N:P ratio) and benthic (macro flora and faunal communities, bottom oxygen condition) indicators based on their effectiveness in assessing eutrophication effects. Our results suggest that the Black Sea and Northern Adriatic appear to be recovering from eutrophication due to economic reorganization in the Black Sea catchment and nutrient abatement measures in the case of the Northern Adriatic. The Baltic is most strongly impacted by eutrophication due to its limited exchange and the prevalence of nutrient recycling. Eutrophication in the North Sea is primarily a coastal problem, but may be exacerbated by climatic changes. Indicator interpretation is strongly dependent on sea-specific knowledge of ecosystem characteristics, and no single indicator can be employed to adequately compare eutrophication state between European seas. Communicating eutrophication-related information to policy-makers could be facilitated through the use of consistent indicator selection and monitoring methodologies across European seas. This work is discussed in the context of the European Commission's recently published Marine Strategy Directive. 相似文献
114.
一种新的土壤冻融特征曲线模型 总被引:1,自引:0,他引:1
土壤冻融特征曲线(SFTC)可以描述冻融过程中未冻水含量随负温的变化关系.准确刻画土壤冻融特征曲线对土壤的冻融过程及相关的水热耦合运移研究至关重要.以往研究中土壤冻融特征曲线适用范围较窄,无法满足实际需要.通过类比水分特征曲线,考虑初始含水量和溶质浓度对未冻水含量的影响,提出了一种新的冻融特征曲线模型,经广泛的文献数据... 相似文献
115.
Michael C. Cooper Jeffrey A. Newman † Alison L. Coil † Darren J. Croton Brian F. Gerke Renbin Yan Marc Davis S. M. Faber Puragra Guhathakurta David C. Koo Benjamin J. Weiner Christopher N. A. Willmer ‡ 《Monthly notices of the Royal Astronomical Society》2007,376(4):1445-1459
Using a sample of 19 464 galaxies drawn from the DEEP2 Galaxy Redshift Survey, we study the relationship between galaxy colour and environment at 0.4 < z < 1.35 . We find that the fraction of galaxies on the red sequence depends strongly on local environment out to z > 1 , being larger in regions of greater galaxy density. At all epochs probed, we also find a small population of red, morphologically early-type galaxies residing in regions of low measured overdensity. The observed correlations between the red fraction and local overdensity are highly significant, with the trend at z > 1 detected at a greater than 5σ level. Over the entire redshift regime studied, we find that the colour–density relation evolves continuously, with red galaxies more strongly favouring overdense regions at low z relative to their red-sequence counterparts at high redshift. At z ≳ 1.3 , the red fraction only weakly correlates with overdensity, implying that any colour dependence to the clustering of ∼ L * galaxies at that epoch must be small. Our findings add weight to existing evidence that the build-up of galaxies on the red sequence has occurred preferentially in overdense environments (i.e. galaxy groups) at z ≲ 1.5 . Furthermore, we identify the epoch ( z ∼ 2) at which typical ∼ L * galaxies began quenching and moved on to the red sequence in significant number. The strength of the observed evolutionary trends at 0 < z < 1.35 suggests that the correlations observed locally, such as the morphology–density and colour–density relations, are the result of environment-driven mechanisms (i.e. 'nurture') and do not appear to have been imprinted (by 'nature') upon the galaxy population during their epoch of formation. 相似文献
116.
Underlying the site of a proposed development is a chalk aquifer, groundwater levels are close to the surface. The storm water from the development is to be drained into a lake in the center of the development. Because of the high groundwater levels the effect of this lake has been studied by means of a simple localized mathematical model. It is shown that such a model can give results of sufficient accuracy with regard to probable changes in groundwater levels caused by the development. 相似文献
117.
B. Miles T. Kalbacher O. Kolditz C. Chen J. Gronewold W. Wang A. Peter 《Environmental Geology》2007,52(7):1399-1412
Ongoing developments in geological and hydrogeological investigation techniques, especially direct-push methods, have led
to an increase in the quality, density and spatial resolution of data available from such investigations. This has created
new challenges in the development of numerical models in terms of accurately and efficiently translating detailed and complex
conceptual models into effective numerical models. Suitable geometrical and numerical modelling tools are essential in order
to meet these challenges. This paper describes the development of a three-dimensional hydrogeological flow model for a contaminated
site near Berlin, Germany, based on high-resolution geological data obtained principally using direct-push methods. The available
data were first interpreted to construct a detailed GIS-based geological model, which formed the basis of the conceptual site
model. The conceptual model was then translated into a geometrical model, which was used to create a finite element numerical
model. An innovative geometry object-based approach enabled the complex structural details of the conceptual model to be accurately
reproduced in the numerical model domain. The resulting three-dimensional steady-state unconfined flow model was successfully
calibrated using external automated calibration software, whereby parameter values for groundwater recharge and hydraulic
conductivity were determined. 相似文献
118.
Research interest in speciation of arsenic stems from its species dependent behavior in the environment and in living organisms. The complexity of the matrix to be analyzed and low concentrations of target arsenic species that may be labile or difficult to chromatogram, indicate that a suitable pre‐treatment methodology is required. This study investigated the usefulness of chelation solvent extraction – high performance liquid chromatography (CSE‐HPLC) for the speciation of arsenic in water. It involved reacting arsenic with the chelant known for its affinity towards arsenic, followed by extraction, separation, and identification of the arsenic‐chelant‐arsenic complex. Arsenic species having different physicochemical properties were investigated. Species, such as, As2O3, As3O5, KH2AsO4, Na2HAsO4, and NaAsO2 were detected as a group of closely eluted peaks with different retention times and spectral properties, whereas, the organic arsenic species CH3Na2AsO3, o‐arsanilic acid, roxarson and triphenyl arsine separated quite well on the EnviroseP‐CM HPLC column. Key method parameters, such as, type of HPLC column, composition of mobile phase and organic solvents affecting peak resolution and sensitivity were optimized. Real environmental matrices contaminated with arsenic were analyzed under varying wavelengths (λmax = 190, 210, 220, 234, 244, and 282 nm), with good precision. Different arsenic species were detected in these samples with excellent background and signal‐to‐noise ratios demonstrating the robustness of the method. The detection limit, reproducibility, selectivity, accuracy, and dynamic range of the calibration curves were evaluated. 相似文献
119.
120.