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91.
K. I. Kellermann R. C. Vermeulen J. A. Zensus M. H. Cohen A. West 《New Astronomy Reviews》1999,43(8-10)
The VLBA has been used over a period of four years to study the internal motions within a sample of quasars and AGN. In most sources, features appear to propagate away from the central engine along a well collimated radio jet with apparent transverse velocities between zero and 10c, with some evidence for apparent accelerations and decelerations. The distribution of apparent velocity is not consistent with any simple ballistic model and appears to require either a spread in intrinsic velocity or a difference between the bulk velocity and pattern velocity. The dependence of apparent angular velocity with redshift is consistent with standard Friedmann world models. Further observations of a larger source sample, especially at large redshift may lead to meaningful constraints on world models. 相似文献
92.
Improving neural network performance on the classification of complex geographic datasets 总被引:4,自引:0,他引:4
Neural Networks are now established computational tools used for search minimisation and data classification. They offer
some highly desirable features for landuse classification problems since they are able to take in a variety of data types,
recorded on different statistical scales, and combine them. As such, neural networks should offer advantages of increased
accuracy. However, a barrier to their general acceptance and use by all but `experts' is the difficulty of configuring the
network initially.
This paper describes the architectural problems of applying neural networks to landcover classification exercises in geography
and details some of the latest developments from an ongoing research project aimed at overcoming these problems. A comprehensive
strategy for the configuration of neural networks is presented, whereby the network is automatically constructed by a process
involving initial analysis of the training data. By careful study of the functioning of each part of the network it is possible
to select the architecture and initial weights on the node connections so the constructed network is `right first time'. Further
adaptations are described to control network behaviour, to optimise functioning from the perspective of landcover classification.
The entire configuration process is encapsulated by a single application which may be treated by the user as a `black box',
allowing the network to the applied in much the same way as a maximum likelihood classifier, with no further effort being
required of the user. 相似文献
93.
Samuel V. Panno Zohreh Askari Walton R. Kelly Thomas M. Parris Keith C. Hackley 《Ground water》2018,56(1):32-45
The conservative nature of chloride (Cl?) in groundwater and the abundance of geochemical data from various sources (both published and unpublished) provided a means of developing, for the first time, a representation of the hydrogeology of the Illinois Basin on a basin‐wide scale. The creation of Cl? isocons superimposed on plan view maps of selected formations and on cross sections across the Illinois Basin yielded a conceptual model on a basin‐wide scale of recharge into, groundwater flow within and through the Illinois Basin. The maps and cross sections reveal the infiltration and movement of freshwater into the basin and dilution of brines within various geologic strata occurring at basin margins and along geologic structures. Cross‐formational movement of brines is also seen in the northern part of the basin. The maps and cross sections also show barriers to groundwater movement created by aquitards resulting in areas of apparent isolation/stagnation of concentrated brines within the basin. The distribution of Cl? within the Illinois Basin suggests that the current chemical composition of groundwater and distribution of brines within the basin is dependent on five parameters: (1) presence of bedrock exposures along basin margins; (2) permeability of geologic strata and their distribution relative to one another; (3) presence or absence of major geologic structures; (4) intersection of major waterways with geologic structures, basin margins, and permeable bedrock exposures; and (5) isolation of brines within the basin due to aquitards, inhomogeneous permeability, and, in the case of the deepest part of the basin, brine density effects. 相似文献
94.
Shin Giek Goh Stéphane Bayen David Burger Barry C. Kelly Ping Han Vladan Babovic Karina Yew-Hoong Gin 《Marine pollution bulletin》2017,114(1):627-634
Water quality in Singapore's coastal area was evaluated with microbial indicators, pathogenic vibrios, chemical tracers and physico-chemical parameters. Sampling sites were grouped into two clusters (coastal sites at (i) northern and (ii) southern part of Singapore). The coastal sites located at northern part of Singapore along the Johor Straits exhibited greater pollution. Principal component analysis revealed that sampling sites at Johor Straits have greater loading on carbamazepine, while turbidity poses greater influence on sampling sites at Singapore Straits. Detection of pathogenic vibrios was also more prominent at Johor Straits than the Singapore Straits. This study examined the spatial variations in Singapore's coastal water quality and provided the baseline information for health risk assessment and future pollution management. 相似文献
95.
The stability of end-bearing piles that are supported laterally along their entire length by an elastic Winkler foundation is investigated for the case when the coefficient of horizontal subgrade reaction varies linearly with depth. A pattern of clustering of buckling modes is shown to occur and the approximate modelling of the elastic foundation by averaging the stiffness of the subgrade is discussed. © 1997 by John Wiley & Sons, Ltd. 相似文献
96.
97.
Chia-Yu Chen Sean D. Willett A. Joshua West Simon Dadson Niels Hovius Marcus Christl J. Bruce H. Shyu 《地球表面变化过程与地形》2020,45(3):548-564
Extreme erosion events can produce large short-term sediment fluxes. Such events complicate erosion rates estimated from cosmogenic nuclide concentrations in river sediment by providing sediment with a concentration different from the long-term basin average. We present a detrital 10Be study in southern Taiwan, with multiple samples obtained in a time sequence bracketing the 2009 Typhoon Morakot, to assess the impact of landslide sediment on 10Be concentrations (N10Be) in river sediment. Sediment samples were collected from 13 major basins, two or three times over the last decade, to observe the temporal variation of N10Be. Landslide inventories with time intervals of 5–6 years were used to quantify sediment flux changes. A negative correlation between N10Be and landslide areal density indicates dilution of N10Be by landslide sediment. Denudation rates estimated from the diluted N10Be can be up to three times higher than the lowest rate derived from the same basins. Observed increases imply that, 3 years after the passage of Typhoon Morakot, fluvial channels still contain a considerable amount of sediment provided by hillslope landslides during the event. However, higher N10Be in 2016 samples indicate that the contribution from landslide sediment at the sampled grain size has decreased with time. The correlation between changes in N10Be and landslide area and volume is not strong, likely resulting from the stochastic and complex nature of sediment transport. To simultaneously evaluate the volume of landslide-derived sediment and estimate the background denudation rate, associated with less impulsive sediment supply, we constructed a sediment-mixing model with the time series of N10Be and landslide inventories. The spatial pattern of background erosion rate in southern Taiwan is consistent with the regional tectonic framework, indicating that the landscape is evolving mainly in response to the tectonic forcing, and this signal is modified, but not obscured by impulsive sediment supply. © 2019 John Wiley & Sons, Ltd. 相似文献
98.
Scott B. Cook Wendy A. Timms Bryce F.J. Kelly S. Lee Barbour 《Hydrogeology Journal》2017,25(5):1451-1463
Measurement of barometric efficiency (BE) from open monitoring wells or loading efficiency (LE) from formation pore pressures provides valuable information about the hydraulic properties and confinement of a formation. Drained compressibility (α) can be calculated from LE (or BE) in confined and semi-confined formations and used to calculate specific storage (S s). S s and α are important for predicting the effects of groundwater extraction and therefore for sustainable extraction management. However, in low hydraulic conductivity (K) formations or large diameter monitoring wells, time lags caused by well storage may be so long that BE cannot be properly assessed in open monitoring wells in confined or unconfined settings. This study demonstrates the use of packers to reduce monitoring-well time lags and enable reliable assessments of LE. In one example from a confined, high-K formation, estimates of BE in the open monitoring well were in good agreement with shut-in LE estimates. In a second example, from a low-K confining clay layer, BE could not be adequately assessed in the open monitoring well due to time lag. Sealing the monitoring well with a packer reduced the time lag sufficiently that a reliable assessment of LE could be made from a 24-day monitoring period. The shut-in response confirmed confined conditions at the well screen and provided confidence in the assessment of hydraulic parameters. A short (time-lag-dependent) period of high-frequency shut-in monitoring can therefore enhance understanding of hydrogeological systems and potentially provide hydraulic parameters to improve conceptual/numerical groundwater models. 相似文献
99.
Naegleria fowleri is a thermophilic free-living amoeba found worldwide in soils and warm freshwater. It is the causative agent of primary amebic meningoencephalitis, a nearly always fatal disease afflicting mainly children and young adults. Humans are exposed to the organism via swimming, bathing, or other recreational activity during which water is forcefully inhaled into the upper nasal passages. Although many studies have looked at the occurrence of N. fowleri in surface waters, limited information is available regarding its occurrence in groundwater and geothermally heated natural waters such as hot springs. This paper reviews the current literature related to the occurrence of N. fowleri in these waters and the methods employed for its detection. Case reports of potential groundwater exposures are also included. Despite increased interest in N. fowleri in recent years due to well-publicized cases linked to drinking water, many questions still remain unanswered. For instance, why the organism persists in some water sources and not in others is not well understood. The role of biofilms in groundwater wells and plumbing in individual buildings, and the potential for warming due to climate change to expand the occurrence of the organism into new regions, are still unclear. Additional research is needed to address these questions in order to better understand the ecology of N. fowleri and the conditions that result in greater risks to bathers. 相似文献
100.
Kelly M. Darnell Tim J. B. Carruthers Patrick Biber Ioannis Y. Georgiou Thomas C. Michot Ronald G. Boustany 《Estuaries and Coasts》2017,40(5):1288-1300
Seagrasses are submerged marine plants that are anchored to the substrate and are therefore limited to assimilating nutrients from the surrounding water column or sediment, or by translocating nutrients from adjacent shoots through the belowground rhizome. As a result, seagrasses have been used as reliable ecosystem indicators of surrounding nutrient conditions. The Chandeleur Islands are a chain of barrier islands in the northern Gulf of Mexico that support the only marine seagrass beds in Louisiana, USA, and are the sole location of the seagrass Thalassia testudinum across nearly 1000 km of the coastline from west Florida to central Texas. Over the past 150 years, the land area of the Chandeleur Islands has decreased by over half, resulting in a decline of seagrass cover. The goals of this study were to characterize the status of a climax seagrass species at the Chandeleur Islands, T. testudinum, in terms of leaf nutrient (nitrogen [N] and phosphorus [P]) changes over time, from 1998 to 2015, and to assess potential drivers of leaf nutrient content. Thalassia testudinum leaf nutrients displayed considerable interannual variability in N and P content and molar ratios, which broadly mimicked patterns in annual average dissolved nutrient concentrations in the lower Mississippi River. Hydrological modeling demonstrated the potential for multiple scenarios that would deliver Mississippi River water, and thus nutrients, to T. testudinum at the Chandeleur Islands. Although coastal eutrophication is generally accepted as the proximate cause for seagrass loss globally, there is little evidence that nutrient input from the Mississippi River has driven the dramatic declines observed in seagrasses at the Chandeleur Islands. Rather, seagrass cover along the Chandeleur Islands appears to be strongly influenced by island geomorphological processes. Although variable over time, the often elevated nutrient levels of the climax seagrass species, T. testudinum, which are potentially driven by river-derived nutrient inputs, raises an important consideration of the potential loss of the ecosystem functions and services associated with these declining seagrass meadows. 相似文献