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471.
Alan Lees 《Geological Journal》1997,32(1):1-36
Recent revision of the biostratigraphy allows the recognition of a stratigraphic entity (here termed Freyrian) between the base of the Moliniacian stage and the base of the Viséan and simplifies sedimentological interpretation of late Tournaisian events around Dinant, in Belgium. Petrographic analysis of Freyrian rocks in Waulsortian buildups and peri-Waulsortian facies reveals a pattern of carbonate sedimentation related to the submarine topography developed by the buildups, and to sea-level changes. Graded beds and thin layers of grainstone in the predominantly fine-grained peri-Waulsortian sediments represent influxes derived mainly from nearby buildups during a period of shallowing. Using the sequence of foraminiferan assemblages which colonized the buildups as a stratigraphic scale, the earlier influxes are shown to have occurred only close to the presumed source, whereas later influxes extended further and marked the culmination of the shallowing phase. Deposition during the latter part of the Freyrian appears to have occurred in rather deeper, less disturbed water. The Moliniacian and Viséan boundary stratotypes, both in peri-Waulsortian facies, are critically assessed because almost all the stratigraphically useful foraminiferans occur in the rare grainstones resulting from sediment influxes. Tetrataxis was one of the few foraminiferans to colonize proximal peri-Waulsortian areas and appears to have ranged to a water depth of about 200 m. Downslope diachronism of colonization is interpreted as evidence of a sea-level fall of about 140 m and is used to draw a sea-level curve for this late Tournaisian regression. Using the same depth scale, microbial coating extended to about 300 m and its development appears to have been related to low sedimentation rates rather than photic conditions. © 1997 John Wiley & Sons, Ltd. 相似文献
472.
473.
An improved age framework for late Quaternary silicic eruptions in northern Central America 总被引:1,自引:0,他引:1
William I. Rose F. Michael Conway Carlos R. Pullinger Alan Deino William C. McIntosh 《Bulletin of Volcanology》1999,61(1-2):106-120
Five new stepwise-heating 40Ar/39Ar ages and one new high-sensitivity 14C date of ash-fall and ash-flow deposits from late Quaternary silicic volcanoes in northern Central America document the eruption
rates and frequencies of five major rhyodacite and rhyolite calderas (Atitlán, Amatitlán, Ayarza, Coatepeque, and Ilopango)
located north of the basalt, andesite, and dacite stratovolcanoes of the Central American volcanic front. These deposits form
extensive time-stratigraphic horizons that intercalate regionally, and knowledge of dates and stratigraphy provides a valuable
framework for age determinations of more localized volcanic and nonvolcanic events. The new data, especially when integrated
with previous stratigraphic and dating work, show that all five calderas erupted several times in the past 200 ka and, despite
a lack of historic activity, should be considered as active centers that could produce highly explosive eruptions again. Because
of their locations near the highly vulnerable economic hearts of Guatemala and El Salvador, the risks of eruptions from these
calderas should be carefully considered along with risks of major earthquakes and volcanic front volcanoes, which are much
more frequent but inflict less severe and extensive damage. This investigation also includes some examples of dating efforts
that failed to produce reasonable results.
Received: 15 May 1998 / Accepted: 18 January 1999 相似文献
474.
Earthquake response of a multiblock nuclear reactor graphite core: Experimental model vs simulations
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Elia Voyagaki Panos Kloukinas Matt Dietz Luiza Dihoru Tony Horseman Olafur Oddbjornsson Adam J. Crewe Colin A. Taylor Alan Steer 《地震工程与结构动力学》2018,47(13):2601-2626
The complex dynamics of a quarter‐scale model of a graphite nuclear reactor core, representative of the second generation of British advanced gas‐cooled nuclear reactors, is investigated numerically and experimentally. Advanced gas‐cooled nuclear reactor cores are polygonal, multilayer, arrays of graphite bricks, with each brick allowed to rock by design relative to each other in accordance with the boundary conditions. A 35 000 DOF, nonlinear finite element model of the core created by Atkins Nuclear, was analysed on a high performance computing facility at the University of Bristol, and a corresponding 8 t physical model, equipped with 3200 data acquisition channels, was built and tested on the University of Bristol 6‐DOF shaking table. In this paper, the two models are subjected to a series of (1) synthetic earthquake and (2) idealised harmonic input motions. The experimental data are used to compare and verify the two models and explore the dynamics of the core. A kinematic model of the response is also developed based solely on geometric constraints. The results are presented in the form of response maps and graphs. Important conclusions are drawn as to the dynamics and earthquake response of such systems, which inform numerical model validation. It is found that contrary to the case of a small number of rocking blocks that exhibit highly complex response patterns, the behaviour of the model at hand is both smooth and repeatable. An analogy between the response of the core and that of dense granular matter exhibiting particle interlocking and dilatancy is highlighted. 相似文献
475.
Costa Carine G. R. Leite José Roberto B. Castro Belmiro M. Blumberg Alan F. Georgas Nickitas Dottori Marcelo Jordi Antoni 《Ocean Dynamics》2020,70(2):257-271
Ocean Dynamics - We describe the design, implementation, and performance of a fully automated Santos Operational Forecasting System (SOFS), built to monitor and predict short-term (< 3 days)... 相似文献
476.
This study addresses the problem of shadows in multi-temporal imagery, which is a key issue with change detection approaches based on image comparison. We apply image-to-image radiometric normalizations including histogram matching (HM), mean-variance (MV) equalization, linear regression based on pseudo-invariant features (PIF-LR), and radiometric control sets (RCS) representing high- and low-reflectance extrema, for the novel purpose of normalizing brightness of transient shadows in high spatial resolution, bi-temporal, aerial frame image sets. Efficient shadow normalization is integral to remote sensing procedures that support disaster response efforts in a near-real-time fashion, including repeat station image (RSI) capture, wireless data transfer, shadow detection (as precursor to shadow normalization), and change detection based on image differencing and visual interpretation. We apply the normalization techniques to imagery of suburban scenes containing shadowed materials of varied spectral reflectance characteristics, whereby intensity (average of red, green, and blue spectral band values) under fully illuminated conditions is known from counterpart reference images (time-1 versus time-2). We evaluate the normalization results using stratified random pixel samples within transient shadows, considering central tendency and variance of differences in intensity relative to the unnormalized images. Overall, MV equalization yielded superior results in our tests, reducing the radiometric effects of shadowing by more than 85 percent. The HM and PIF-LR approaches showed slightly lower performance than MV, while the RCS approach proved unreliable among scenes and among stratified intensity levels. We qualitatively evaluate a shadow normalization based on MV equalization, describing its utility and limitations when applied in change detection. Application of image-to-image radiometric normalization for brightening shadowed areas in multi-temporal imagery in this study proved efficient and effective to support change detection. 相似文献
477.
Along with rapid global urbanization, cities are challenged by environmental risks and resource scarcity. Sustainable urban planning is central to address the dilemma of economic growth and ecosystem protection, where the use of land is critical. Sustainable land use patterns are spatially explicit in nature, and can be structured and addressed using spatial optimization integrating GIS and mathematical models. This research discusses prominent sustainability concerns in land use planning and suggests a generalized multi‐objective spatial optimization model to facilitate conventional planning. The model is structured to meet land use demand while satisfying the requirements of the physical environment, society and economy. Unlike existing work relying on raster data, due to its simple data structure and ease of spatial relationship evaluation, this research develops an approach for identifying land use solutions based on vector data that better reflects the actual shape and spatial layout of land parcels as well as the ways land use information is managed in practice. An evolutionary algorithm is developed to find the set of efficient (Pareto) solutions given the complexity of vector‐based representations of space. The proposed approach is applied in an empirical study of Dafeng, China in order to support local urban growth and development. The results demonstrate that spatial optimization can be a powerful tool for deriving effective and efficient land use planning strategies. A comparison to results using a raster data approach supports the superiority of land use optimization using vector data as part of planning practice. 相似文献
478.
479.
Alan E. Rubin 《Meteoritics & planetary science》2018,53(10):2181-2192
The number ratio of carbonaceous to ordinary chondrites (the CC/OC ratio) varies with mass. It is very high (≳90) in small mass ranges (10−8 to 10−12 kg) among interplanetary dust particles and micrometeorites; it is moderately high (~5 to 30) for 1 to 10 m size fireball meteoroids (with estimated masses between ~103 and ~106 kg). In the range of most normal-sized meteorite falls (0.01–20 kg), the ratio is low (0.04–0.05); the ratio increases at greater mass ranges: at ≥200 kg, the ratio is 0.09; at ≥500 kg, the ratio is 0.20. The CC/OC ratio also increases from 0.05 to 0.16 for small meteorite finds (10−3 to 10−4 kg). High CC/OC ratios at low and high mass ranges are due to the predominance of CC material in the outer solar system. Small particles from this region spiral into the inner solar system typically in ≤106 years due to Poynting–Robertson drag. Meter-sized meteoroids in this region are affected by Yarkovsky forces, pushing them into resonances where they are efficiently transferred to the inner solar system. Normal-sized meteorites are derived from centimeter-to-decimeter-sized meteoroids that have sluggish drift rates (i.e., they are less affected by the seasonal Yarkovsky effect) compared to larger bodies. Consequently, the centimeter-to-decimeter-sized meteoroids spend more time in interplanetary space (where they are subject to collisions) than larger objects. The greater friability of carbonaceous chondrites relative to ordinary chondrites tends to winnow the carbonaceous chondrites out in this size/mass range during their long interplanetary sojourn, thereby decreasing the CC/OC ratio. 相似文献
480.
Chlorpyrifos and dimethoate are overused agricultural pesticides that can trigger trophic cascades, resulting in toxicity to both terrestrial and aquatic organisms as well as altered ecosystems. In previous studies, substantial attention has been given to the effects of pesticides on vertebrate species and, to a lesser extent, species of zooplankton. The present study was designed to show that the fission time effective concentration in ciliates is a potential aquatic detection index for environmental monitoring. The ciliate Urostyla grandis was treated with doses of chlorpyrifos and dimethoate. After exposed to the pesticides, the LC_(50)(i.e., concentration that killed 50% of the ciliate cells within 24 h) values were 0.029 mg L~(-1) for chlorpyrifos and 0.0685 mg L~(-1) for dimethoate. The fission time effective concentrations after 168 h of exposure were 0.0075–0.0093 mg L~(-1) for chlorpyrifos and 0.2640–0.2750 mg L~(-1) for dimethoate. These results show that the fission time effective concentration is lower than the LC_(50) value in ciliates, indicating that fission time effective concentration is more suitable than the LC_(50) value for environmental monitoring using ciliates. The effects of chlorpyrifos and dimethoate on ciliate cell ultrastructures included agglutination of chromatin in the macronucleus, protruded and discontinuous macronuclear and micronuclear membranes, loss of integrity of mitochondrial membranes and contents, and abscission and deformation of the adoral zone of membranelles. 相似文献