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91.
92.
The European Gravity Field and Steady-State Ocean Circulation Explorer Satellite Mission Its Impact on Geophysics 总被引:1,自引:0,他引:1
J. A. Johannessen G. Balmino C. Le Provost R. Rummel R. Sabadini H. Sünkel C.C. Tscherning P. Visser P. Woodworth C. Hughes P. Legrand N. Sneeuw F. Perosanz M. Aguirre-Martinez H. Rebhan M. Drinkwater 《Surveys in Geophysics》2003,24(4):339-386
Current knowledge of the Earth's gravity field and its geoid, as derived from various observing techniques and sources, is incomplete. Within a reasonable time, substantial improvement will come by exploiting new approaches based on spaceborne gravity observation. Among these, the European Space Agency (ESA) Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite mission concept has been conceived and designed taking into account multi-disciplinary research objectives in solid Earth physics, oceanography and geodesy. Based on the unique capability of a gravity gradiometer combined with satellite-to-satellite high-low tracking techniques, an accurate and detailed global model of the Earth's gravity field and its corresponding geoid will be recovered. The importance of this is demonstrated by a series of realistic simulation experiments. In particular, the quantitative impact of the new and accurate gravity field and geoid is examined in studies of tectonic composition and motion, Glaciological Isostatic Adjustment, ocean mesoscale variability, water mass transport, and unification of height systems. Improved knowledge in each of these fields will also ensure the accumulation of new understanding of past and present sea-level changes. 相似文献
93.
Paleomagnetic data from lavas and dikes of the Unkar igneous suite (16 sites) and sedimentary rocks of the Nankoweap Formation (7 sites), Grand Canyon Supergroup (GCSG), Arizona, provide two primary paleomagnetic poles for Laurentia for the latest Middle Proterozoic (ca. 1090 Ma) at 32°N, 185°E (dp=6.8°, DM=9.3°) and early Late Proterozoic (ca. 850–900 Ma) at 10°S, 163°E (dp=3.5°, DM=7.0°). A new 40Ar/39Ar age determination from an Unkar dike gives an interpreted intrusion age of about 1090 Ma, similar to previously reported geochronologic data for the Cardenas Basalts and associated intrusions. The paleomagnetic data show no evidence of any younger, middle Late Proterozoic tectonothermal event such as has been revealed in previous geochronologic studies of the Unkar igneous suite. The pole position for the Unkar Group Cardenas Basalts and related intrusions is in good agreement with other ca. 1100 Ma paleomagnetic poles from the Keweenawan midcontinent rift deposits and other SW Laurentia diabase intrusions. The close agreement in age and position of the Unkar intrusion (UI) pole with poles derived from rift related rocks from elsewhere in Laurentia indicates that mafic magmatism was essentially synchronous and widespread throughout Laurentia at ca. 1100 Ma, suggesting a large-scale continental magmatic event. The pole position for the Nankoweap Formation, which plots south of the Unkar mafic rocks, is consistent with a younger age of deposition, at about 900 to 850 Ma, than had previously been proposed. Consequently, the inferred 200 Ma difference in age between the Cardenas Basalts and overlying Nankoweap Formation provides evidence for a third major unconformity within the Grand Canyon sequence. 相似文献
94.
Andrew Swales R. Bruce Williamson Laurence F. Van Dam Morag J. Stroud Matt S. McGlone 《Estuaries and Coasts》2002,25(1):43-56
Cores were collected from the length of Pakuranga estuary, a small urban estuary in Auckland, New Zealand, to determine sedimentation
and contaminant history, and in particular the impact of urbanization. Catchment sediment loads for the most recent history
(1953–1995), including urbanization since 1960, were reconstructed using the landcover history and soil erosion modeling.
Pollen and14C dating and pre-urban landcover history were used to reconstruct early estuary sedimentation (i.e., post-3000 yr BP to 1960).
Heavy metal concentrations, particle size,137Cs, pollen, and catchment sediment loads were all needed to disentangle the complex estuarine response to urbanization.137Cs profiles did not reflect the historical fallout pattern, but deposition of137Cs-labelled eroded catchment soil, coinciding with peaks in urban construction. Temporal variations in stormwater137Cs concentrations are likely due to varying contributions from137Cs-rich topsoil and137Cs-poor subsoils. A similar pattern was observed in heavy-metal concentrations and attributed to street runoff rather than
topsoil being diluted by metal-poor subsoils. Dating of the sediment profiles showed that during urbanization sedimentation
rates in the tidal creek and estuary were higher than sedimentation rates associated with past agricultural landuse and the
original forest landcover. Urbanization has brought about substantial environmental changes in the upper estuary through continued
infilling of shallow, intertidal areas, contamination by heavy metals to levels of ecological concern, sediment textural changes,
and rapid mangrove colonization of formerly bare intertidal sediments. 相似文献
95.
A Holocene record of pollen, macrofossils, testate amoebae and peat humification is presented from a small montane bog. Sediment accumulation began before 9000 yr BP, but peat growth not until ca. 7000 BP. From 12 000 to 7000 yr BP, a shrub–grassland dominated under a dry climate, with increasing conifer forest and tall scrub from ca. 9600 yr BP. At 7000 yr BP a dense montane–subalpine low conifer forest established under a moist, cool climatic regime. Between 7000 and 700 yr BP the bog surface was shrubby, tending to be dry but with highly variable surface wetness. The catchment was affected by major fire at least four times between 4000 and 1000 yr BP. Both fire and bog surface wetness may have been linked to ENSO-caused variations in rainfall. Cooler, cloudier winters and disturbance by fire promoted the expansion of the broadleaf tree Nothofagus menziesii between 4000 yr BP and 1300 yr BP at the expense of the previous conifer forest–scrub vegetation. Polynesian fires (ca. 700 yr BP) reduced the vegetation to tussock grassland and bracken. Deforestation did not markedly affect the hydrology of the site. European pastoralism since ad 1860 has increased run-off and rising water tables in the bog have led to a Sphagnum-dominated cover. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
96.
δ13C was determined in lignin extracted from present-day cladodes of Phyllocladus alpinus (a small coniferous tree) from seven well-lit sites across New Zealand. The δ13C values ranged from −30.9‰ to −23.6‰ and were compared with monthly means of temperature, precipitation, relative humidity and vapour pressure deficit from the nearest recording stations. Of these parameters, the leaf-to-air vapour pressure deficit of the first month of cladode growth and expansion proved to be the most significantly correlated with lignin δ13C, over a range of 0.3 to 0.8 kPa, confirming the importance of atmospheric moisture content on stomatal conductance. The carbon isotopic signature of lignin from fossilised cladodes preserved under the Kawakawa Tephra (22.6 k 14C yr BP) on the North Island is identical to that of the whole tissue, suggesting that for this species at least, fossil material can be used to approximate the lignin δ13C. The δ13C of species- and organ-specific fossil terrestrial plant material therefore provides an excellent method to quantify past changes in leaf-to-air vapour pressure deficit. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
97.
Jinglang Feng Ron Noomen Xiyun Hou Pieter Visser Jianping Yuan 《Celestial Mechanics and Dynamical Astronomy》2017,127(1):67-93
Using a gravitational field truncated at the 4th degree and order, the 1:1 ground-track resonance is studied. To address the main properties of this resonance, a 1-degree of freedom (1-DOF) system is firstly studied. Equilibrium points (EPs), stability and resonance width are obtained. Different from previous studies, the inclusion of non-spherical terms higher than degree and order 2 introduces new phenomena. For a further study about this resonance, a 2-DOF model which includes a main resonance term (the 1-DOF system) and a perturbing resonance term is studied. With the aid of Poincaré sections, the generation of chaos in the phase space is studied in detail by addressing the overlap process of these two resonances with arbitrary combinations of eccentricity (e) and inclination (i). Retrograde orbits, near circular orbits and near polar orbits are found to have better stability against the perturbation of the second resonance. The situations of complete chaos are estimated in the \(e-i\) plane. By applying the maximum Lyapunov Characteristic Exponent (LCE), chaos is characterized quantitatively and similar conclusions can be achieved. This study is applied to three asteroids 1996 HW1, Vesta and Betulia, but the conclusions are not restricted to them. 相似文献
98.
Craig E. Divine Tracy Roth Michelle Crimi Abrahm C. DiMarco Matt Spurlin Jeff Gillow Gastón Leone 《Ground Water Monitoring & Remediation》2018,38(1):56-65
A new in‐situ remediation concept termed a Horizontal Reactive Media Treatment Well (HRX Well®) is presented that utilizes horizontal wells filled with reactive media to passively treat contaminated groundwater in‐situ. The approach involves the use of large‐diameter directionally drilled horizontal wells filled with granular reactive media generally installed parallel to the direction of groundwater flow. The design leverages natural “flow‐focusing” behavior induced by the high in‐well hydraulic conductivity of the reactive media relative to the aquifer hydraulic conductivity to passively capture and treat proportionally large volumes of groundwater within the well. Clean groundwater then exits the horizontal well along its downgradient sections. Many different types of solid granular reactive media are already available (e.g., zero valent iron, activated carbon, ion exchange resins, zeolite, apatite, chitin); therefore, this concept could be used to address a wide range of contaminants. Three‐dimensional flow and transport simulations were completed to assess the general hydraulic performance, capture zones, residence times, effects of aquifer heterogeneity, and treatment effectiveness of the concept. The results demonstrate that capture and treatment widths of up to tens of feet can be achieved for many aquifer settings, and that reductions in downgradient concentrations and contaminant mass flux are nearly immediate. For a representative example, the predicted treatment zone width for the HRX Well is approximately 27 to 44 feet, and contaminant concentrations immediately downgradient of the HRX Well decreased an order of magnitude within 10 days. A series of laboratory‐scale physical tests (i.e., tank tests) were completed that further demonstrate the concept and confirm model prediction performance. For example, the breakthrough time, peak concentration and total mass recovery of methylene blue (reactive tracer) was about 2, 35, and 20 times (respectively) less than chloride (conservative tracer) at the outlet of the tank‐scale HRX Well. 相似文献
99.
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
100.
Martin G. Shepley Nick Schmidt Matt J. Senior Stephen R. H. Worthington Ron B. Scheckenberger 《Ground water》2020,58(2):269-277
Storm sewer systems and their associated utility trenches may strongly influence the effects of urbanization on a groundwater system. This study was undertaken to identify the causes of district-wide basement infiltration in an aquitard system. It comprised widespread continuous monitoring of utility trench wells and dye tracing from storm sewer system exfiltration tests. The results indicate that a major effect of urbanization on shallow groundwater is related to storm sewer system exfiltration, which is marked by a characteristic pattern of head variations in the aquitard unrelated to distributed surface infiltration. The aquitard constrains flow from storm sewer system exfiltration to the utility trench, creating an urban flow path for groundwater discharge. Temporary buildup of water levels in the utility trench drives relatively high-velocity flow through the permeable sewer bedding material of the utility trench to a separate foundation drainage collector system, ultimately causing a severe “urban karst” effect that produces system surcharging and widespread basement water infiltration. The main conditions causing the “urban karst” are the large hydraulic conductivity ratio between the utility trench material and the aquitard, and the shallow depth and low gradient of the storm sewer system imposed by a very flat drainage basin. 相似文献