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241.
Xavier Emery 《Mathematical Geosciences》2008,40(1):83-99
This paper presents random field models with Gaussian or gamma univariate distributions and isofactorial bivariate distributions,
constructed by composing two independent random fields: a directing function with stationary Gaussian increments and a stationary
coding process with bivariate Gaussian or gamma distributions. Two variations are proposed, by considering a multivariate
directing function and a coding process with a separable covariance, or by including drift components in the directing function.
Iterative algorithms based on the Gibbs sampler allow one to condition the realizations of the substitution random fields
to a set of data, while the inference of the model parameters relies on simple tools such as indicator variograms and variograms
of different orders. A case study in polluted soil management is presented, for which a gamma model is used to quantify the
risk that pollutant concentrations over remediation units exceed a given toxicity level. Unlike the multivariate Gaussian
model, the proposed gamma model accounts for an asymmetry in the spatial correlation of the indicator functions around the
median and for a spatial clustering of high pollutant concentrations. 相似文献
242.
Paulo Pereira Xavier Úbeda Deborah Martin Jorge Mataix‐Solera Artemi Cerdà Maria Burguet 《水文研究》2014,28(11):3681-3690
The aim of this work is to study the effects of a wildfire on water‐extractable elements in ash from a Pinus pinaster forest located in Portugal. The pH, electrical conductivity (EC), calcium (Ca2+), magnesium (Mg2+), sodium (Na+), potassium (K+), sodium and potassium adsorption ratio (SPAR), aluminium (Al3+), manganese (Mn2+), iron (Fe2+), zinc (Zn2+), sulphur (S), silica (Si) and phosphorous (P) were analysed in ash sampled from a sloped area burned in a wildfire and from litter from a contiguous unburned area, with similar morphological conditions. The results showed that ash leachates had higher pH and EC, and were significantly richer in water‐extractable Ca2+, Mg2+, Na+, K+, SPAR, S and Si and significantly poorer in water‐extractable Al3+, Fe2+, Mn2+ and Zn2+ than litter solutions. No significant differences were observed in water‐extractable P. The fire changed the ash solute chemistry compared with the unburned litter and increased the sample variability of nutrient distribution with potential implications for plant recovery. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
243.
High-resolution modelling of the Antarctic surface mass balance, application for the twentieth, twenty first and twenty second centuries 总被引:1,自引:1,他引:0
Cécile Agosta Vincent Favier Gerhard Krinner Hubert Gallée Xavier Fettweis Christophe Genthon 《Climate Dynamics》2013,41(11-12):3247-3260
About 75 % of the Antarctic surface mass gain occurs over areas below 2,000 m asl, which cover 40 % of the grounded ice-sheet. As the topography is complex in many of these regions, surface mass balance modelling is highly dependent on horizontal resolution, and studying the impact of Antarctica on the future rise in sea level requires physical approaches. We have developed a computationally efficient, physical downscaling model for high-resolution (15 km) long-term surface mass balance (SMB) projections. Here, we present results of this model, called SMHiL (surface mass balance high-resolution downscaling), which was forced with the LMDZ4 atmospheric general circulation model to assess Antarctic SMB variability in the twenty first and the twenty second centuries under two different scenarios. The higher resolution of SMHiL better reproduces the geographical patterns of SMB and increase significantly the averaged SMB over the grounded ice-sheet for the end of the twentieth century. A comparison with more than 3200 quality-controlled field data shows that LMDZ4 and SMHiL reproduce the observed values equally well. Nevertheless, field data below 2,000 m asl are too scarce to efficiently show the added value of SMHiL and measuring the SMB in these undocumented areas should be a future scientific priority. Our results suggest that running LMDZ4 at a finer resolution (15 km) may give a future increase in SMB in Antarctica that is about 30 % higher than by using its standard resolution (60 km) due to the higher increase in precipitation in coastal areas at 15 km. However, a part (~15 %) of these discrepancies could be an artefact from SMHiL since it neglects the foehn effect and likely overestimates the precipitation increase. Future changes in the Antarctic SMB at low elevations will result from the competition between higher snow accumulation and runoff. For this reason, developing downscaling models is crucial to represent processes in sufficient detail and correctly model the SMB in coastal areas. 相似文献
244.
Françoise Andrieux-Loyer Afi Azandegbé Florian Caradec Xavier Philippon Roger Kérouel Agnès Youenou Jean-Louis Nicolas 《Aquatic Geochemistry》2014,20(6):573-611
This study aims to compare the impact of oyster cultures on diagenetic processes and the phosphorus cycle in the sediments of the Aber Benoît and the Rivière d’Auray, estuary of Brittany, France. Our results showed clear evidence of the seasonal impact of oyster cultures on sediment characteristics (grain size and organic matter parameters) and the phosphorus cycle, especially in the Aber Benoît. At this site, seasonal variations in sulfide and Fe concentrations in pore waters, as well as Fe–P concentrations in the solid phase, highlighted a shift from a system governed by iron reduction (Reference) to a system governed by sulfate reduction (beneath oyster). This could be partly explained by the increase in labile organic matter (i.e., biodeposits) beneath oysters, whose mineralization by sulfate led to high sulfide concentrations in pore waters (up to 4,475 µmol l?1). In turn, sulfide caused an enhanced release of phosphate in the summer, as adsorption sites for phosphate decreased through the formation of iron–sulfide compounds (FeS and FeS2). In the Aber Benoît, dissolved Fe/PO4 ratios could be used as an indicator of phosphate release into oxic water. Low Fe/PO4 ratios in the summer indicated higher effluxes of phosphate toward the water column (up to 47 µmol m?2 h?1). At other periods, Fe/PO4 ratios higher than 2 mol/mol indicated very low phosphate fluxes. In contrast, in the Rivière d’Auray, the occurrence of macroalgae, stranding regularly all over the site, clearly masked the impact of oyster cultures on sediment properties and the phosphorus cycle and made the use of Fe/PO4 ratios more difficult in terms of indicators of phosphate release. 相似文献
245.
Simulating Large Gaussian Random Vectors Subject to Inequality Constraints by Gibbs Sampling 总被引:4,自引:2,他引:2
The Gibbs sampler is an iterative algorithm used to simulate Gaussian random vectors subject to inequality constraints. This algorithm relies on the fact that the distribution of a vector component conditioned by the other components is Gaussian, the mean and variance of which are obtained by solving a kriging system. If the number of components is large, kriging is usually applied with a moving search neighborhood, but this practice can make the simulated vector not reproduce the target correlation matrix. To avoid these problems, variations of the Gibbs sampler are presented. The conditioning to inequality constraints on the vector components can be achieved by simulated annealing or by restricting the transition matrix of the iterative algorithm. Numerical experiments indicate that both approaches provide realizations that reproduce the correlation matrix of the Gaussian random vector, but some conditioning constraints may not be satisfied when using simulated annealing. On the contrary, the restriction of the transition matrix manages to satisfy all the constraints, although at the cost of a large number of iterations. 相似文献
246.
247.
A new nonsingular analytical theory for the motion of near Earth satellite orbits with the air drag effect is developed for long term motion in terms of the KS uniformly regular canonical elements by a series expansion method, by assuming the atmosphere to be symmetrically spherical with constant density scale height. The series expansions include up to third order terms in eccentricity. Only two of the nine equations are solved analytically to compute the state vector and change in energy at the end of each revolution, due to symmetry in the equations of motion. Numerical comparisons of the important orbital parameters semi major axis and eccentricity up to 1000 revolutions, obtained with the present solution, with KS elements analytical solution and Cook, King-Hele and Walker's theory with respect to the numerically integrated values, show the superiority of the present solution over the other two theories over a wide range of eccentricity, perigee height and inclination. 相似文献
248.
Xavier Leyre Michel Sghedoni Francis Arbusti Primo Attina Peter von Ballmoos 《Experimental Astronomy》2005,20(1-3):455-464
Until recently, focusing of gamma-radiation was regarded as an impracticable task. Today, gamma-ray lenses have become feasible and present promising perspectives for future instrumentation. For the first time in high energy astronomy the signal/noise ratio will be dramatically improved as gamma-rays are collected on the large area of a lens from where they are focused onto a small detector. Besides an unprecedented sensitivity, MAX will feature very high angular and energy resolution. The conjunction of this new technique of Gamma Ray focusing and the new possibilities brought by the developping formation flying technology will allow a great step for Gamma Astronomy. This paper will describe after a short recall of the scientific objectives of MAX, the design of the MAX formation flying concept and the associated main design drivers and constraint. 相似文献
249.
The homogenization method is used to determine the formulation of the behaviour of both saturated and unsaturated porous media. This approach makes it possible to assess the validity of the effective stress concept as a function of the properties of the porous media at the microscopic scale. Furthermore, the influence of the morphologies of the solid and fluid phases on the macroscopic behaviour is studied. The strain induced by drying is examined as a function of the morphological properties. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
250.
Daniele L. Pinti Xavier Quidelleur Pierre Lahitte Cyril Aznar Sergio Chiesa Pierre-Yves Gillot 《地学学报》2003,15(3):176-186
ABSTRACT Distal tephra are a valuable record of the volcano-tectonic evolution of an area under study. Here, we document the case of the Early Middle Pleistocene rhyolitic tephra of Piànico, discovered in the Southern Italian Alps. The geochemical characteristics of Piànico are unique among the distal tephra outcropping in Italy and indicate an intraplate volcanism at the source. The alkali composition and trace elements show a striking resemblance with the rhyolitic complex of the Euganean Hills, located 170 km from Piànico. However, these rhyolites are much older (Oligocene). Alternatively, the source of this intraplate volcanic episode could be located in the southern Tyrrhenian Sea. 相似文献