Biogeochemistry, transport fluxes and emission of greenhouse gases from the Niger River (West Africa): preliminary results after two years of monitoring

Poster (2013, April 07)

Variability of North Sea pH and CO2 pumping in response to North Atlantic Oscillation forcing

Poster (2013, April 07)

Branch-and-price: principles and its application to a 2-period vehicle routing problem

Scientific conference (2013, April 05)

We review branch-and-price as an efficient algorithm to solve integer programming problems with huge number of variables. In particular, we discuss column generation algorithm as the main engine in branch ... [more ▼]

We review branch-and-price as an efficient algorithm to solve integer programming problems with huge number of variables. In particular, we discuss column generation algorithm as the main engine in branch-and-price. Implementation of branch-and-price to solve the most basic version of the well-known vehicle routing problem (VRP) is investigated, and some common tricks are introduced. Then, a new extension of VRP is introduced and exploitation of branch-and-price to solve it is discussed. We consider a 2-period vehicle routing problem where each vertex of the network has a positive demand for period 1, 2, or both. Each demand on period 1 can be postponed to period 2 in order to decrease sum of the routing costs on two periods, but it is penalized in the objective function. Similarly, each demand on period 2 can be advanced to period 1, and yet penalized, with the hope of reducing routing costs. We have used many of the classic tricks to implement branch-and-price for solving our 2-period VRP. We have also used new tricks to (1) possibly improve the upper bound during the course of column generation in each node and (2) decrease the computations time to solve pricing problem in column generation. [less ▲]

Evidence and analysis of 2012 Greenland records from spaceborne observations, a regional climate model and reanalysis data

in Cryosphere (The) (2013), 7

A combined analysis of remote sensing observations, regional climate model (RCM) outputs and reanalysis data over the Greenland ice sheet provides evidence that multiple records were set during summer ... [more ▼]

A combined analysis of remote sensing observations, regional climate model (RCM) outputs and reanalysis data over the Greenland ice sheet provides evidence that multiple records were set during summer 2012. Melt extent was the largest in the satellite era (extending up to ∼97% of the ice sheet) and melting lasted up to ∼2 months longer than the 1979–2011 mean. Model results indicate that near surface temperature was ∼3 standard deviations (σ) above the 1958–2011 mean, while surface mass balance (SMB) was ∼3σ below the mean and runoff was 3.9σ above the mean over the same period. Albedo, exposure of bare ice and surface mass balance also set new records, as did the total mass balance with summer and annual mass changes of, respectively, −627 Gt and −574 Gt, 2σ below the 2003–2012 mean. We identify persistent anticyclonic conditions over Greenland associated with anomalies in the North Atlantic Oscillation (NAO), changes in surface conditions (e.g., albedo, surface temperature) and preconditioning of surface properties from recent extreme melting as major driving mechanisms for the 2012 records. Less positive if not increasingly negative SMB will likely occur should these characteristics persist. [less ▲]

Gestion à long terme des déchets nucléaires belges moyennement et hautement radioactifs: Construire un dispositif communicationnel mais comment?

Conference (2013, April 04)

Aujourd’hui, personne ne remet plus en cause l’intégration d’une approche participative dans le processus de gestion des déchets moyennement et hautement radioactifs belges (de catégorie B&C). Cette ... [more ▼]

Aujourd’hui, personne ne remet plus en cause l’intégration d’une approche participative dans le processus de gestion des déchets moyennement et hautement radioactifs belges (de catégorie B&C). Cette dimension apparaît indispensable compte tenu, d’une part, du consensus international et européen en la matière, et d’autre part, des écueils qu’a déjà connu le programme belge de gestion des déchets faiblement radioactifs. Ce constat d’une nécessaire intégration de la dimension sociétale dans la problématique a nécessité la mobilisation d’un nouveau champ d’expertise dans le champ des déchets radioactifs : celui des sciences sociales. Mais comment le chercheur en sciences sociales a-t-il été mobilisé ? Cette communication propose de retracer l’évolution de l’engagement du chercheur en sciences sociales depuis le lancement des activités participatives mises en place lors de l’élaboration du programme de gestion jusqu’à aujourd’hui. D’une situation initiale qui attribuait un rôle prédéfini au chercheur, celui d’évaluateur externe réagissant à la demande du commanditaire public, on est passé à une relation plus étroite, mais aussi plus ambivalente, de co-construction de connaissance entre le chercheur et l’ONDRAF, devenu à la fois sujet et objet d’étude. Pour analyser ce changement de configuration, l’auteure adopte une démarche principalement inductive et pragmatique en partant de ses expériences empiriques menées au sein de l’organisme de gestion des déchets radioactifs. L’auteure s’inscrit dans le cadre d’analyse proposé par Laurent and Van Oudheusden (2013 à paraitre) pour rendre compte du rôle du chercheur dans le cadre des nanotechnologies. L’évolution de la situation de chercheure en sciences sociales décrite ici sera ainsi déclinée sur les trois dimensions mises en évidence par les auteurs: la relation du chercheur en sciences sociales aux acteurs qu’il étudie ; la pertinence politique de son travail ; enfin, les problèmes auxquels le chercheur doit faire face. [less ▲]

Automatic target recognition using passive bistatic radar signals

Doctoral thesis (2013)

We present the design, development, and test of three novel, distinct automatic target recognition (ATR) systems for the recognition of airplanes and, more specifically, non- cooperative airplanes, i.e ... [more ▼]

We present the design, development, and test of three novel, distinct automatic target recognition (ATR) systems for the recognition of airplanes and, more specifically, non- cooperative airplanes, i.e. airplanes that do not provide information when interrogated, in the framework of passive bistatic radar systems. Passive bistatic radar systems use one or more illuminators of opportunity (already present in the field), with frequencies up to 1 GHz for the transmitter part of the systems considered here, and one or more receivers, deployed by the persons managing the system, and not co-located with the transmitters. The sole source of information are the signal scattered on the airplane and the direct-path signal that are collected by the receiver, some basic knowledge about the transmitter, and the geometrical bistatic radar configuration. The three distinct ATR systems that we built respectively use the radar images, the bistatic complex radar cross-section (BS-RCS), and the bistatic radar cross-section (BS- RCS) of the targets. We use data acquired either on scale models of airplanes placed in an anechoic, electromagnetic chamber or on real-size airplanes using a bistatic testbed consisting of a VOR transmitter and a software-defined radio (SDR) receiver, located near Orly airport, France. We describe the radar phenomenology pertinent for the problem at hand, as well as the mathematical underpinnings of the derivation of the bistatic RCS values and of the construction of the radar images. For the classification of the observed targets into pre-defined classes, we use either extremely randomized trees or subspace methods. A key feature of our approach is that we break the recognition problem into a set of sub-problems by decomposing the parameter space, which consists of the frequency, the polarization, the aspect angle, and the bistatic angle, into regions. We build one recognizer for each region. We first validate the extra-trees method on the radar images of the MSTAR dataset, featuring ground vehicles. We then test the method on the images of the airplanes constructed from data acquired in the anechoic chamber, achieving a probability of correct recognition up to 0.99. We test the subspace methods on the BS-CRCS and on the BS-RCS of the airplanes extracted from the data acquired in the anechoic chamber, achieving a probability of correct recognition up to 0.98, with variations according to the frequency band, the polarization, the sector of aspect angle, the sector of bistatic angle, and the number of (Tx,Rx) pairs used. The ATR system deployed in the field gives a probability of correct recognition of 0.82, with variations according to the sector of aspect angle and the sector of bistatic angle. [less ▲]

Probability perturbation method applied to the inversion of groundwater flow models using HydroGeoSphere

Conference (2013, April 04)

Solving spatial inverse problems in Earth Sciences remains a big challenge given the high number of parameters to invert for and the complexity of non-linear forward models. Techniques were developed to ... [more ▼]

Solving spatial inverse problems in Earth Sciences remains a big challenge given the high number of parameters to invert for and the complexity of non-linear forward models. Techniques were developed to reduce the number of parameters to invert for or to produce geologically consistent simulations from an initial guess. These techniques ask for a prior model to constrain the spatial distribution of the solution. Geostatistical models contain, by nature, information to control the spatial features of the inverse solutions, but the integration of dynamic data into such models remains difficult. We adapted, the “probability perturbation algorithm” (PPM) using Matlab® to invert hydrogeological data using multiple-point geostatistics to build models of pre-defined hydrofacies. The algorithm uses HydroGeoSphere (HGS) to compute the forward response of the model and SGems to produce geostatistical realizations. The algorithm only needs the proper definition of all the parameters to be used by HydroGeoSphere (grid matching with SGems, position of the wells, pumping rate, facies properties, boundary conditions, etc.). The PPM algorithm will automatically seek solutions fitting both hydrogeological data and geostatistical constraints. Through the inversion process, the initial geostatistical realization is perturbed. Only geometrical features of the model are affected, i.e. we do not attempt to directly find the optimal value of hydrogeological parameters, but the optimal spatial distribution of facies whose prior distribution is quantified in a training image. The algorithm can be divided in three steps. In the first step, we use SGems to generate an initial facies model with the multiple-point geostatistical algorithm SNESIM (single normal equation simulation). The facies model is composed of several categories representing hydrological facies (e.g. gravel, sand and clay). It can be conditioned using hard data (borehole data) and/or soft data (e.g. geophysical data). We then run a first flow simulation with HydroGeoSphere. This requires defining hydrogeological parameters (porosity, hydraulic conductivity, etc.) for each category of the facies model to create a hydrogeological model. The response of the latter model is compared to the expected one through an objective function. In the second step, a perturbation to the facies model is computed using a single parameter called rD. This perturbation is used to generate a new facies model with SGems and calculate a new objective function value via HGS, as done in the first step. An inner loop optimizes the value of rD. In the third step, we verify if the objective function of the best fitting model is smaller than a predefined value. If it is the case, we stop the algorithm, otherwise we go back to step 2 until convergence. We illustrate the methodology with a synthetic example in an alluvial aquifer. The model is based on a training image depicting gravel channels and clay lenses in a coarse sand aquifer. We simulate a pumping test and inverse water level data recorded at 9 wells using our implementation of the PPM algorithm. Using this method, it is possible to generate multiple solutions and to derive a posterior probability of the facies distribution. [less ▲]

La question de l'activité dans le vieillissement normal et pathologique

Scientific conference (2013, April 03)

A multiscale model of the influence of oxygen during bone fracture healing.

Poster (2013, April 03)

An XMM-Newton view of the M 17 nebula

in New Astronomy (2013), 20

We present the analysis of an XMM-Newton observation of the M 17 nebula. The X-ray point source population consists of massive O-type stars and a population of probable low-mass pre-main sequence stars ... [more ▼]

We present the analysis of an XMM-Newton observation of the M 17 nebula. The X-ray point source population consists of massive O-type stars and a population of probable low-mass pre-main sequence stars. CEN 1a,b and OI 352, the X-ray brightest O-type stars in M 17, display hard spectra (kT of 3.8 and 2.6 keV) consistent with a colliding wind origin in binary/multiple systems. We show that the strong interstellar reddening towards the O-type stars of M 17 yields huge uncertainties on their L[SUB]X[/SUB]/L[SUB]bol[/SUB] values. The low-mass pre-main sequence stars exhibit hard spectra resulting from a combination of high plasma temperatures and very large interstellar absorption. We find evidence for considerable long term (months to years) variability of these sources. M 17 is one of the few star formation complexes in our Galaxy producing diffuse X-ray emission. We analyze the spectrum of this emission and compare it with previous studies. Finally, we discuss the Optical Monitor UV data obtained simultaneously with the X-ray images. We find very little correspondence between the UV and X-ray sources, indicating that the majority of the UV sources are foreground stars, whilst the bulk of the X-ray sources are deeply embedded in the M 17 complex. [less ▲]