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See detailGeometry and Symmetries in Coordination Control
Sarlette, Alain ULg

Doctoral thesis (2009)

The present dissertation studies specific issues related to the coordination of a set of “agents” evolving on a nonlinear manifold, more particularly a homogeneous manifold or a Lie group. The viewpoint is ... [more ▼]

The present dissertation studies specific issues related to the coordination of a set of “agents” evolving on a nonlinear manifold, more particularly a homogeneous manifold or a Lie group. The viewpoint is somewhere between control algorithm design and system analysis, as algorithms are derived from simple principles — often retrieving existing models — to highlight specific behaviors. With a fair amount of approximation, the objective of the dissertation can be summarized by the following question: Given a swarm of identical agents evolving on a nonlinear, nonconvex configuration space with high symmetry, how can you define specific collective behavior, and how can you design individual agent control laws to get a collective behavior, without introducing hierarchy nor external reference points that would break the symmetry of the configuration space? Maintaining the basic symmetries of the coordination problem lies at the heart of the contributions. The main focus is on the global geometric invariance of the configuration space. This contrasts with most existing work on coordination, where either the agents evolve on vector spaces — which, to some extent, can cover local behavior on manifolds — or coordination is coupled to external reference tracking such that the reference can serve as a beacon around which the geometry is distorted towards vector space-like properties. A second, more standard symmetry is to treat all agents identically. Another basic ingredient of the coordination problem that has important implications in this dissertation is the reduced agent interconnectivity: each agent only gets information from a limited set of other agents, which can be varying. In order to focus on issues related to geometry / symmetry and reduced interconnectivity, individual agent dynamics are drastically simplified to simple integrators. This is justified at a “planning” level. Making the step towards realistic dynamics is illustrated for the specific case of rigid body attitude synchronization. The main contributions of this dissertation are I. an extensive study of synchronization on the circle, (a) highlighting difficulties encountered for coordination and (b) proposing simple strategies to overcome these difficulties; II. (a) a geometric definition and related control law for “consensus” configurations on compact homogeneous manifolds, of which synchronization — all agents at the same point — is a special case, and (b) control laws to (almost) globally reach synchronization and “balancing”, its opposite, under general interconnectivity conditions; III. several propositions for rigid body attitude synchronization under mechanical dynamics; IV. a geometric framework for “coordinated motion” on Lie groups, (a) giving a geometric definition of coordinated motion and investigating its implications, and (b) providing systematic methods to design control laws for coordinated motion. Examples treated for illustration of the theoretical concepts are the circle S^1 (sometimes the sphere S^n ), the rotation group SO(n), the rigid-body motion groups SE(2) and SE(3) and the Grassmann manifolds Grass(p,n). The developments in this dissertation remain at a rather theoretical level; potential applications are briefly discussed. [less ▲]

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See detailGeometry module for next-generation computational electromagnetics software
Geuzaine, Christophe ULg

in Proceedings of the 4th Advanced Computational Electromagnetics workshop (2009, January 13)

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See detailGeometry of the mass-outflows around broad absorption line QSOs and formation of the complex Ly-alpha + N V line profile
Surdej, Jean ULg; Hutsemekers, Damien ULg

in Astronomy and Astrophysics (1987), 177

The possibility that the observed broad absorption line (BAL) profiles are formed in spherically symmetric atmospheres is investigated, and it is found that BAL and non-BAL QSOs form two distinct classes ... [more ▼]

The possibility that the observed broad absorption line (BAL) profiles are formed in spherically symmetric atmospheres is investigated, and it is found that BAL and non-BAL QSOs form two distinct classes of quasars. It is shown that the small residual intensity and large equivalent width noted for some absorption troughs may be reproduced by the resonance scattering of line photons across spherically symmetric BAL regions. N V emission strength is found to be higher in the spectrum of BAL QSOs than in that of non-BAL QSOs. Good agreement between theory and the observed attenuation of the Ly-alpha emission line, the observed enhancement of the N V emission strength, and the possible presence of a shoulderlike feature in the red wing of the N V emission profile is obtained. [less ▲]

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See detailThe geometry of the quadruply imaged quasar PG 1115+080: implications for H_0_.
Courbin, F.; Magain, Pierre ULg; Keeton, C. R. et al

in Astronomy and Astrophysics (1997), 324

Time delay measurements have recently been reported for the lensed quasar PG 1115+080. These measurements can be used to derive H_0_, but only if we can constrain the lensing potential. We have applied a ... [more ▼]

Time delay measurements have recently been reported for the lensed quasar PG 1115+080. These measurements can be used to derive H_0_, but only if we can constrain the lensing potential. We have applied a recently developed deconvolution technique to analyze sub-arcsecond I band images of PG 1115+080, obtained at the Nordic Optical Telescope (NOT) and the Canada France Hawaii Telescope (CFHT). The high performance of the deconvolution code allows us to derive precise positions and magnitudes for the four lensed images of the quasar, as well as for the lensing galaxy. The new measurement of the galaxy position improves its precision by a factor of 3 and thus strengthens the constraints on the lensing potential. With the new data, a range of models incorporating some of the plausible systematic uncertainties yields H_0_=53^+10^_-7_km/s/Mpc. [less ▲]

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See detailGéomorphologie du milieu littoral et aménagement
Ozer, André ULg; Ozer, Pierre ULg

in Fierro, Giuliano (Ed.) MED PICTURE : Mediterranean Project for the Improvement of Cooperation in Training between Universities, Regions and Enterprises (2002)

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See detailGeomorphology of streams and rivers
Bravard, J. P.; Petit, François ULg

in Encyclopedia of Inland Waters (2009)

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See detailGeophysical characterisation of a former waste disposal site in the context of landfill mining
Dumont, Gaël ULg; Robert, Tanguy ULg; Pilawski, Tamara et al

in EarthDoc - Near Surface Geoscience 2013 – 19th European Meeting of Environmental and Engineering Geophysics (2013, September 11)

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See detailGeophysical identification, characterization, and monitoring of preferential groundwater flow paths in fractured media
Robert, Tanguy ULg

Doctoral thesis (2012)

This thesis investigates fractured zones leading to preferential groundwater flow paths. In this context, we used the electrical resistivity tomography (ERT) and the self-potential (SP) methods to ... [more ▼]

This thesis investigates fractured zones leading to preferential groundwater flow paths. In this context, we used the electrical resistivity tomography (ERT) and the self-potential (SP) methods to identify, characterize, monitor, and finally model preferential flow in hydraulically-active fractured zones at a scale representative of real world applications. From an experimental point of view, we first identified the magnitude of self-potential signature, a -15 mV anomaly that could be associated with preferential groundwater flow in a shallow quartzite aquitard whereas the streaming potential theory was originally developed for porous media. This signature was confirmed experimentally in limestone aquifers at greater depths. The joint use of surface ERT and SP allowed the identification of water-bearing fractured areas which were electrically more conductive, presenting contrasts from 1 to 10 and which were hydraulically-active presenting negative SP anomalies ranging from -10 to -30 mV. We were also able to correlate hydraulic heads and SP gradients during a low and a high groundwater level period leading to interesting perspectives in understanding the dynamics of complex groundwater flow systems. Finally, a preferential flow and rapid transport path, over 10 m/h, was highlighted in a 20 m deep fractured and karstified limestone valley by monitoring a salt tracer test with only surface ERT. This methodology was being mostly used for relatively shallow and homogeneous aquifers up to now. Such information is crucial to set up new monitoring wells or to define the sampling rates of classic tracer test. From a methodological point of view, we quantitatively assessed the efficiency of blocky and minimum-gradient-support regularizations in electrical imaging to recover sharp interfaces on numerical benchmarks and with field data. The usefulness of resolution indicators such as the cumulative sensitivity matrix and the resolution matrix were also assessed in this context. We demonstrated that noise characterization is crucial in time-lapse inversion and may supplant the choice of the time-lapse inversion scheme, calling for a systematic analysis of reciprocal measurements (or a subset of them). We also showed that, when using data differences in an inversion scheme, the data error, as estimated by time-lapse reciprocal measurements, depends on the mean measured resistance. These error characterization studies should always be performed if one wants to avoid wrong interpretations about the hydrodynamics. We further showed that focused inversion techniques (blocky inversion, minimum-gradient-support) may offer great perspectives when recovering model changes in time-lapse inversion. Finally, ERT and SP were jointly used to conceptualize a physically-based and spatially distributed hydrogeological model, in particular to characterize the preferential flow paths. Predicted hydraulic heads and SP-derived hydraulic heads using the water table model showed a clear correlation, leading to perspectives in terms of hydrogeological model calibration. Further experiments are however needed to fully estimate the streaming potential apparent coupling coefficient, but the use of the full SP signals for hydrogeological model calibration is a clear perspective to this work. [less ▲]

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See detailGeophysical investigation and numerical modelling of unstable slopes: case-study of Kainama (Kyrgyzstan)
Danneels, Gaëlle ULg; Bourdeau, Céline ULg; Torgoev, I. et al

in Geophysical Journal International (2008), 175

The presence of massive Quaternary loess units at the eastern border of the Fergana Basin (Kyrgyzstan, Central Asia) makes this area particularly prone to the development of catastrophic loess earthflows ... [more ▼]

The presence of massive Quaternary loess units at the eastern border of the Fergana Basin (Kyrgyzstan, Central Asia) makes this area particularly prone to the development of catastrophic loess earthflows, causing damages and injuries almost every year. Efficient disaster management requires a good understanding of the main causes of these mass movements, that is, increased groundwater pressure and seismic shaking. This paper focuses on the Kainama earthflow, mainly composed of loess, which occurred in 2004 April. Its high velocity and the long run-out zone caused the destruction of 12 houses and the death of 33 people. In summer 2005, a field survey consisting of geophysical and seismological measurements was carried out along the adjacent slope. By combination and geostatistical analysis of these data, a reliable 3-D model of the geometry and properties of the subsurface layers, as shown in the first part of the paper, was created. The analysis of the seismological data allowed us to point out a correlation between the thickness of the loess cover and the measured resonance frequencies and associated amplification potential. The second part of this paper is focused on the study of the seismic response of the slope by numerical simulations, using a 2-D finite difference code named FLAC.Modelling of the seismic amplification potential along the slope confirmed the results obtained from the seismological survey—strong amplifications at the crest and bottom of the slope where there is a thick loess cover and almost no amplification in the middle part of the slope. Furthermore, dynamic slope stability analyses were conducted to assess the influence of local amplifications and increased groundwater pressures on the slope failure. The results of the dynamic modelling, although preliminary, show that a combination of seismic and hydrologic origin (pore pressure build-up during the seismic shaking) is the most probable scenario responsible for the 2004 failure. [less ▲]

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See detailGEOPHYSICAL INVESTIGATIONS OF SEISMICALLY INDUCED SURFACE EFFECTS: CASE STUDY OF A LANDSLIDE IN THE SUUSAMYR VALLEY, KYRGYZSTAN
Havenith, Hans-Balder ULg; Jongmans, D.; Abdrakhmatov, K. et al

in Surveys in Geophysics (2000), 21

In summer 1998, a geophysical survey including seismic profiles and electrical tomography has been carried out in the Suusamyr valley, Kyrgyzstan. The scope was to investigate surface effects induced by ... [more ▼]

In summer 1998, a geophysical survey including seismic profiles and electrical tomography has been carried out in the Suusamyr valley, Kyrgyzstan. The scope was to investigate surface effects induced by the Ms = 7.3 Suusamyr earthquake, the 19th of August, 1992. In this paper, special attention is paid to the case study of a debris slide triggered by the earthquake. Seismic data are analysed by P-wave refraction technique and by surface wave inversion. Electrical tomographic profiles are processed by 2D-inversion. Using geotechnical and geological information, P-velocity models and resistivity sections are interpreted in terms of geological materials, in order to build a geological 3D model. On the basis of the latter, we carried out static finite element computations as well as static and pseudo-static calculations with Janbu’s method. Newmark displacement was computed, considering or not the influence of the shallow soft deposits. The results are compared to the real displacement observed in the field and conclusions are drawn about the mechanism of the landslide. [less ▲]

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See detailGeophysical monitoring of a thermal response test
Daoudi, Moubarak; Hermans, Thomas ULg; Vandenbohede, Alexander et al

Conference (2011, October 20)

Geothermal energy systems efficiency relies on the quantification of the thermal properties of the ground. The estimation of those properties can be done by means of thermal response tests (TRT) or thanks ... [more ▼]

Geothermal energy systems efficiency relies on the quantification of the thermal properties of the ground. The estimation of those properties can be done by means of thermal response tests (TRT) or thanks to values found in literature tables; the first approach provides relatively limited information since the measures are done in boreholes and the values from general tables can present a lack of accuracy. Geophysical methods might be useful in order to yield additional information for thermal properties estimation on higher investigation scales. In this study, electrical resistivity tomography (ERT) is used to monitor the evolution of heated water injected into an unconfined aquifer. Both heat injection and storage were monitored during a total period of 17 days. The principal conclusion is that the use of ERT is efficient to monitor the progressive temperature increase and decrease in the aquifer due to the presence of the heated plume. The resistivity distributions calculated by inversion were compared with electromagnetic borehole measurements and were consistent in the case of the storage phase, whereas a higher gap remained between measured and calculated resistivities for the injection phase. A conversion of the calculated resistivities into temperature values enabled a validation with temperature logs for the storage phase, while the temperature deduced for the injection profiles were too low. Those differences are justified by the fact that different error models were used to proceed to the imaging of the heat injection and storage. This work demonstrates the ability of ERT to monitor geothermal experiment in shallow aquifer. Keywords: geothermal energy, electrical resistivity, electromagnetic method, heat transfer, inversion process, thermal response test. [less ▲]

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See detailThe geophysical prospecting methods applied in engineering geology
Dassargues, Alain ULg

Scientific conference (1989, October)

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See detailGeophysical records in recent (Cenozoic) and ancient (Devonian – Palaeozoic) carbonate mound systems.
Foubert, Aneleen; Da Silva, Anne-Christine ULg; De MolB, Ben et al

in Book of Abstracts - 18th ISC Congress (2010, October 28)

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See detailGEOPHYSICAL SEARCHES FOR THREE NEUTRINO OSCILLATIONS
Cudell, Jean-René ULg; Gaisser, T. K.

in 19th Int. Cosmic Ray Conf. (1985)

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See detailGeophysical validation of MIPAS-ENVISAT operational ozone data
Cortesi, U.; Lambert, J. C.; De Clercq, C. et al

in Atmospheric Chemistry and Physics (2007), 7(18), 4807-4867

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), on-board the European ENVIronmental SATellite (ENVISAT) launched on 1 March 2002, is a middle infrared Fourier Transform spectrometer ... [more ▼]

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), on-board the European ENVIronmental SATellite (ENVISAT) launched on 1 March 2002, is a middle infrared Fourier Transform spectrometer measuring the atmospheric emission spectrum in limb sounding geometry. The instrument is capable to retrieve the vertical distribution of temperature and trace gases, aiming at the study of climate and atmospheric chemistry and dynamics, and at applications to data assimilation and weather forecasting. MIPAS operated in its standard observation mode for approximately two years, from July 2002 to March 2004, with scans performed at nominal spectral resolution of 0.025 cm(-1) and covering the altitude range from the mesosphere to the upper troposphere with relatively high vertical resolution (about 3 km in the stratosphere). Only reduced spectral resolution measurements have been performed subsequently. MIPAS data were re-processed by ESA using updated versions of the Instrument Processing Facility (IPF v4.61 and v4.62) and provided a complete set of level-2 operational products (geo-located vertical profiles of temperature and volume mixing ratio of H2O, O-3, HNO3, CH4, N2O and NO2) with quasi continuous and global coverage in the period of MIPAS full spectral resolution mission. In this paper, we report a detailed description of the validation of MIPAS-ENVISAT operational ozone data, that was based on the comparison between MIPAS v4.61 (and, to a lesser extent, v4.62) O-3 VMR profiles and a comprehensive set of correlative data, including observations from ozone sondes, ground-based lidar, FTIR and microwave radiometers, remote-sensing and in situ instruments on-board stratospheric aircraft and balloons, concurrent satellite sensors and ozone fields assimilated by the European Center for Medium-range Weather Forecasting. A coordinated effort was carried out, using common criteria for the selection of individual validation data sets, and similar methods for the comparisons. This enabled merging the individual results from a variety of independent reference measurements of proven quality (i.e. well characterized error budget) into an overall evaluation of MIPAS O-3 data quality, having both statistical strength and the widest spatial and temporal coverage. Collocated measurements from ozone sondes and ground-based lidar and microwave radiometers of the Network for the Detection Atmospheric Composition Change (NDACC) were selected to carry out comparisons with time series of MIPAS O-3 partial columns and to identify groups of stations and time periods with a uniform pattern of ozone differences, that were subsequently used for a vertically resolved statistical analysis. The results of the comparison are classified according to synoptic and regional systems and to altitude intervals, showing a generally good agreement within the comparison error bars in the upper and middle stratosphere. Significant differences emerge in the lower stratosphere and are only partly explained by the larger contributions of horizontal and vertical smoothing differences and of collocation errors to the total uncertainty. Further results obtained from a purely statistical analysis of the same data set from NDACC ground-based lidar stations, as well as from additional ozone soundings at middle latitudes and from NDACC ground-based FTIR measurements, confirm the validity of MIPAS O-3 profiles down to the lower stratosphere, with evidence of larger discrepancies at the lowest altitudes. The validation against O-3 VMR profiles using collocated observations performed by other satellite sensors (SAGE II, POAM III, ODIN-SMR, ACE-FTS, HALOE, GOME) and ECMWF assimilated ozone fields leads to consistent results, that are to a great extent compatible with those obtained from the comparison with ground-based measurements. Excellent agreement in the full vertical range of the comparison is shown with respect to collocated ozone data from stratospheric aircraft and balloon instruments, that was mostly obtained in very good spatial and temporal coincidence with MIPAS scans. This might suggest that the larger differences observed in the upper troposphere and lowermost stratosphere with respect to collocated ground-based and satellite O-3 data are only partly due to a degradation of MIPAS data quality. They should be rather largely ascribed to the natural variability of these altitude regions and to other components of the comparison errors. By combining the results of this large number of validation data sets we derived a general assessment of MIPAS v4.61 and v4.62 ozone data quality. A clear indication of the validity of MIPAS O-3 vertical profiles is obtained for most of the stratosphere, where the mean relative difference with the individual correlative data sets is always lower than +/- 10%. Furthermore, these differences always fall within the combined systematic error (from 1 hPa to 50 hPa) and the standard deviation is fully consistent with the random error of the comparison (from 1 hPa to similar to 30-40 hPa). A degradation in the quality of the agreement is generally observed in the lower stratosphere and upper troposphere, with biases up to 25% at 100 hPa and standard deviation of the global mean differences up to three times larger than the combined random error in the range 50-100 hPa. The larger differences observed at the bottom end of MIPAS retrieved profiles can be associated, as already noticed, to the effects of stronger atmospheric gradients in the UTLS that are perceived differently by the various measurement techniques. However, further components that may degrade the results of the comparison at lower altitudes can be identified as potentially including cloud contamination, which is likely not to have been fully filtered using the current settings of the MIPAS cloud detection algorithm, and in the linear approximation of the forward model that was used for the a priori estimate of systematic error components. The latter, when affecting systematic contributions with a random variability over the spatial and temporal scales of global averages, might result in an underestimation of the random error of the comparison and add up to other error sources, such as the possible underestimates of the p and T error propagation based on the assumption of a 1 K and 2% uncertainties, respectively, on MIPAS temperature and pressure retrievals. At pressure lower than 1 hPa, only a small fraction of the selected validation data set provides correlative ozone data of adequate quality and it is difficult to derive quantitative conclusions about the performance of MIPAS O-3 retrieval for the topmost layers. [less ▲]

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See detailGeophysical Validation of SCIAMACHY NO2 Vertical Columns: Overview of Early 2004 Results
Lambert, Jean-Christopher; Blumenstock, Thomas; Boersma, F. et al

in Proceedings of the Second Workshop on the Atmospheric Chemistry Validation of ENVISAT (ACVE-2), ESA-ESRIN, Frascati, Italy, 3-7 May 2004 (ESA SP-562, August 2004) ESC01JL1 (2004, August)

Following the recommendations drawn after the Commissioning Phase of the ENVISAT satellite in 2002, SCIAMACHY near real time data processors were upgraded to version 5.01 in early 2004. Before public ... [more ▼]

Following the recommendations drawn after the Commissioning Phase of the ENVISAT satellite in 2002, SCIAMACHY near real time data processors were upgraded to version 5.01 in early 2004. Before public release of the new SCIAMACHY nitrogen dioxide (NO2) vertical column data product, several validation teams investigated its improvement and assessed its geophysical consistency by means of correlative studies involving NDSC-affiliated ground-based networks of DOAS UV-visible and FTIR spectrometers and the ERS-2 GOME satellite. In parallel, preliminary SCIAMACHY NO2 column data products generated by research processors under development at scientific institutes were also tested, using the same correlative data and validation procedures. Digesting the results obtained by a list of validation teams and SCIAMACHY data processing teams, this overview paper draws a preliminary quality assessment of the SCIAMACHY NO2 column data sets available in spring 2004. [less ▲]

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See detailGéopolitique du Changement Climatique
Gemenne, François ULg

Book published by Armand Colin (2009)

LE CHANGEMENT CLIMATIQUE EST AUJOURD’HUI DEVENU UN SUJET DE POLITIQUE INTERNATIONALE dont les enjeux dépassent largement la seule question écologique pour englober l’ensemble des équilibres mondiaux, et ... [more ▼]

LE CHANGEMENT CLIMATIQUE EST AUJOURD’HUI DEVENU UN SUJET DE POLITIQUE INTERNATIONALE dont les enjeux dépassent largement la seule question écologique pour englober l’ensemble des équilibres mondiaux, et notamment des rapports Nord-Sud. Le présent ouvrage, solidement référencé et assorti d’une dizaine de cartes en couleurs, examine cette dimension géopolitique du réchauffement global. Quels sont les pays responsables ou réputés tels, quels sont ceux qui en subiront l’impact le plus fortement ? Quels sont les déplacements de populations engagés ou à prévoir ? Les risques pour la sécurité internationale ? L’auteur présente également les différents mécanismes de coopération internationale mis en place, et fait le point sur l’état actuel des négociations. Nous sommes en train d’entrer dans la phase véritablement critique, voire potentiellement dramatique de la question. Ce livre permet à chacun d’en comprendre les clés et les ressorts. [less ▲]

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See detailGeorges ADINS His Story and Series
Sacré, Robert ULg

Article for general public (1983)

Detailed reference viewed: 26 (3 ULg)
See detailGEORGES SIMENON - Comprendre et ne pas juger
Sacré, Robert ULg

in Télé Moustique (1983)

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See detailGEORGES ADINS - roadrunner van de Blues
Sacré, Robert ULg

in BLOCK (1984), (49), 17-23

Interview Georges Adins + Photos ( G. Adins)

Detailed reference viewed: 9 (1 ULg)