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See detailInverse soil parameter identification of an earth and rockfill dam by genetic algorithm optimization
Vahdati, Pooya; Levasseur, Séverine ULg; Mattsson, Hans et al

in Sharing Experience for Safe and Sustainable Water Storage (2013)

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See detailInverse star, borders, and palstars
Rampersad, Narad ULg; Shallit, Jeffrey; Wang, Ming-wei

in Information Processing Letters (2011), 111

A language L is closed if L = L*. We consider an operation on closed languages, L-*, that is an inverse to Kleene closure. It is known that if L is closed and regular, then L-* is also regular. We show ... [more ▼]

A language L is closed if L = L*. We consider an operation on closed languages, L-*, that is an inverse to Kleene closure. It is known that if L is closed and regular, then L-* is also regular. We show that the analogous result fails to hold for the context-free languages. Along the way we find a new relationship between the unbordered words and the prime palstars of Knuth, Morris, and Pratt. We use this relationship to enumerate the prime palstars, and we prove that neither the language of all unbordered words nor the language of all prime palstars is context-free. [less ▲]

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See detailInverse vegetation modelling : a tool to reconstruct palaeoclimates under changed CO2 conditions
Guiot, J.; Torre, F.; Jolly, D. et al

in Ecological Modelling (2000), 127

Atmospheric CO2 concentration has greatly fluctuated during the Quaternary. These variations have influenced the vegetation changes. The assumption that the relationship vegetation–climate sensu stricto ... [more ▼]

Atmospheric CO2 concentration has greatly fluctuated during the Quaternary. These variations have influenced the vegetation changes. The assumption that the relationship vegetation–climate sensu stricto was constant through time should be reconsidered taking into account the impact of the atmospheric CO2 content on the plants. Here we propose to use a process-based vegetation model (BIOME3) in an inverse mode to reconstruct from pollen data the most probable climate under precipitation seasonality change and under lowered CO2 concentration in the biosphere. Appropriate tools to match the model outputs with the pollen data are developed to generate a probability distribution associated with the reconstruction (Monte Carlo sampling and neural network techniques). The method is validated with modern pollen samples from Greece and Italy: it proves to be able to reconstruct modern climate with a more or less large error bar from pollen data. The error bar depends in fact on the tolerance of the vegetation to the corresponding climatic variable. The application to six pollen assemblages from Greece and Italy, representing the last glacial maximum (LGM: 18 000 14C-year B.P.), is done into three experiments: (1) modern CO2 concentration; (2) LGM CO2 concentration; (3) LGM CO2 concentration and high winter precipitation. The latter experiment is motivated by evidence of high lake-levels in Greece during the LGM which has been attributed to winter rainfall. These experiments show that winter was ca. 15–20°C colder than the present, in agreement with previous climate reconstruction. The apparent discrepancy between the high lake-levels and the steppe vegetation during the LGM, can be explained by an increase of the winter precipitation (which leads to high lake level) while the summer season is mild and dry (which affects the vegetation). The summer temperature has three stable states (−16°C, −10°C, −2°C), but the warmest one is the most probable if we take into account the lowered CO2 and the high lake-levels. [less ▲]

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See detailInversion et caractérisation de profils de constituants atmosphériques fluorés à partir de spectres infrarouges enregistrés au Jungfraujoch
Duchatelet, Pierre ULg

Master of advanced studies dissertation (2005)

La composition de l’atmosphère terrestre évolue, suite à l’accumulation d’un nombre croissant de constituants gazeux chimiquement et radiativement actifs, émis au sol par les activités humaines. Ces ... [more ▼]

La composition de l’atmosphère terrestre évolue, suite à l’accumulation d’un nombre croissant de constituants gazeux chimiquement et radiativement actifs, émis au sol par les activités humaines. Ces émissions croissantes influencent l’ozone atmosphérique, l’équilibre radiatif de la Terre (et dès lors son climat) et modifient le niveau d’oxydation de son atmosphère. Une meilleure connaissance de la composition atmosphérique et une compréhension approfondie des processus physiques et chimiques qui prévalent dans la troposphère et la stratosphère sont essentielles pour établir l’état actuel de notre environnement, pour en comprendre les changements récents et en cours, et pour en prédire l’évolution future. Elle est également indispensable pour fournir une solide base scientifique aux mesures régulatoires de pratiques industrielles et humaines, liées aux Protocoles de Montréal et de Kyoto. L’acquisition de cette connaissance nécessite des mesures troposphériques et stratosphériques, basées notamment sur des observations récurrentes à partir du sol. A moyenne latitude, ces observations sont réalisées par l’Université de Liège à la Station Scientifique Internationale du Jungfraujoch (ISSJ, 46.55°N, 8.98°E, 3580m d’altitude, Alpes suisses) à l’aide de deux spectromètres par transformée de Fourier à haute résolution opérant dans l’infrarouge. Les observations FTIR systématiques de l’atmosphère terrestre au Jungfraujoch ont débuté dès 1985. Il a en effet été démontré que la continuation ininterrompue de mesures à long terme joue un rôle primordial dans la recherche sur les changements atmosphériques globaux focalisés sur la chimie et la dynamique de l’atmosphère et le climat terrestre. Les données ainsi recueillies au Jungfraujoch par l’Université de Liège se sont déjà révélées extrêmement utiles pour l’étude de la variabilité et de l’évolution à long terme d’un grand nombre d’espèces. La base de données constituée au fil des années d’observation s’exprime pour chaque constituant en termes de colonne verticale totale, déduite à partir de l’algorithme SFIT-1. Cependant, depuis le début des années 2000, un nouveau code, SFIT-2, permet, en plus d’obtenir la colonne verticale de l’espèce cible, de déduire son profil de distribution verticale. La détermination de colonnes partielles a également été rendue possible. De cet essor, s’en est accompagné le développement de bon nombre d’outils de caractérisation de profils FTIR et de formalismes de comparaison entre distributions verticales issues d’instruments différents. L’analyse du contenu en information et l’analyse d’erreur des profils inversés constituent les principaux outils permettant de caractériser les données FTIR. L’un des objectifs premiers du présent travail est par conséquent de fournir un aperçu des possibilités liées à l’algorithme d’inversion SFIT-2 ainsi qu’à ses outils de caractérisation. Ces outils, décrits en détail dans ce travail, ont été appliqués à l’inversion et à la caractérisation des distributions verticales de deux gaz atmosphériques, HF et COF2 (Chapitre 4). Ces gaz ont été choisis car ils représentent les deux principaux réservoirs de fluor stratosphérique dont l’importance permet de quantifier indirectement l’impact de l’activité humaine sur l’érosion de la couche d’ozone. Les profils ainsi déduits ont également été comparés aux mesures réalisées par le satellite canadien ACE lors de ses passages au-dessus du Jungfraujoch. Auparavant, après un bref aperçu des propriétés physiques et chimiques de l’atmosphère, nous évoquons toute la problématique des phénomènes d’effet de serre et d’érosion de couche d’ozone, dans lesquels les constituants fluorés sont directement ou indirectement impliqués (Chapitre 1). Nous décrivons ensuite les réactions qui régissent la chimie du fluor atmosphérique pour montrer comment les réservoirs prennent naissance à partir des gaz sources émis en surface. L’importance de ces réservoirs ainsi que leur évolution sont également largement abordées (Chapitre 2). Après avoir évoqué les notions fondamentales liées à la spectrométrie par transformée de Fourier, nous décrivons les instruments et les bases de données observationnelles qui leur sont associées (Chapitre 3) et qui nous ont aidés à produire les résultats géophysiques présentés dans la dernière partie de ce travail. [less ▲]

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See detailInversion of a part of the numerator relationship matrix using pedigree information
Faux, Pierre ULg; Gengler, Nicolas ULg

in Genetics, Selection, Evolution (2013), 45

Background. In recent theoretical developments, the information available (e.g. genotypes) divides the original population into two groups: animals with this information (selected animals) and animals ... [more ▼]

Background. In recent theoretical developments, the information available (e.g. genotypes) divides the original population into two groups: animals with this information (selected animals) and animals without this information (excluded animals). These developments require inversion of the part of the pedigree-based numerator relationship matrix that describes the genetic covariance between selected animals (A22). Our main objective was to propose and evaluate methodology that takes advantage of any potential sparsity in the inverse of A22 in order to reduce the computing time required for its inversion. This potential sparsity is brought out by searching the pedigree for dependencies between the selected animals. Jointly, we expected distant ancestors to provide relationship ties that increase the density of matrix A22 but that their effect on A22i might be minor. This hypothesis was also tested. Methods. The inverse of A22 can be computed from the inverse of the triangular factor (T-1 ) obtained by Cholesky root-free decomposition of A22 . We propose an algorithm that sets up the sparsity pattern of T-1 using pedigree information. This algorithm provides positions of the elements of T-1 worth to be computed (i.e. different from zero). A recursive computation of A22i is then achieved with or without information on the sparsity pattern and time required for each computation was recorded. For three numbers of selected animals (4000; 8000 and 12 000), A22 was computed using different pedigree extractions and the closeness of the resulting A22i to the inverse computed using the fully extracted pedigree was measured by an appropriate norm. Results. The use of prior information on the sparsity of T-1 decreased the computing time for inversion by a factor of 1.73 on average. Computational issues and practical uses of the different algorithms were discussed. Cases involving more than 12 000 selected animals were considered. Inclusion of 10 generations was determined to be sufficient when computing A22. Conclusions. Depending on the size and structure of the selected sub-population, gains in time to compute A22 are possible and these gains may increase as the number of selected animals increases. Given the sequential nature of most computational steps, the proposed algorithm can benefit from optimization and may be convenient for genomic evaluations. [less ▲]

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See detailInversion of asteroid lightcurves: determination of the pole orientation.
Hainaut, O.; Detal, Alain ULg; Surdej, Jean ULg

in 3. International Symposium on Asteroids, Comets, Meteors, Abstr. 56 (1989, February 01)

Not Available

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See detailInversion of multi-temporal geoelectrical data sets: insights from several case studies
Nguyen, Frédéric ULg; Kemna, Andreas; Robert, Tanguy ULg et al

Conference (2011, December)

Time-lapse inversion of geoelectrical data is increasingly growing as remote monitoring systems are being used in more applications such as seawater intrusions, landslides, bioremediation of contaminated ... [more ▼]

Time-lapse inversion of geoelectrical data is increasingly growing as remote monitoring systems are being used in more applications such as seawater intrusions, landslides, bioremediation of contaminated sites, landfill operations, shallow geothermal systems, or water resources. To date, several inversion strategies exist for taking into account the temporal dimension of the data. Among the most used ones are the independent inversion of multi-temporal data sets, the difference inversion, the temporally-constrained inversion, and the more recent process-based inversion. The success of a particular time-lapse inversion scheme depends on the validity of several assumptions made by these inversion schemes. Difference inversion schemes generally assume that part of the noise contained in the data cancels out when working with temporal data differences. Process-based inversion requires a more advanced knowledge of the system prior the inversion. Temporally-constrained inversion on the other hand assumes that the changes are localized and minor. We show in this paper using data sets with different time and spatial scales, and with different degrees of geological complexity and resistivity contrasts, that the particular success of a time-lapse inversion scheme is highly dependent on the temporal behaviour of the noise estimation in the time-lapse data set and of the model-dependent resolution pattern of the survey. We attempt to provide guidelines for successful quantitative interpretation of time-lapse data sets whenever possible. [less ▲]

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See detailInversion of multi-temporal geoelectrical field data sets: insights on noise characterization and regularization
Nguyen, Frédéric ULg; Kemna, Andreas; Robert, Tanguy ULg et al

Poster (2012, July 11)

Inversion of geoelectrical time-lapse data sets is increasingly growing as monitoring systems are being used in more applications such as seawater intrusion, landslides, remediation of contaminated sites ... [more ▼]

Inversion of geoelectrical time-lapse data sets is increasingly growing as monitoring systems are being used in more applications such as seawater intrusion, landslides, remediation of contaminated sites, landfill operation, shallow geothermal systems, or management of water resources. To date, several inversion strategies exist for taking into account the temporal dimension of the data. The most used nowadays are the independent inversion of multi-temporal data sets, the difference inversion, the temporally-constrained inversion, and the more recent process-based inversion. However, difference inversion schemes generally assume that part of the noise contained in the data cancels out when working with temporal data differences. Temporally-constrained inversion on the other hand assumes that the changes are localized and minor. Process-based inversion requires a more advanced knowledge of the system prior the inversion. In this study we demonstrate that the resolution of the time-lapse inversion scheme is mostly dependent on the quantification of the temporal behavior of the data error, on the resolution of the model-dependent pattern of the survey, and not on the regularization strategy. Our study is based on the imaging results of different data sets with different time and spatial scales, and with different degrees of geological complexity and resistivity contrast, The considered sites are a shallow sandy aquifer and a fractured hard rock aquifer where tracer experiments were performed and monitored using surface arrays. The two studied transport processes are advection, with velocities on the order of 10 m/hour and slower advection/diffusion processes. The strongest improvements were brought by using the data difference and a quantitative estimation of the data error. We found in particular a dependence of the time-lapse data error to the measured resistance (i.e., signal-to-noise-ratio), permitting to formulate an error model to describe the data error present in time-lapse data sets. We used minimum gradient support regularization to invert for model changes with enhanced contrast and found this technique more suited to time-lapse studies than for static images. Noise characterization and error models appear therefore as essential and the most impacting for a successful inversion both for static and time-lapse data whereas different spatio-temporal regularization techniques allowed to decrease artefacts but needs to be coherent with the process. [less ▲]

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See detailInversion of probabilistic models of structures using measured transfer functions
Arnst, Maarten ULg

Doctoral thesis (2007)

The aim of this thesis is to develop a methodology for the experimental identification of probabilistic models for the dynamical behaviour of structures. The inversion of probabilistic structural models ... [more ▼]

The aim of this thesis is to develop a methodology for the experimental identification of probabilistic models for the dynamical behaviour of structures. The inversion of probabilistic structural models with minimal parameterization, introduced by Soize, from measured transfer functions is in particular considered. It is first shown that the classical methods of estimation from the theory of mathematical statistics, such as the method of maximum likelihood, are not well-adapted to formulate and solve this inverse problem. In particular, numerical difficulties and conceptual problems due to model misspecification are shown to prohibit the application of the classical methods. The inversion of probabilistic structural models is then formulated alternatively as the minimization, with respect to the parameters to be identified, of an objective function measuring a distance between the experimental data and the probabilistic model. Two principles of construction for the definition of this distance are proposed, based on either the loglikelihood function, or the relative entropy. The limitation of the distance to low-order marginal laws is demonstrated to allow to circumvent the aforementioned difficulties. The methodology is applied to examples featuring simulated data and to a civil and environmental engineering case history featuring real experimental data. [less ▲]

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See detailInversion of probabilistic structural models using measured transfer functions
Arnst, Maarten ULg; Clouteau, Didier; Bonnet, Marc

in Computer Methods in Applied Mechanics & Engineering (2008), 197(6-8), 589-608

This paper addresses the inversion of probabilistic models for the dynamical behaviour of structures using experimental data sets of measured frequency-domain transfer functions. The inversion is ... [more ▼]

This paper addresses the inversion of probabilistic models for the dynamical behaviour of structures using experimental data sets of measured frequency-domain transfer functions. The inversion is formulated as the minimization, with respect to the unknown parameters to be identified, of an objective function that measures a distance between the data and the model. Two such distances are proposed, based on either the loglikelihood function, or the relative entropy. As a comprehensive example, a probabilistic model for the dynamical behaviour of a slender beam is inverted using simulated data. The methodology is then applied to a civil and environmental engineering case history involving the identification of a probabilistic model for ground-borne vibrations from real experimental data. [less ▲]

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See detailInversion of the Biot-Savart Law: an approach based on discrete sine and cosine transforms
Dirickx, Michel; Denis, Samuel ULg; Vanderheyden, Benoît ULg et al

in Johansen, T. H.; Shantsev, D. V. (Eds.) Magneto-Optical Imaging, Proceedings of the NATO Advanced Research Workshop, held in Øystese, Norway, 28-30 August 2003 (2004)

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See detailInversion of Venus NO nightglow limb profiles
Stiepen, Arnaud ULg; Soret, Lauriane ULg; Gérard, Jean-Claude ULg et al

Conference (2012, July)

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See detailInversions Estrangeres Directes i Innovació a Catalunya
Artige, Lionel ULg; Nicolini, Rosella

in Busom, Isabel (Ed.) La situació de la innovació a Catalunya (2006)

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See detailLes invertébrés des carrières souterraines de craie de la Montagne Saint-Pierre (Province de Liège, Belgique). Note préliminaire
Dethier, Michel; Willems, Luc ULg

in Notes Fauniques de Gembloux (2005), 57

During the 1930’s, Leruth collected 145 species in the underground chalk quarries at the Montagne Saint-Pierre. None was troglobitic. To evaluate the biological interest of the sites and the faunal drift ... [more ▼]

During the 1930’s, Leruth collected 145 species in the underground chalk quarries at the Montagne Saint-Pierre. None was troglobitic. To evaluate the biological interest of the sites and the faunal drift 70 years after, we have undertaken in 2004 a new faunistic survey. [less ▲]

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See detailInverted patterns of genetic diversity in continental and island populations of the heather Erica scoparia s.l.
Desamore, Aurélie ULg; Laenen, Benjamin ULg; González-Mancebo, JM et al

in Journal of Biogeography (2012), 39(3), 574--584

Aim  Using the heather Erica scoparia s.l. as a model, this paper aims to test theoretical predictions that island populations are genetically less diverse than continental ones and to determine the ... [more ▼]

Aim  Using the heather Erica scoparia s.l. as a model, this paper aims to test theoretical predictions that island populations are genetically less diverse than continental ones and to determine the extent to which island and continental populations are connected by pollen- and seed-mediated gene flow.Location  Macaronesia, Mediterranean, Atlantic fringe of Europe.Methods  Patterns of genetic diversity are described based on variation at two chloroplast DNA (cpDNA) loci and one nuclear DNA (nDNA) locus for 109 accessions across the entire distribution range of the species. Global patterns of genetic differentiation were investigated using principal coordinates analysis. Genetic differentiation between island and continental areas, estimations of pollen- and seed-mediated gene flow, and the presence of phylogeographical signal were assessed by means of Fst/NST (continental scale) and Fij/Nij (local scale). Extant and past distribution ranges of the species were inferred from niche modelling using layers describing present and Last Glacial Maximum (LGM) macroclimatic conditions.Results  The Azores exhibited a significantly higher genetic diversity than the continent. The lowest levels of genetic differentiation were observed between the Azores and the western Mediterranean, and the diversity observed in the Azores resulted from at least two colonization waves. Within the Azores, kinship coefficients showed a significant and much steeper decrease with geographical distance in the cpDNA than in the nDNA. The distribution predicted by LGM models was markedly different from the current potential distribution, particularly in western Europe, where no suitable areas were predicted by LGM models, and along the Atlantic coast of the African continent, where LGM models predicted highly suitable climatic conditions.Main conclusions  The higher diversity observed in Azorean than in continental populations is inconsistent with MacArthur and Wilson’s equilibrium model and derived theoretical population genetic expectations. This inverted pattern may be the result of extinction on the continent coupled with multiple island colonization events and subsequent allopatric diversification and lineage hybridization in the Azores. The results highlight the role of allopatric diversification in explaining diversification on islands and suggest that this process has played a much more significant role in shaping Azorean biodiversity than previously thought. [less ▲]

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See detailInverting Hydraulic Heads In An Alluvial Aquifer Constrained With Electrical Resistivity Tomography Data Through Multiple-Point Statistics And Probability Perturbation Method: A Case Study
Hermans, Thomas ULg; Scheidt, Celine; Caers, Jef et al

Conference (2014, July)

Solving spatial inverse problems in the Earth Sciences remains a considerable challenge given the large number of parameters to invert for, the non-linearity of forward models and as a result the ill ... [more ▼]

Solving spatial inverse problems in the Earth Sciences remains a considerable challenge given the large number of parameters to invert for, the non-linearity of forward models and as a result the ill-posedness of the problem. Geostatistics is therefore needed to specify prior models, more particularly, information to control the spatial features of the inverse solutions. We used multiple-point statistics (MPS) to build models of pre-defined hydrofacies: clay, sand and gravel facies constrained to geological data (hard data) and geophysical data (soft data). The electrical resistivity tomography method was chosen to bring relevant spatially distributed information on the presence of the facies, given its sensitivity to variations in lithology and porosity. The comparison of the geophysical signature of the deposits with direct observations in boreholes enables to derive the conditional probability of observing a facies given its electrical resistivity. This is used to produce probability maps for each facies and constrain stochastic simulations of the alluvial aquifer. Then, the probability perturbation method (PPM) is used to integrate hydraulic heads data, using MPS to generate models. This process enables us to obtain calibrated models of the aquifer. The PPM algorithm will automatically seek solutions fitting both hydrogeological data and training-image based geostatistical constraints. Only geometrical features of the model are affected by the perturbation, i.e. we do not attempt to directly find the optimal value of hydrogeological parameters (chosen a priori), but the optimal spatial distribution of facies whose prior distribution is quantified in a training image. The methodology is first tested with a synthetic benchmark. The tests performed show that the choice of the training image is a major source of uncertainty. Therefore, one first needs to select those training images consistent with the geophysical data (and hence reject the inconsistent ones). Then, we proceed with them to hydrogeological inversions. Geophysical data (soft constraints) acts as an accelerator of convergence by reducing prior uncertainty. The hydraulic conductivity of each facies is a sensitive parameter, but it can be easily optimized prior to the PPM process. The stochastic method is then successfully applied within the context of an alluvial aquifer submitted to a pumping experiment. We show how the integration of various sources of data (borehole logs, geophysics, hydraulic heads) aids in calibrating hydrogeological models, locating high hydraulic conductivity zones and reducing uncertainty. The developed methodology proposes a common framework (multiple-point statistics) to integrate various information sources with variable resolutions relevant for hydrogeology: geological, geophysical and hydrogeological data. The method can be extended to integrate tracer tests to enable the calibration of transport parameters as well. The originality of the method is to use geophysical data both to refine the choice of the training image and to constrain the inversion of hydrogeological models. [less ▲]

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See detailA Investigacao Sociologica os "pragamatic" americanos
Stassart, Pierre M ULg

Scientific conference (2011)

Detailed reference viewed: 12 (0 ULg)
See detailLa investigación experimental en educación
De Landsheere, Gilbert ULg

Book published by UNESCO (1982)

Detailed reference viewed: 78 (3 ULg)
See detailLa investigación pedagógica
de Landsheere, Gilbert ULg; Mialaret, Gaston

Book published by Angel Estrada - 1ère éd. (1971)

Detailed reference viewed: 10 (0 ULg)
See detailInvestigación y Desarrollo más Innovación
Artige, Lionel ULg; Nicolini, Rosella

Book published by Consejo Económico y Social de España (2008)

Detailed reference viewed: 30 (5 ULg)