ANALYSE DES PROGRAMMES D’ACTIONS PARIS ÉLABORÉS PAR LES GESTIONNAIRES DE COURS D’EAU POUR LA PÉRIODE 2022-2027

En application de nouvelles dispositions du Code de l’Eau entrées en vigueur fin 2018, les gestionnaires de cours d’eau publics wallons ont élaboré des programmes d’actions détaillés (PARIS) pour la période 2022-2027, à l’échelle d’unités de gestion physiquement homogènes appelées « secteurs ». Ces PARIS ont pour vocation de contribuer à une gestion intégrée, durable et coordonnée des cours d’eau, ainsi qu’à la prise en compte de leurs quatre fonctions : hydraulique, écologique, économique et socioculturelle. L'objectif de cet article est d'analyser ces PARIS, c'est-à-dire les enjeux identifiés, les objectifs de gestion fixés ainsi que les travaux planifiés par les gestionnaires pour chacun des 6 254 secteurs du réseau hydrographique. Le premier enseignement est le taux d’adhésion élevé des gestionnaires de cours d’eau, d'autant plus remarquable qu’il s’agit d’une nouvelle approche, impliquant l’utilisation d’un nouvel outil informatique. Il ressort également que les enjeux les plus fréquemment encodés sont les enjeux inondation et biodiversité, très souvent combinés l’un avec l’autre au sein des mêmes secteurs. Les enjeux socioculturel et économique viennent loin derrière. L’analyse révèle une distribution équilibrée entre travaux visant à lutter contre les inondation et ceux visant à préserver ou à restaurer la qualité écologique des cours d’eau. La moitié des secteurs se sont vu planifier comme unique mesure « visite et surveillance », car ils ne nécessitent pas de gestion active. Enfin, la variabilité des enjeux, objectifs de gestion et mesures définis sur les secteurs a pu être mise en évidence à l’échelle des différents sous-bassins hydrographiques

Subsurface imaging and characterization using full-wave inversion of near-field GPR data

Non-destructive characterization of the structural and physical properties of soils and materials has remained a major challenge in the scientific community. In that respect, ground-penetrating radar (GPR) has progressively become a key technology. The ever increasing computational performances available together with recent theoretical advances have made full-wave modeling and inversion a rational choice nowadays. Through full-wave inversion, information retrieval capabilities are inherently maximized. Approaching the radar antenna to the medium permits deeper characterization and higher resolution but also increases the complexity of the modeling problem. In this thesis, we further analyzed and advanced the processing of near-field GPR data. In particular, the impact of the number of frequencies and multi-offset antenna configurations on multilayered media reconstruction was investigated using objective function analyses. Focus was then dedicated to the development, optimization and validation of new radar data processing strategies. We introduced a novel numerical approach to remove antenna effects from the radar data, including near-field interactions with the medium, and, thereby, to provide much better radar images. This approach inherently normalizes the measurements, which makes straightforward the fusion of multi-frequency radar data for improved resolution. Finally, the near-field antenna effects removal and inverse modeling procedures were integrated in a GPR-based methodology to help detecting leaks in water distribution networks. This thesis opens new research and application avenues of GPR in civil and environmental engineering, e.g., for road inspection, among others.(AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 201

Fusion of Multifrequency GPR Data Freed From Antenna Effects

Several data fusion approaches have been developed to optimize both resolution and characterization depth for multifrequency ground-penetrating radar (GPR). In this study, we propose a novel physically basedmethod tomerge radar data coming from antennas operating in different frequency ranges. The strategy relies on the removal of the source and antenna effects from GPR data and the subsequent fusion of the resulting signals, which are now normalized, in the frequency domain. The approach used to filter out antenna effects resorts to an intrinsic, closed-form solution of Maxwell’s equations to describe the radar-antenna-medium system. We validated the multifrequency GPR data fusion approach through laboratory experiments with measurements performed in far- and near-field conditions above a copper plane and pipes buried at different depths in a sandbox. The results demonstrated the benefit of filtering the frequencydependent antennas effects before data fusion. Enhanced radargrams were subsequently obtained as a result of the broadening of the spectral bandwidth. This physically based fusion approach appears to be very promising to improve subsurface imaging

Estimation de la position de la phase pure de PCE impactant la qualité de la nappe souterraine du captage 1 de l'UCL à Louvain-la-Neuve

En 2001, des analyses furent réalisées sur les eaux des puits de captage creusés par l’UCL entre 1971 et 1979 (PC1, PC2, PC3 et PC4). On y a mesuré des concentrations en tétrachloroéthylène (PCE) s’élevant à respectivement 290 μg/l et 5,1 μg/l dans le PC1 et PC3. Ce produit anthropique, également appelé perchloroéthylène, est un liquide utilisé comme solvant ou intermédiaire dans l’industrie chimique et appartient à la catégorie des liquides denses nonaqueux (DNAPL). Sa solubilité relativement faible et sa grande densité lui permettent de s’accumuler au fond des aquifères. En outre, il possède une mobilité non-négligeable dans le sol et une faible capacité d’adsorption. La concentration retrouvée au niveau du PC1 est bien supérieure à la norme indiquée par la directive 98/83/CE relative à la qualité des eaux destinées à la consommation, soit 10 μg/l pour la somme du tri- et du tétrachloroéthylène (Andres, 2012). Suite à cette découverte, les eaux provenant du PC1 furent isolées du reste du réseau. Pour comprendre le devenir de cette pollution et évaluer son évolution potentielle en vue d’un assainissement potentiel, une étude de simulation du panache de pollution dans la partie non-saturée de la formation géologiqe, à partir d’une source virtuelle, a été réalisée. Cette étude a été réalisée grâce au similateur multiphasique "DNAPL simulator". Le rapport explique la paramétrisation du modèle pour les sols du site des captages de l’UCL et les résultats de la simulation de la dispersion du panache de la pollution dans la partie non-saturée du sol, pour quelques scénarios potentiels de pollution du site

Full-Wave Removal of Internal Antenna Effects and Antenna-Medium Interactions for Improved Ground-Penetrating Radar Imaging

Antenna effects alter the detection of buried objects during ground-penetrating radar (GPR) surveys. In this paper, we propose a novel approach based on full-wave inversion to filter out antenna effects from GPR data. The approach, which is exact for locally planar layered media, resorts to a recently developed electromagnetic model that takes advantage of an intrinsic, closed-form solution of Maxwell’s equations to describe the antenna–medium system. As any multilayered medium can be reduced to a half-space medium with effective, frequency-dependent, global reflection coefficients, the method consists in inverting the radar data to retrieve a frequencydependent, half-space complex conductivity. Converted into the time domain, this quantity represents the filtered radar image. We validated the approach through numerical simulations and laboratory experiments with pipes buried in a sandbox. The results demonstrated the validity of the concept and showed that the filtered radar images include only medium reflections, which means an easier interpretation in terms of medium structures. Antenna radiation pattern effects are, however, not removed. This physically based approach including the full-wave antenna model appears to be very promising for improved subsurface imaging and provides the basis for multifrequency GPR data fusion as the source is inherently normalized

Nouvelles avancées scientifiques dans le domaine de l'évaluation de la pollution des eaux souterraines, cas des études réalisées en Belgique

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Multi-frequency GPR data fusion

Several data fusion approaches have been developed to maximize both resolution and characterization depth for GPR. In this study, we propose a new physicallybased method to merge multi-frequency radar data coming from different antennas. The strategy relies on the removal of antenna effects and the subsequent fusion of the resulting Green’s functions in the frequency domain. We validated the approach in far-field conditions using two different antennas operating in the ranges 0.8-4.4 GHz and 2.6-6.2 GHz, respectively. We performed radar measurements above a sandbox wherein a number of objects were buried. The physically-based data fusion provided enhanced radargrams due to the broadening of the spectral bandwidth. Future research will focus on near-field conditions, for which the removal of antenna effects should be performed numerically

Fundamental Analyses on Layered Media Reconstruction Using GPR and Full-Wave Inversion in Near-Field Conditions

Multifrequency and multioffset ground-penetrating radar data acquisition modes are used to maximize the information content and parameter retrieval capabilities. However, they also increase the computational cost dedicated to the inversion procedure. In this paper, the impact of the number of frequencies and the multistatic configurations on the information retrieval capabilities is investigated through the response surface topographies of the objective functions.We resort to a full-wave-inversion procedure and a recently developed electromagnetic model which takes advantage of a closed-form solution of Maxwell’s équations to describe the antenna–medium system. We show with numerical and laboratory experiments the possibility of reducing the number of frequencies from several hundreds to one or several tens of components without affecting the information retrieval capabilities. We also show through several scenarios that the presence of a perfect electrical conductor increases the number of frequencies required to ensure an acceptable retrieval of the subsurface properties whereas the conductivity of the first layer and the relative permittivity of the second layer do not affect it. The results highlight that information content analyses are important in order to study and optimize data acquisition and inversion procedures, and thereby the computation time

Information content in frequency-dependent, multi-offset GPR data for layered media reconstruction using full-wave inversion

Water lost through leaks can represent high percentages of the total production in water supply systems and constitutes an important issue. Leak detection can be tackled with various techniques such as the ground-penetrating radar (GPR). Based on this technology, various procedures have been elaborated to characterize a leak and its evolution. In this study, we focus on a new full-wave radar modelling approach for near-field conditions, which takes into account the antenna effects as well as the interactions between the antenna(s) and the medium through frequency-dependent global transmission and reflection coefficients. This approach is applied to layered media for which 3-D Green’s functions can be calculated. The model allows for a quantitative estimation of the properties of multilayered media by using full-wave inversion. This method, however, proves to be limited to provide users with an on-demand assessment as it is generally computationally demanding and time consuming, depending on the medium configuration as well as the number of unknown parameters to retrieve. In that respect, we propose two leads in order to enhance the parameter retrieval step. The first one consists in analyzing the impact of the reduction of the number of frequencies on the information content. For both numerical and laboratory experiments, this operation has been achieved by investigating the response surface topography of objective functions arising from the comparison between measured and modelled data. The second one involves the numerical implementation of multistatic antenna configurations with constant and variable offsets in the model. These two kinds of analyses are then combined in numerical experiments. To perform the numerical analyses, synthetic Green’s functions were simulated for different multilayered medium configurations. The results show that an antenna offset increase leads to an improvement in the response surface topography, which is more or less marked according to the initial information content. It also highlights the theoretical possibility of significantly reducing the number of frequencies without degrading the information content. This last statement is confirmed with the laboratory experiment which incorporates measurements done with a Vivaldi antenna above a medium composed of one or more sand layers characterized by different water contents. As a conclusion, the offset and frequency analyses highlight the great potential of the model for improving the soil parameter retrieval while reducing the computation time for a given antenna(s) - medium configuration. Acknowledgments: This work benefited from networking activities carried out within the EU funded COST Action TU1208 "Civil Engineering Applications of Ground Penetrating Radar" and was supported by the Walloon Region through the "SENSPORT" project (Convention n◦1217720) undertook in the framework of the WBGreen research program

Impact of the antenna offset and the number of frequencies on layered media reconstruction using full-wave inversion in near-field conditions

Pipes and water leaks can potentially be detected using ground-penetrating radar (GPR). Recently, a new generalized model was developed to quantify the medium electrical properties by full-wave inverse modelling. It takes advantage of a closed form solution of Maxwell’s equations for describing the coupled antenna-medium system. In this study, we used that model to evaluate the inverse problem for different radar configurations and various multilayered medium layouts. Numerical experiments were performed by generating and inverting synthetic Green’s functions in order to show the influence of the antenna offset on the response surface topography of the objective functions. The impact of a reduced number of frequencies on these objective functions was also investigated for a monostatic configuration before combining this approach with a multioffset configuration. The results showed an improvement in the response surface topography when the number of antennas was increased. However, the importance of this enhancement varied according to the number of frequencies taken into account. It also pointed out the possibility of reducing the number of frequencies used for the inversion while preserving the information content. This work highlights the great potential of the model to improve the medium parameter retrieval while reducing the computation time