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The epigenetic uranium deposits in the eastern part of the Athabasca Basin are classified as unconformity-related ore deposits. Their explicit spatial association to reactivated basement faults is observed within the regional structural NNE trend Wollaston-Mudjatik transition zone, marked by elongated dravite, illite, and chlorite alteration zones. Accordingly, geochemical studies have advocated a circulation and focalization of large amount of one or more fluids to carry and precipitate aqueous chemical materials. At the deposit-scale, the uranium deposits are found mainly at the intersection between two or more fault sets, and described as elongated-like bodies varying in orientation from E-W to NNE direction along the regional transitional zone. Furthermore, some orebodies show a change of orientation and dip of their structures. Thus, what is the hydro-mechanical response of reactivated and inherited fault architecture (e.g., intersection zone) under different stress states (e.g., reverse, strike-slip, and normal faulting regime), and its potential contribution to the shape and orientation of orebodies at deposit scale?Using hydro-mechanical numerical modeling, this project demonstrates the role that fault intersections play in controlling mineralized fluids by examining the various fluid flow patterns observed when reactivated intersected faults are under various stress states. Numerical modeling is performed using 3-Dimensional Distinct Element Code (3DEC). The numerical models are subdivided into two categories: 1) simplified 3-D models of two intersecting faults, 2) 3-D complex models of fault network at different deposits sites (e.g., the Cigar Lake deposit). While the first simple models attempt to evaluate the effects of intersection angle, burial depth, fluid pressure, basin permeability and stress states on the fluid flow patterns; the second models investigate the stress state under which certain orebodies may have formed.Our preliminary results from simplified models show that at defined intersection angles, the fluid flow deviates from the main fault toward the secondary fault at their intersection point. The deviation in fluid flow is referred to the value of intersection angle at which the shear stress varies along the secondary fault, leading to the opening of secondary fault. Additionally, the burial depth does not affect the flow along the basement faults, whereas, the overlying highly permeable basin reduces the horizontal flow along the basement faults toward the intersection zone, and reorients a part of the flow toward the basin. In the complex models (the Cigar Lake model), considering a compressional regime, the E-W fault set is reactivated once the maximum stress is oriented N40W to N65W, which is in agreement with field observations.

International audience; The deposits of meandering fluvial successions constitute important hosts for natural resources including hydrocarbons, water and ore deposits. Their successful exploitation requires knowledge of both the architecture of the preserved meander-belt depositional elements and their internal facies composition. However, these features are complex as meandering fluvial systems undertake a variety of types of channel migration including expansion, translation, rotation and combinations thereof. Such behaviour results in the generation and accumulation of bar deposits that are characterised by a range of external geometries and internal lithofacies compositions, giving rise to considerable stratigraphic complexity in the depositional record. Typically, point-bar and channel-fill deposits are preserved as fragments of abandoned meanders. Attempts to reconstruct the evolution and migratory behaviour of formative river courses therefore requires careful analysis and reconstructive modelling before predicting the arrangement of preserved facies. This study uses inverse and forward numerical stratigraphic modelling approaches (ChaRMigS and PB-SAND), statistically informed by a database (FAKTS) that records facies architectures from 290 analogue studies of fluvial systems, to predict three-dimensional internal facies distributions within preserved fluvial meander belts and associated point-bar elements. ChaRMigS is an inverse modelling method that uses the plan-view shape of remnant abandoned meander loops and the last active channel path as inputs, to propose multiple possible geometries of the formative river courses back through a series of reverse time steps. Stochastic modelling outputs consist of different scenarios of temporal steps that record channel and point-bar evolution, including episodes of loop abandonment due to neck or chute cutoff. In contrast, PB-SAND is a forward model that reconstructs point-bar facies architecture based on channel trajectories in plan-view at key time steps. It then uses this information to predict the internal anatomy of resultant channel and point-bar deposits, and the associated lithofacies distributions. This model is constrained by reference to examples documented from outcrop data, seismic images, or relationships derived from a quantitative sedimentological database (FAKTS). PB-SAND returns stochastic realizations of 3D facies distributions in meander-belt deposits.

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International audience; Lhoist Industries Ltd. plan to deepen the Boverie Quarry (Rochefort, Belgium), in which two Frasnian limestone reefs are exploited. If this project is allowed, the dewatering will dry up the Tridaine gallery that provides drinking water for the city and Saint-Remy's Abbey of Rochefort. Therefore, the project of deepening has to conciliate both quarry dewatering and drinking water production. The solution found by the quarry owner is to achieve the dewatering using three wells, for technical safety or water quality reasons, located in the periphery of the quarry and to distribute the pumped water to both the city and abbey. Hydrogeological characterization and a model using the SUFT3D research code tend to show that quarry dewatering and drinking water production could be achieved during and after the quarry operation, even by using a single well.

International audience; La géomodélisation, représentation numérique 3D d'objets géologiques, est née à la fin des années 1980 avec l'apparition des premiers géomodeleurs. D'abord développée pour ses applications dans les industries pétrolière et minière, la géomodélisation a profité de l'essor du numérique et des avancées théoriques des années 90 et 2000 pour se démocratiser. Aujourd'hui, de nombreux logiciels commerciaux existent et tous les domaines d'application des géosciences sont concernés. Néanmoins, à l'heure du « one-click », les logiciels proposés sont souvent des « boites noires » pour leurs utilisateurs, alors que tous n'utilisent pas les mêmes méthodes. Dans ce domaine en perpétuelle évolution, comment démystifier les technologies et former les étudiants à produire des modèles de qualité ? Comment les aider à garder un oeil critique sur un modèle qui n'est par définition qu'une approximation de la réalité ? C'est avec ces préoccupations que nous avons mis en place l'enseignement de la géomodélisation à l'Ecole Nationale Supérieure de Géologie dès le début des années 2000. D'abord enseigné dans un format classique « Cours-TD » alternant théorie et pratique, l'organisation du module a évolué vers une forme de pédagogie par projet qui s'inscrit notamment dans la continuité d'un stage de cartographie de terrain. En revenant sur plus de dix années de pratique, cette présentation résume et commente les choix de compétences, d'outils techniques et de méthodes pédagogiques effectués ainsi que leurs évolutions. Ce retour d'expérience se veut aussi comme un moyen d'échanger autour des pratiques pédagogiques en géologie numérique.

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International audience; Despite intensive explorations by speleologists, karstic systems remain only partially described as many conduits are not accessible to humans. Paleokarsts are buried karstic systems with a significant reservoir potential but they are not easily identifiable on seismic images, which leads to a huge uncertainty on the corresponding underground flow simulations. Stochastic simulations of karstic networks allow to better assess that uncertainty but only if the simulated networks are comparable to those observed in the field. One way to ensure some realism is to reproduce the topologies and geometries of explored networks, as proposed in various recent works. The height and width of the conduits are directly measured by speleologists. The general width-height ratio of different networks was already studied (Pardo-Iguzquiza et al., 2011; Jouves et al., 2017), but no generic study seems to have be done on their spatial repartition. It leads to a lack of constraints when simulating conduit dimensions (sections or volumes) during modeling, which is, however, a crucial parameter for further flow simulations. In this study, we analyze a set of different explored karstic systems to check if there are typical distributions of height, width and section of conduits within the networks. For the analysis, the karstic network is considered as a graph, whose nodes are the measurement stations while the edges correspond to lines-of-sight. Instead of directly working on the height and width, we chose to work on the average radius of the conduits. The distributions tend to be close to log-normal and the range of variation is rather similar from a network to another, both for the width-height ratio and the average radius. A direct correlation between these metrics and the degree of the nodes (i.e., the number of neighbors) also seems to exist. Besides, we propose a new method to spatially hierarchize the networks and check the consistency of usual approximations made when performing flow simulations. Even so, no relation between the point location inside the network and the studied metrics was observed. The spatial continuity of the conduit geometrical properties was also characterized by the use of 1-dimensional curvilinear variograms inside the whole network and its different branches. This method is more adapted to the case of karstic networks than the use of variogramms based on Cartesian coordinates. Based on these results, we propose a method to stochastically simulate the average radius and width-heigth ratio of karstic conduits. These simulated average radiuses can be directly taken into account when running flow simulations on explicit conduit networks (SWMM, Epanet…), while the WH ratios allow keeping information about the form of the conduits.

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La modélisation numérique des réservoirs souterrains est un outil essentiel à la compréhension de leur organisation, de leur fonctionnement et à la prévision des impacts des activités anthropiques sur leur évolution. Certains réservoirs se caractérisent par des géométries complexes qui vont directement influencer la connectivité hydrodynamique en leur sein, et plus largement l'hétérogénéité des propriétés du milieu et tous les processus physiques qui s'y déroulent. C'est le cas des bassins miniers ennoyés, des karsts, des chenaux, ou des dépôts salifères. L'objectif de mes travaux est de définir de nouvelles méthodologies de modélisation numérique adaptées à ces réservoirs à géométrie complexe et ce, en fonction de l'échelle spatiale et du processus physique d'intérêt. Dans une première partie, l'accent est mis sur le lien entre la stratégie de modélisation retenue et le phénomène étudié lorsque deux processus principaux, ou plus, entrent en jeu. Ceci est principalement illustré à travers des travaux menés sur la gestion de l'après-mine lorrain. La deuxième partie est consacrée à l'analyse et à la caractérisation des objets géologiques à géométrie complexe, et notamment des systèmes karstiques et chenalisés. La troisième partie s'intéresse aux méthodes de simulation stochastiques de ces milieux : l'objectif n'est pas de trouver la solution unique, mais d'identifier une/des solutions honorant les données disponibles et rendant compte de la réalité, de sa complexité et des incertitudes qui lui sont associées. Le dernier chapitre dresse enfin une rapide conclusion et présente les perspectives de ces travaux à court et moyen termes.

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International audience; The existence of double-peaked breakthrough curves (BTC), which are the result of the transport of a dye tracer through underground lakes, is reported. Investigations were undertaken on the Furfooz karst sys- tem in southern Belgium. In this system, the River Lesse sinks partially into a swallow hole. The water follows a solitary conduit leading to an underground lake that is directly connected to a second underground lake. Double-peaked BTCs were detected in the resurgent wa- ter, downstream of this second lake. The report first de- scribes field data (tracer tests in various hydrologic con- ditions) which point towards the double peak being linked to a nonlinear process that originates within the lakes. Complementary investigations within the lakes show a complex behavior of the dye tracer related to a specific hydrodynamic feature that leads to the separation of the solute plume. A conceptual model of the solute transport within the lakes is proposed. This model emphasizes the physical effect of the lakes on the dye flow-through process.Note : A correction of this paper has also been published in Sep 2019 and can be downloaded here as supplementary material

International audience; Karstic systems are geological structures that strongly impact underground flows. Despite intensive explorations by speleologists, they remain partially described as many conduits are not accessible to humans. Paleokarsts are buried karstic systems with a significant reservoir potential. But they are not easily identifiable on seismic images. In those contexts, a huge uncertainty subsists on the network location and the conduit geometry. Stochastic simulations help to better assess that uncertainty. The difficulty is to reproduce the system connectivity at different scales while integrating as much geological knowledge as possible without involving poorly constrained parameters (e.g. paleo-climate, boundary conditions...). In this paper we propose to work on two aspects and scales of karstic systems. At large scale, we stochastically simulate karst network skeletons with a new method based on a formal grammar, the Lindenmayer-system. Based on an alphabet, an axiom and user-defined rules, the method puts together segments to build the network skeleton. The definition of proper rules and the introduction of karst-dedicated parameters generate curves reproducing the complex architectures encountered in those systems, mixing branchwork and anastomotic patterns. At the conduit scale, we propose to build a 3D envelope around these skeletons with an enhanced Object Distance based Simulation Method. It uses a custom distance field from the skeleton which takes into account geological features influencing karstogenesis (horizons, faults or fractures). This controls the first-order shape of the conduits. It is then combined to a custom random threshold controlling finer-scale features of the conduits. This threshold is generated with several parameter values depending on the involved geological structures. This workflow is demonstrated on a synthetic case, showing the potentialities of the approach at both scales. Results are encouraging and various improvements are in focus. Data conditioning, both to karst observations and local shape information has to be enhanced. The network simulation has the advantage to be grid-free, meaning that no background grid is needed to perform the simulation. Thus, it avoids the stair-step effect that can be observed in other techniques. On the opposite, the method used to simulate the conduit shapes relies on a grid, necessary to compute the distance fields and to perform the threshold geostatistical simulation. For detailed conduit geometry, the grid requires a high resolution, which impacts directly the computational efficiency. Finally, it would be interesting to test the approach on a real dataset and to develop a coupling with a flow simulator to evaluate the impact of the shape and of the network connections on the flow response.

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