Résultat de la recherche
Probabilistic estimation of tunnel inflow from a karstic conduit network
When planning infrastructures such as tunnels in karstified formations, a risk assessment of groundwater inflow must be conducted. The aim of this paper is to present a workflow for the probabilistic estimation of the water inflow from karst conduits using a Monte-Carlo approach. The procedure involves three main steps. First, realistic stochastic karstic conduit network geometries are generated based on fracture and stratigraphic information using the Stochastic Karstic Simulation approach (SKS). To represent the geological uncertainty, different scenarios are considered. Then, a discrete–continuum numerical modeling approach is employed, allowing the flow calculation to account for the exchange between the matrix and the conduits as well as the transition between turbulent and laminar flow in the conduits. Because it is not known if and where (at which depths) the tunnel may hit a karst conduit, and what will be the pressure gradient in the system, different hydrogeological scenarios are considered in the uncertainty analysis phase including a randomized location of the tunnel, a range of possible pressure gradients, and a range of possible matrix permeability values. The final step consists of the statistical analysis of the results. The proposed workflow allows estimating the range of plausible inflows and studying how the inflows are related to the network geometry properties and to the hydrodynamic parameters of the aquifer. This method is illustrated in a simple synthetic but realistic case of a rather deep and confined karstic formation. In that situation, the results show that even if the pressure difference in the system and the matrix permeability value are important factors controlling the long-term inflow, the karstic conduit network geometry and connectivity also play a critical role in the determination of the potential discharge. Overall, this study demonstrates the possibility and advantages of using stochastic analysis in the early phases of project planning to predict possible long-term water inflow in tunnel after its construction.
Ice volume and basal topography estimation using geostatistical methods and GPR measurements: Application on the Tsanfleuron and Scex Rouge glacier, Swiss Alps
Ground Penetrating Radar (GPR) is nowadays widely used for determining glacier thickness. However, this method provides thickness data only along the acquisition lines and therefore interpolation has to be made between them. Depending on the interpolation strategy, calculated ice volumes can differ and can lack an accurate error estimation. Furthermore, glacial basal topography is often characterized by complex geomorphological features, which can be hard to reproduce using classical 5 interpolation methods, especially when the conditioning data are sparse or when the morphological features are too complex. This study investigates the applicability of multiple-point statistics (MPS) simulations to interpolate glacier bedrock topography using GPR measurements. In 2018, a dense GPR data set was acquired on the Tsanfleuron Glacier (Switzerland). The results obtained with the direct sampling MPS method are compared against those obtained with kriging and sequential Gaussian simulations (SGS) on both a synthetic data set – with known reference volume and bedrock topography – and the real data 10 underlying the Tsanfleuron glacier. Using the MPS modelled bedrock, the ice volume for the Scex Rouge and Tsanfleuron Glacier is estimated to be 113.9 ± 1.6 Miom3 . The direct sampling approach, unlike the SGS and the kriging, allowed not only an accurate volume estimation but also the generation of a set of realistic bedrock simulations. The complex karstic geomorphological features are reproduced, and can be used to significantly improve for example the precision of under-glacial flow estimation.
Hydro-geological modeling of the Roussillon aquifer : integrating geological knowledge uncertainties and geostatistical methods in groundwater modeling
Ce travail de thèse porte sur la modélisation géologique et hydrodynamique de l’aquifère du Roussillon, en mettant l’accent sur la transition d’un modèle géologique détaillé, utilisant la méthode géostatistique de simulation multipoint (MPS), vers des modèles hydrodynamiques. La première étape de ce travail de thèse a consisté à créer les enveloppes du modèle géologique 3D du Roussillon. Les principales unités géologiques de l’aquifère du Roussillon comprennent le Pliocène marin, le Pliocène continental et le Quaternaire. La compilation d’une base de données géologiques, composés de logs géophysiques et de ligne sismiques, a permis de comprendre les structures de l’aquifère et d’interpoler les surfaces 2D qui délimitent le modèle géologique 3D. Une fois les enveloppes interpolées, la seconde étape de modélisation de ce travail s’est concentrée sur la simulation des faciès sédimentaire composant l’aquifère du Pliocène Continental. L’utilisation de l’approche de simulation multipoint (MPS) a permis de créer des modèles réalistes de faciès sédimentaire dans l’unité du Pliocène continental, en reproduisant des structures alluviales à l’échelle régionale. En complément de la simulation MPS, deux autres modèles sédimentaires ont été créés. Le premier est déterministe et se base sur l’interprétation d’essais de pompage pour caractériser les propriétés physiques du Pliocène Continental. Le second utilise une approche géostatistique appelée simulations séquentielles d’indicateurs (SIS) pour générer les propriétés hydrodynamiques de l’aquifère. Cette seconde approche géostatistique est plus couramment utilisée que le MPS et est plus simple à mettre en œuvre. La troisième étape de ce travail consiste en la définition du modèle hydrodynamique de l’aquifère du Roussillon. Le modèle hydrodynamique a été réalisé en considérant les conditions aux limites, les budgets de prélèvement asso- ciés, les observations piézométriques disponibles, et a été pré-calibrer en régime d’écoulement permanent dans une première phase de modélisation. Les modèle d’écoulement ont été réalisés avec le logiciel MODFLOW 6. La dernière étape de modélisation consiste en la création de modèle d’écoulement en régime transitoire ainsi que dans la création d’une approche de calibration des paramètres physique du modèle MPS du Pliocène Continental. Un défi important de ce travail réside dans la conciliation des modèles géologiques avec les données hydrodynamiques, ce qui nécessite une approche spécifique pour garantir de préserver les structures sédimentaires simulées, lors du processus de calibration. Il convient de noter que peu d’études existent sur la calibration des modèles MPS régionaux, et que souvent, les processus de calibration ne prennent pas en compte les éléments structuraux géologiques. La comparaison des approches de modélisation sédimentologique, effectuée en régime d’écoulement permanent et transitoire, met en avant une homogénéité des résultats entre les différentes approches. Les résultats en régime permanent sont satisfaisants pour les trois approches, mais peine à reproduire certains signaux en régime transitoire. Les problèmes des modèles en régime transitoire sont probablement dus à un problème d’initialisation du système hydrodynamique et de calibration des conditions limites. Ce travail propose donc une comparaison d’approches de modélisation sédimentologique et de leur impact sur les simulations hydrodynamiques. Il met en évidence des améliorations potentielles pour le modèle hydrogéologique de l’aquifère du Roussillon. Des données d’observation plus fiables et des informations sédimentologiques supplémentaires sont fortement recommandées, en particulier dans les zones présentant des différences significatives par rapport aux niveaux d’eau simulés, afin d’améliorer le modèle hydrogéologique. Cela permettrait de mieux comprendre le fonctionnement du système et de faciliter les ajustements locaux du modèle sédimentologique et des conditions hydrodynamiques. Malgré les difficultés rencontrées, notamment concernant la reproduction de certain signal piézométrique lors des simulations en régime transitoire, cette étude contribue à la compréhension de l’état de l’aquifère en identifiant les principales sources d’incertitude dans le modèle actuel de l’aquifère du Roussillon. ABSTRACT The presented study focuses on the geological and hydrodynamic modeling of the Roussillon aquifer. Located in southern France, near the Mediterranean Sea, the Roussillon plain covers an area of over 800 km2 and serves as the most important source of fresh water for the local community, supporting various needs such as irrigation, drinking water, and industrial usage. This aquifer is situated in one of the driest regions of France. Additionally, the aquifer experiences heavy water abstraction, mainly for drinking and agricultural purposes, leading to a steady decline in its water level over the years. The region is also affected by climatic changes, including rising sea levels and potential disruptions in precipitation patterns, which further impact the aquifer’s water availability. Balancing water management and conservation in the face of increasing population and climate change poses significant challenges for the Roussillon aquifer. The primary aim of the thesis is to enhance the geological understanding of the Roussillon aquifer and develop a hydrodynamic model to gain deeper insights into the functioning of the aquifer system. Additionally, the study aimed to create a solid foundation for investigating the potential consequences of climate change on this essential regional resource. The geological model consists of three main units, starting with the deepest unit, the Marine Pliocene unit, followed by the Continental Pliocene unit, and finally at the top the Quaternary unit. The initial phase of this work involved compiling a comprehensive geological database using onshore and offshore data sets to develop a conceptual understanding of the aquifer’s structures and to interpolate the main 2D surfaces that separate the 3D geological model. Within the Continental Pliocene layer, four subintervals were defined, and the elevation map of the three surfaces dividing these subintervals was mapped and interpolated using geophysical logs and offshore seismic data. The geological data set, although limited in resolution and coverage, served as conditioning data for the geostatistical simulation of the Continental Pliocene layer. We then used the multiplepoint simulation approach (MPS) to simulate realistic lithofacies patterns representative of the sediment spatial distribution in the Continental Pliocene layer. The 3D model of the Continental Pliocene layer was created by stacking 2D simulations controlled by vertical conditioning sampling. The results demonstrated satisfactory reproduction of sedimentary structures at the regional scale. In addition to the MPS simulation, two other approaches, a depth related approach and a Sequential Indicator Simulation (SIS) set, were used to generate hydro physical property fields for the aquifer. The depth related approach is based on the interpretation of hydraulic pumping tests, to assign hydraulic conductivity values based on the cell’s depth in the grid. The SIS employed a variogram based algorithm to simulate simple lithofacies structures (more simple compared to the MPS models). These three sets of hydraulic conductivity and specific storage values are used to feed the hydrodynamic simulations and estimate the propagated uncertainty of the sedimentological models on the hydrodynamic simulations. This work then focuses on defining the conceptual hydrodynamical model of the Roussillon aquifer. We present the main boundary conditions, their associated budgets, available piezometric observations, and the main modeling assumptions, linked to the different components of the MODFLOW 6 hydrodynamic model. In the first modeling step, a steady state calibration is performed to calibrate river parameters and mean hydraulic conductivity of simulated facies with the goal of preserving the simulated lithofacies patterns while matching the hydrodynamic observations. Once calibrated, we used these parameters for transient hydrodynamic models over a 20 years period. The three model approaches are used and compared in this study. It appears that reproducing the piezometric transient observation series presented some difficulties, with the models failing to capture the main trend of the piezometric levels on some locations. The reproduction of these piezomet ric series suffered from limited data availability, simplified river systems, and uncertainties regarding the local hydraulic conductivity and specific storage parameters. To better reproduce the piezometric series, this work ends with a short study on the use of the ES-MDA approach to attempt local corrections of the hydraulic conductivity and specific storage parameters. These initial tests faced limitations, as many forward models failed to converge during the process, limiting the applicability of the calibration process. Overall, this work proposes a unique regional comparison of sedimentological modeling approaches and their influence on hydrodynamic simulations. It also identifies directions for improving the aquifer model’s performance. Obtaining more reliable observation data series, as well as more onshore sedimentological information, especially in areas with significant deviations from simulated water levels, is highly recommended for improving the Roussillon hydrogeological model. This would aid in better understanding the system’s behavior and facilitate localized modifications of the sedimentological model and the hydrodynamic conditions. Despite the challenges faced, the study contributes to understanding the aquifer’s transient state, emphasizing the importance of sedimentological models in hydrodynamic studies, and identifying major sources of uncertainty in the current model of the Roussillon aquifer.
3D multiple-point statistics simulations of the Roussillon Continental Pliocene aquifer using DeeSse
This study introduces a novel workflow to model the heterogeneity of complex aquifers using the multiplepoint statistics algorithm DeeSse. We illustrate the approach by modeling the Continental Pliocene layer of the Roussillon aquifer in the region of Perpignan (southern France). When few direct observations are available, statistical inference from field data is difficult if not impossible and traditional geostatistical approaches cannot be applied directly. By contrast, multiple-point statistics simulations can rely on one or several alternative conceptual geological models provided using training images (TIs). But since the spatial arrangement of geological structures is often non-stationary and complex, there is a need for methods that allow to describe and account for the non-stationarity in a simple but efficient manner. The main aim of this paper is therefore to propose a workflow, based on the direct sampling algorithm DeeSse, for these situations. The conceptual model is provided by the geologist as a 2D non-stationary training image in map view displaying the possible organization of the geological structures and their spatial evolution. To control the non-stationarity, a 3D trend map is obtained by solving numerically the diffusivity equation as a proxy to describe the spatial evolution of the sedimentary patterns, from the sources of the sediments to the outlet of the system. A 3D continuous rotation map is estimated from inferred paleoorientations of the fluvial system. Both trend and orientation maps are derived from geological insights gathered from outcrops and general knowledge of processes occurring in these types of sedimentary environments. Finally, the 3D model is obtained by stacking 2D simulations following the paleotopography of the aquifer. The vertical facies transition between successive 2D simulations is controlled partly by the borehole data used for conditioning and by a sampling strategy. This strategy accounts for vertical probability of transitions, which are derived from the borehole observations, and works by simulating a set of conditional data points from one layer to the next. This process allows us to bypass the creation of a 3D training image, which may be cumbersome, while honoring the observed vertical continuity.
Probabilistic prediction of karst water inflow during construction of underground structures
AbstractVarious methods have been developed in recent decades to predict hazards associated with karst voids in underground construction. Common to all these methods is that the predicted range of water inflow is often insufficient for the purpose of implementing the planned construction works. This is usually due to an incomplete knowledge of the karst conduit system within a project area, making it difficult to predict the position and characteristics of karst voids. The method presented in this paper permits a robust prediction of karst water inflow. It is based on a combination of stochastically generated, pseudo‐genetic karst conduit systems and hydraulic modelling of the hydrogeological conditions using a Monte Carlo approach. This approach facilitates a plausible estimation of the expected range of karst‐induced water inflows and also enables the probability of encountering a karst voids. to be determined. The predictions allow for differentiated treatment of the hazards associated with karst water during the construction and operation phase of underground structures. In concrete terms, this relates to the planning and implementation of exploratory measures and ground‐improvement measures, the design of the dewatering system and its monitoring during the construction and operation phase.