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Signification physique et hydrologique de l'information spectrale contenue dans le signal hydrodynamique à l'exutoire des systèmes karstiques.

Abstract : Globally, major socio-economic challenges lie on karst systems, both for drinking water supply to populations and for deposits exploitation. Karst systems are defined, by De Marsily (1984), as environments in which "heterogeneity reaches its paroxysm": the physical patterns characterizing karst domains are particularly varied and the hydrodynamic responses are undeniably complex and non-linear. Today, it is still not possible to precisely define the heterogeneities organization of these aquifers although many studies have been carried out on their overall functioning. These studies are based on the only information generally available on these systems: precipitation on the studied watershed, flow rates recorded at the outlet, well-located monitoring of water levels, concentrations or turbidity, and local estimated hydrodynamic properties such as the hydraulic conductivity of the aquifer or the storage coefficient. In recent years, several works have shown the interest of applying correlative and spectral analyzes on time series recorded on karst systems in order to interpret them hydro(geo)logical functioning. This thesis work explored the potential of this hydrological signal analysis methods as an aid to the interpretation and inference of physical and hydrogeological characteristics (karst networks geometry, exchanges between conduit networks and the surrounding porous matrix, recharge mode including diffuse and point-source one). This study was based on a coupling approach of direct modeling flows through synthetic karst systems, and the statistical and spectral processing of these simulated signals. The modeled domains were built according to different complexity degrees: from very simple arbitrary cases to complex cases corresponding to realistic systems. The flows in these synthetic networks were simulated with the physics-based hydrogeological model developed by the BRGM, MARTHE (Thiéry, 2015), and more particularly thanks to the “Drains-Conduits” package allowing to couple matrix, karst conduits and exchanges between these two entities. The employed signal processing methods compared the statistical and spectral properties of a climatic signal (precipitations) with those of the simulated discharges. They also give an understanding of how these properties vary according to changes in the domains hydraulic and physical characteristics. Even if the auto- and cross-correlation functions may exhibit almost similar behaviors (i.e. similar “memory effects” or decorrelation times, for different karst networks or matrix/conduit exchange properties), subtle but statistically significant differences allow the distinction between the karstification degree of the modeled domains. The scalar behaviors, and more particularly the different ranges of time scale invariance, can be used to distinguish the models in the spectral domain. These spectral properties reflect the possible filtering of the input signal by the considered hydrosystem, and therefore express different flow kinetics. Using discrete wavelet decomposition methods ultimately allows to reconstruct the hydrodynamic variability associated with these flow kinetics, completing thus a statistical decomposition of the hydrograph at the outlet.
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Adeline Dufoyer. Signification physique et hydrologique de l'information spectrale contenue dans le signal hydrodynamique à l'exutoire des systèmes karstiques.. Sciences de la Terre. Normandie Université, 2019. Français. ⟨NNT : 2019NORMR147⟩. ⟨tel-02976892⟩

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