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Apport des méthodes sismiques à l'hydrogéophysique : importance du rapport Vp/Vs et contribution des ondes de surface

Abstract : Characterisation and monitoring of groundwater resources and associated flow and transport processes mainly rely on the implementation of wells (piezometers). The interpretation of hydrogeological observations is however limited by the variety of scales at which these processes occur and by their variability in space and in time. In such a context, using geophysical methods often improves the very low spatial resolution of borehole data and limits their destructive nature. Among the geophysical tools applied to hydrogeology, seismic methods are commonly used at different scales. However, the seismic response in the context of aquifer characterisation remains complex. The interpretation of the estimated velocities is often difficult because of their variability depending on the aquifer lithology (intrinsic mechanical parameters and geometry of the constituting porous media, influence of the degree of saturation, etc). The permeability of the medium also affects the geometry of a hydrological reservoir whose contours may vary in space and in time, thus complicating the interpretation of seismic data. Geophysicists seek to overcome these limitations, especially through the joint study of compression (P-) and shear (S-) wave velocities (Vp and Vs), whose evolution is by definition highly decoupled in the presence of fluids. From a theoretical point of view, this approach proves suitable for the characterisation of aquifers, especially by estimating Vp/Vs or Poisson's ratio. The evaluation of these ratios can be systematically carried out with seismic refraction tomography using both P- and S-waves. However, retrieving Vs remains practically delicate because S-waves are usually difficult to generate and identify on seismic records. As an alternative, indirect estimation of Vs is commonly achieved thanks to surface-wave dispersion inversion, carried out from measurements of surface waves phase velocities contained in typical seismic records. Although it is usually proposed for the characterisation of 1D media, surface-wave prospecting can be deployed along linear sections in order to build 2D models of Vs distribution in the ground. A specific methodology has been developed for the combined and optimised exploitation of P- and surface waves present on single seismic records. When deployed on the field, this "P-wave" acquisition has been systematically followed by a "SH-wave" acquisition in order to compare Vs models obtained from surface-wave dispersion analysis and SH-wave refraction tomography. The use of this methodology in several geological and hydrogeological contexts allowed for estimating Vp/Vs ratio lateral and temporal variations in good agreement with a priori geological information and existing geophysical and piezometric data. Laser-based ultrasonic techniques were also proposed to put these processing techniques in practice on perfectly controlled physical models and study elastic wave propagation in partially saturated porous media.
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  • HAL Id : tel-01127635, version 1

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Sylvain Pasquet. Apport des méthodes sismiques à l'hydrogéophysique : importance du rapport Vp/Vs et contribution des ondes de surface. Sciences de la Terre. Université Pierre et Marie Curie - Paris VI, 2014. Français. ⟨NNT : 2014PA066495⟩. ⟨tel-01127635⟩

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