Development of a hydrocarbon reservoir requires information about the type of fluid that saturates the pore space, and the permeability distribution that determines how the fluid can be extracted. The presence of fractures in a reservoir can be useful for obtaining this information. The main objectives of this thesis are to investigate how fracturing can be detected remotely using exploration seismology. Fracturing will effect seismic data in a number of ways. Firstly, if the fractures are aligned preferentially in some direction, the medium will exhibit long wavelength anisotropy. In turn, if wave propagation is not aligned with one of the symmetry axes of the effective medium then shear wave splitting will depend upon the properties of the fracture filling fluid. Secondly, elastic waves will experience attenuation and dispersion due to scattering and wave-induced fluid flow between the fractures and matrix porosity. This occurs because the fractures are more compliant than the background medium and therefore there will be a pressure gradient formed during passage of the wave, causing fluid to flow between fractures and background. If the direction of shear-wave propagation is not perpendicular or parallel to the plane of fracturing, the wave polarized in the plane perpendicular to the fractures is a quasi-shear mode, and therefore the shear-wave splitting will be sensitive to the fluid bulk modulus. / The magnitude of this sensitivity depends upon the extent to which fluid pressure can equilibrate between pores and fractures during the period of the deformation. In this thesis I use the anisotropic Gassmann equations and existing formulations for the excess compliance due to fracturing to estimate the splitting of vertically propagating shear-waves as a function of the fluid modulus for a porous medium with a single set of dipping fractures and with two conjugate fracture sets dipping with opposite dips to the vertical. This is achieved using two alternative approaches. In the first approach it is assumed that the deformation taking place is quasi-static. That is, the frequency of the elastic disturbance is low enough to allow enough time for fluid to flow between both the fractures and the pore space throughout the medium. In the second approach I assume that the frequency is low enough to allow fluid flow between a fracture set and the surrounding pore space, but high enough so that there is not enough time during the period of the elastic disturbance for fluid flow between different fracture sets to occur. It is found that the second approach yields a much stronger dependency of shear-wave splitting on the fluid modulus than the first one. This is a consequence of the fact that at higher wave frequencies there is not enough time for fluid pressure to equilibrate and therefore the elastic properties of the fluid have a greater effect on the magnitude of the shear-wave splitting. I conclude that the dependency of the shear-wave splitting on the fluid bulk modulus will be at its minimum for quasi-static deformations, and will increase with increasing wave frequency. / In order to treat the problem of dispersion and attenuation due to wave-induced fluid flow I consider interaction of a normally incident time-harmonic longitudinal plane wave with a circular crack imbedded in a porous medium governed by Biot’s equations of dynamic poroelasticity. The problem is formulated in cylindrical coordinates as a system of dual integral equations for the Hankel transform of the wave field, which is then reduced to a single Fredholm integral equation of the second kind. It is found that the scattering that takes place is predominantly due to wave induced fluid flow between the pores and the crack. The scattering magnitude depends on the size of the crack relative to the slow wave wavelength and has its maximum value when they are of the same order. I conclude that this poroelastic effect should not be neglected, at least at seismic frequencies. Using the solution of the scattering problem for a single crack and multiple-scattering theory I estimate the attenuation and dispersion of elastic waves taking place in a porous medium containing a sparse distribution of such cracks. I obtain from this analysis an effective velocity which at low frequencies reduces to the known static Gassmann result and a characteristic attenuation peak at the frequency such that the crack size and the slow wave wavelength are of the same order. / When comparing with a similar model in which multiple scattering effects are neglected I and that there is agreement at high frequencies and discrepancies at low frequencies. I conclude that the interaction between cracks should not be neglected at low frequencies, even in the limit of weak crack density. Since the models only agree with each other at high frequencies, when the time available for fluid diffusion is small, I conclude that the interaction between cracks that takes place as a result of fluid diffusion is negligible at high frequencies. I also compare my results with a model for spherical inclusions and find that the attenuation for spherical inclusions has exactly the same dependence upon frequency, but a difference in magnitude that depends upon frequency. Since the attenuation curves are very close at low frequencies I conclude that the effective medium properties are not sensitive to the shape of an inclusion at wavelengths that are large compared to the inclusion size. However at frequencies such that the wavelength is comparable to or smaller than the inclusion size the effective properties are sensitive to the greater compliance of the flat cracks, and more attenuation occurs at a given frequency as a result.

La caractérisation de la proche surface est un enjeu majeur pour l'industrie pétrolière. Lors des acquisitions terrestres et Ocean Bottom Cable (OBC), les couches superficielles généralement altérées ou peu consolidées, présentent des structures géologiques complexes et ont éventuellement des variations topographiques importantes. Les ondes de surface, énergétiques, se propagent dans ce milieu complexe et dominent les sismogrammes, ce qui masque le signal utile pour le traitement sismique classique et rend difficile l'imagerie à la profondeur du réservoir.Il est donc important de pouvoir atténuer ces ondes, éventuellement d'appliquer des corrections statiques et/ou d'amplitude. Ceci qui nécessite une connaissance précise du modèle de vitesse de la proche surface. L'étude de la dispersion des ondes de surface est couramment utilisée en sismologie globale et à l'échelle géotechnique pour évaluer les propriétés des milieux terrestres. Il existe néanmoins des limitations: la mesure de cette dispersion est souvent difficile et les profils de vitesses obtenus sont 1D. A l'échelle pétrolière, l'hypothèse 1D n'est pas toujours adaptée, ce qui motive l'utilisation d'une méthode alternative d'imagerie plus haute résolution, la méthode d'inversion de la forme d'onde (FWI). Cependant, le modèle de vitesse initial doit être assez précis pour éviter le "cycle-skipping" et permettre la convergence vers la solution optimale.Cette étude explore différentes alternatives de fonctions coûts pour résoudre le "cycle-skipping" et diminuer la dépendance de l'inversion à la qualité du modèle initial. En exprimant les fonctions coûts dans le domaine f-k (fréquence-nombre d'onde) et le domaine f-p (fréquence-lenteur), la FWI est plus robuste. A l'aide d'exemples synthétiques, nous démontrons l'efficacité de ces nouvelles approches qui permettent bien de retrouver les variations latérales de vitesses d'onde S.Dans une seconde partie, nous développons une inversion FWI en "layer stripping", adaptée spécifiquement à la physique des ondes de surface. Comme la profondeur de pénétration de ces ondes dépend de leur longueur d'onde, et donc, de leur contenu fréquentiel, nous proposons d'inverser séquentiellement des plus hautes aux plus basses fréquences de ces ondes pour contraindre successivement les couches superficielles jusqu'aux plus profondes. Un fenêtrage selon la distance source-station est également appliqué. Dans un premier temps seules les courtes distances sont inversées, au fur à mesure les données associées à des plus grandes distances sont rajoutées, plus fortement impactées par le "cycle-skipping". Nous démontrons à l'aide d'exemples synthétiques l'avantage de cette méthode par rapport aux méthodes multi-échelles conventionnelles inversant des basses vers les hautes fréquences.Enfin, l'inversion des ondes de surface pour la caractérisation de la proche surface est confrontée à un cas réel. Nous discutons la construction et la pertinence du modèle initial et les difficultés rencontrées lors de l'inversion. / The characterization of the near surface is an important topic for the oil and gas industry. For land and Ocean Bottom Cable (OBC) acquisitions, weathered or unconsolidated top layers, prominent topography and complex shallow structures may make imaging at target depth very difficult. Energetic and complex surface waves often dominate such recordings, masking the signal and challenging conventional seismic processing. Static corrections and the painstaking removal of surface waves are required to obtain viable exploration information.Yet surface waves, which sample the near surface region, are considered as signal on both the engineering and geotechnical scale as well as the global seismology scale. Their dispersive property is conventionally used in surface wave analysis techniques to obtain local shear velocity depth profiles. But limitations such as the picking of dispersion curves and poor lateral resolution have lead to the proposal of Full Waveform Inversion (FWI) as an alternative high resolution technique. FWI can theoretically be used to explain the complete waveforms recoded in seismograms, but FWI with surface waves has its own set of challenges. A sufficiently accurate initial velocity model is required or otherwise cycle-skipping problems will prevent the inversion to converge.This study investigates alternative misfit functions that can overcome cycle-skipping and decrease the dependence on the initial model required. Computing the data-fitting in different domains such as the frequency-wavenumber (f-k) and frequency-slowness (f-p) domains is proposed for robust FWI, and successful results are achieved with a synthetic dataset, in retrieving lateral shear velocity variations.In the second part of this study a FWI layer stripping strategy, specifically adapted to the physics of surface waves is proposed. The penetration of surface waves is dependent on their wavelength, and therefore on their frequency. High-to-low frequency data is therefore sequentially inverted to update top-to-bottom layer depths of the shear velocity model. In addition, near-to-far offsets are considered to avoid cycle-skipping issues. Results with a synthetic dataset show that this strategy is more successful than conventional multiscale FWI in using surface waves to update the shear velocity model.Finally inversion of surface waves for near surface characterization is attempted on a real dataset at the oil and gas exploration scale. The construction of initial models and the difficulties encountered during FWI with real data are discussed.