Prise en compte d’une anisotropie réaliste dans l’imagerie sismique des zones complexes

devant le jury: Véronique FARRA ........................ Directrice de thèse Gilles LAMBARE ......................... Examinateur Jean-Paul MONTAGNER .................... Examinateur Laurence Nicolétis ..................... Co-directrice de thèse Ivan Psencik ........................... Raporteur Jean VIRIEUX ........................... Raporteur Résumé: In order to respond to world energy demand in the years to come and renew reserves, it is essential to support ongoing exploration efforts by developing new methods. In this way the zones with complex geological structures are one of the main exploration zones to study. To explore these zones and reduce the risk of dry wells we need to develop high performance methods through seismic imaging which could be also used for surveying the new exploration zones and re-evaluating the potential of oil fields already exploited. Anisotropy is an omnipresent property in the sedimentary layers which is mainly originated from fine stratification, effect of mineral particles and in-situ stress conditions. For the complex geological structures, the layers dip is largely variable which can lead the principal directions of sedimentary anisotropy to vary through the layers as a function of stratification. The main goal of this work is to provide a 3D modeling software which can be used for realistic layered anisotropic media in the complex tectonics area, to study the effects of such anisotropy on seismic imaging, and proposing some solutions for taking into account the anisotropy in the treatment procedure. For defining the anisotropic media, we consider here the special case of transverse isotropy (TI) with a spatially varying symmetry axis conformed with the structure (STI). The first chapter of this thesis is consecrated to a brief introduction to seismic anisotropy. Different types of seismic anisotropy and their geologic aspects are explained. Also the way we define the STI media is presented. In the second section we present the approximate ray tracing for qP waves in inhomogeneous layered STI media. All needed equations for calculating cinematic and dynamic parameters using first-order ray tracing are given in this chapter. All the quantities required for evaluation of the Green s function are calculated to first order, except the traveltime that is calculated to second order. In this chapter the accuracy of developed algorithm is verified on simple models with respect to exact ray-tracing results, then the method is applied to simple STI models and to a realistic overthrust model to study the cinematic and dynamic effects of STI media. The two next sections are consecrated to applications of developed algorithm to AVAZ study and tomography. The third chapter includes an application of our dynamic ray tracing algorithm to AVA analysis. The relative importance of several factors affecting the amplitude versus azimuth in a realistic 3D model is analyzed and the results obtained by using our dynamic ray tracing algorithm are compared with 1D analytical solutions which are current in the industry. Then we present the first attempts to STI inversion using our ray tracing algorithm. It is explained how we can calculate the partial derivatives of traveltimes with respect to the model parameters in TI media. Then the formulation used by the 3D reflection tomography software (called Jerry) developed by KIM research consortium (IFP) is presented, and the inversion method is applied to a synthetic STI example. Finally, the main points are summarized in the fifth chapter of thesis which is consecrated to conclusions and some perspectives to this study.