The understanding of the effect of overpressure and anisotropic in-situ stress on elastic properties is important for the interpretation of 4D reflection seismic data with respect to stress, production effect and pore content. Reservoir rocks and cap rocks of most siliclastic hydrocarbon reservoirs are composed of sedimentary rocks ranging from pure sandstones over siltstones to shale. Recently gas shales have attracted increasing interest due to their potential economic value. Most of these reservoir rocks are seismically anisotropic. Only a few publications exist, which investigate experimentally the elastic anisotropy of clay bearing rocks and shales under changing stress conditions. In particular the change of an initially transverse isotropic rock (e.g. shale) which turns into an orthorhombic one by a triaxial load is insufficiently understood. The aim of the project is to improve the estimation of pressure dependent velocity in sedimentary rocks, taking into account lithology (e.g. clay content) and pressure conditions. The possible deduction of stress conditions using the measured velocity anisotropy will also be covered. The project will include theoretical and phenomenological analysis of intrinsic and stress induced seismic anisotropy. In order to estimate and understand the pressure dependency of seismic velocity in various anisotropic sedimentary rocks we will apply and develop further the anisotropic piezosensitivity (also called porosity - deformation) approach. Here we will concentrate on the change of symmetry of anisotropic rocks due to axial loading. Special emphasis will be laid upon the reduction of free parameters for modelling and the physical interpretation of effective porosity and anellipticity. The verification and adjustment of the model will be supported by literature data and new laboratory measurements considering various sedimentary rocks ranging from pure sandstones over siltstones to shale. The proposed project will contribute to a better interpretation of reflection seismic data with respect to anisotropy and in situ stress conditions and lead to a better construction of velocity models for seismic imaging.