Full-azimuth subsurface angle domain wavefield decomposition and imaging Part I: Directional and reflection image gathers

Abstract

We present a new subsurface angle-domain seismic imaging system for generating and extracting high-resolution information about subsurface angle-dependent reflectivity. The system enables geophysicists to use all recorded seismic data in a continuous fashion directly in the subsurface local angle domain (LAD), resulting in two complementary, full-azimuth, common-image-angle gather systems: directional and reflection. The complete set of information from both types of angle gathers leads to accurate, high-resolution, reliable velocity model determination and reservoir characterization. The directional angle decomposition enables the implementation of specular and diffraction imaging in real 3D isotropic/anisotropic geological models, leading to simultaneous emphasis on continuous structural surfaces and discontinuous objects such as faults and small-scale fractures. Structural attributes at each subsurface point, e.g., dip, azimuth and continuity, can be derived directly from the directional anglegathers. The reflection-angle gathers display reflectivity as a function of the opening angle and opening azimuth. These gathers are most meaningful in the vicinity of actual local reflecting surfaces, where the reflection angles are measured with respect to the derived background specular direction. The reflection-angle gathers are used for automatic picking of full-azimuth angle-domain residual moveouts (RMO) which, together with the derived background orientations of the subsurface reflection horizons, provide a complete set of input data to isotropic/anisotropic tomography. The full-azimuth, angle-dependent amplitude variations are used for reliable and accurate amplitude versus angle and azimuth (AVAZ) analysis and reservoir characterization. The proposed system is most effective for imaging and analysis below complex structures, such as subsalt and subbasalt, high-velocity carbonate rocks, shallow low-velocity gas pockets, and others. In addition, it enables accurate azimuthal anisotropic imaging and analysis, providing optimal solutions for fracture detection and reservoir characterization.