| Seismic migration based on isotropic downward continuation is an effective technique for imaging complex subsurface geologic structure. However, this approach has difficulty in imaging steep reflectors, such as faults and salt flanks, that are often important in hydrocarbon exploration. Waves illuminating steep reflectors usually have nearly horizontal or overturned raypaths. This thesis addresses two major challenges in propagating these types of waves using isotropic downward continuation methods. First, when anisotropy is present in the subsurface, the errors associated with propagating near-horizontal waves under the isotropic assumption become severe. Second, downward continuation methods using the one-way wave equation have difficulty propagating near-horizontal or overturned waves. In theory, both issues can be resolved by using anisotropic reverse-time migration, which uses the two-way wave-equation, but such schemes are significantly more computationally expensive.; I present a new one-way wave-equation based method that efficiently images steep reflectors in anisotropic media. First, I incorporate anisotropy in wave propagation by developing a new one-way wavefield extrapolation scheme, optimized finite-difference, for anisotropic media. The scheme is designed by fitting the dispersion relation with a rational function using the weighted least-squares methods, and it propagates waves accurately and efficiently in anisotropic media. Synthetic data examples show that with anisotropy taken into account, reflectors are positioned correctly with this scheme.; Second, I develop a new migration algorithm, plane-wave migration in tilted coordinates. The recorded surface data are transformed by slant stacking to a collection of plane-wave source datasets with all possible take-off angles. Each plane-wave source dataset is migrated independently in a tilted coordinate system with the tilting angle determined by the take-off angle of the plane-wave source. For waves illuminating steep reflectors, the propagation direction is close to the extrapolation direction, thus they are accurately propagated by plane-wave migration in tilted coordinates. For steep reflectors in tilted coordinates, the subsurface offset direction is close to the dip direction of reflectors, thus plane-wave migration in tilted coordinates generates robust angle-domain common-image gathers for steep reflectors. I apply plane-wave migration in tilted coordinates to synthetic and 3D field datasets. Results show that steep salt flanks and faults are well imaged. |
