Abstract
Scattering-angle migration maps seismic prestack data directly into angle-dependent reflectivity at the image point. The method automatically accounts for triplicated rayfields and is easily extended to handle anisotropy. We specify scattering-angle migration integrals for PP and PS ocean-bottom seismic (OBS) data in 3D and 2.5D elastic media exhibiting weak contrasts and weak anisotropy. The derivation is based on the anisotropic elastic Born-Kirchhoff-Helmholtz surface scattering integral. The true-amplitude weights are chosen such that the amplitude versus angle (AVA) response of the angle gather is equal to the Born scattering coefficient or, alternatively, the linearized reflection coefficient.
We implement scattering-angle migration by shooting a fan of rays from the subsurface point to the acquisition surface, followed by integrating the phase- and amplitude-corrected seismic data over the migration dip at the image point while keeping the scattering-angle fixed. A dense summation over migration dip only adds a minor additional cost and enhances the coherent signal in the angle gathers.
The 2.5D scattering-angle migration is demonstrated on synthetic data and on real PP and PS data from the North Sea. In the real data example we use a transversely isotropic (TI) background model to obtain depth-consistent PP and PS images. The aim of the succeeding AVA analysis is to predict the fluid type in the reservoir sand. Specifically, the PS stack maps the contrasts in lithology while being insensitive to the fluid fill. The PP large-angle stack maps the oil-filled sand but shows no response in the brine-filled zones. A comparison to common-offset Kirchhoff migration demonstrates that, for the same computational cost, scattering-angle migration provides common image gathers with less noise and fewer artifacts.
