Near-surface Imaging Challenges for Marine PS-wave Data

Abstract

Sparse seafloor seismic acquisitions present new challenges when processing and imaging converted wave (PS-wave) data. The severe anti-symmetric nature of PS-wave reflections for deepwater acquisitions, and the strong aliasing and low fold derived from sparse shot or receiver arrays, necessitate the solution of significant practical problems to achieve high-quality seismic images near the seafloor. The problems of PS-wave imaging with deep-water, sparse ocean-bottom receiver arrays (OBN or OBC) has motivated new investigation and led to interesting results. For sparse acquisition geometries, wavefield aliasing is more acute when processing in some domains than others. For example, shot-carpet acquisitions recorded with sparse ocean-bottom nodes are well sampled in the common-receiver (CR) domain but poorly sampled in the common shot (CS) domain. Migration of these data is more attractive in the CR domain, in which the process of anti-aliasing during migration may be more effectively carried out than in the CS domain. Nevertheless, the summation of all the migrated CR wavefields still suffers from aliasing caused by the large receiver spacing. Further, low S-wave velocities are responsible for narrow illumination cones for all the CR migrated images, and the coarse receiver spacing cannot “heal” these images to produce a coherent stacked image near the seafloor. Here we propose some practical solutions to mitigate the effects of receiver spacing. We use Gaussian beam migration (GBM) to track surface ray parameters into the subsurface. We use this information both to redatum CR recorded wavefields above the seafloor and to anti-alias CR migration. Also, we show that PS-wave common-offset-vector (COV) GBM can provide adequate anti-aliasing when the receivers are relatively close together (OBC). Synthetic examples illustrate the effectiveness and limitations of these solutions for both OBC and OBN data.