PS-wave Processing and Velocity-model Building from a Sparse OBN Array

PS-wave processing is particularly challenging for sparse ocean-bottom acquisitions in deep water. Problems in signal processing revolve around the extreme asymmetry of raypaths (due both to the mode conversion and also to the acquisition geometry), as well as severe aliasing and low fold in conversion-point gathers that would be well focused with denser acquisitions. Aliasing and low fold also cause problems in PS-wave velocity-model building, since migrated gathers are typically noisy and multidimensional interpolation or regularisation methods produce inaccurate images in the shallow subsurface. These problems require methodological modifications both in the processes applied and in the workflows with which they are implemented. For example, PS-wave directional-designature and surface-related multiple suppression are particularly effective with receiver-domain radial-down deconvolution. Meanwhile, meaningful updates of principle shear velocity and anisotropy without use of image gathers for tomography or quality control can be achieved using common-receiver controlled-beam migration straight to stack. Examples from data acquired in deep water to the west of the Shetland Isles (UK) show the efficacy of such modifications, and indicate that PS data from such sparse acquisitions can provide useful images of the subsurface.