Borehole‐calibrated anisotropic processing of converted modes — A sub‐chalk imaging study

Abstract

Summary Processing multi-component ocean bottom cable (OBC) data for converted mode imaging is fraught with challenges. In addition to the usual challenges confronting conventional processing are the problems of determining a second (shear) velocity field, asymmetric ray paths, much stronger anisotropy effects and often increased attenuation, to name a few. In this project the goal of imaging beneath a weathered, high velocity chalk layer added further complexity. Borehole data, including dipole sonic logs, vertical and offset VSPs and walkaways can greatly assist in multicomponent OBC processing. We build a calibrated, anisotropic (vertical transverse isotropic or VTI), anelastic 1D model at a well location through a workflow that includes OBC data from near the well and walkaway VSP direct arrival times. This forms the basis for model-based converted mode processing and interpretation. In addition to VTI ray-based moveout correction and CCP binning, we use the calibrated 1D model in several other ways. We apply prestack Q-compensation using the VSP-derived Q values, use the model to compensate for geometrical spreading out to long offsets, carry out simulations to design the optimum mute gates and use borehole matching to guide parameter selection in demultiple operations. This calibrated model-based approach has led to significant improvements in P-s processing, but has also allowed us to exploit a P-S-s mode, with mode conversion taking place at the top of the chalk. We show a depth converted P-S-s stack that images the sub-chalk reservoir targets.