Compensating for the effects of gas clouds on <i>C</i> -wave imaging: A case study from Valhall

Abstract

P-to-S converted waves (or C-waves) have been successfully used to image beneath gas clouds in many areas. However, C-waves often suffer from severe diodic effects due to the gas clouds; that is, the C-wave amplitude and traveltime may be different in the forward and reverse shooting directions, giving rise to different C-wave stacking velocities (diodicVC) and velocity ratios (diodicγeff). In some cases, whether a horizon (a geologic target) beneath a gas cloud can be undershot also depends on the shooting directions (diodic illumination).