Compensating for the effects of gas clouds on C-wave imaging : a case study from Valhall

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 (diodic VC) and velocity ratios (diodic γeff). In some cases, whether a horizon (a geologic target) beneath a gas cloud can be undershot also depends on the shooting directions (diodic illumination). These effects, compounded with the asymmetric raypath of the C-wave and the uncertainties in the P-wave data due to the gas clouds, will further increase the difficulties and costs of processing C-wave data. In this article, using the 2-D Valhall data as an example, we examine these effects and discuss ways to compensate for them during processing for improving the C-wave imaging.