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 (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,...

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