SIGN-CHANGE CORRECTION FOR PRESTACK MIGRATION OF P-S CONVERTED WAVE REFLECTIONS

The polarization direction or 'sign' of reflected converted P-S waves depends upon the angle of incidence of the incident P-wave. Sign reversal due to reversal of the angle of incidence is often encountered and is an impediment to P-S wave processing and imaging, because when P-S events or P-S migrated images with mixed signs are stacked, destructive interference occurs. We have created and demonstrated a means of correcting for this reversal. To do this, a P-wave angle of incidence is calculated for every point in the image space. This is done by calculating a P-S reflected waveform for every point, by extrapolating the reflected S-wavefield backwards from the receiver line, and then cross-correlating this waveform with the S-wave reflections observed at the receiver line. A multiplier, (sgn alpha) is assigned to each point in the image space, where alpha is the angle of incidence of the P-wave. The multiplier was applied to a set of prestack reverse time migration images derived from a cross-borehole physical elastic model data set. The improvement in the stacked image when the sign correction is applied is spectacular. The P-S image quality is comparable to, or better than, stacked migrated P-P images. The method appears to be applicable to all reflection modes and to all recording geometries