Sign‐change correction for prestack migration of P‐S converted wave reflections 1

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 α) is assigned to each point in the image space, where α 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.