Prestack imaging of overturned and prismatic reflections by reverse time migration

I present a simple method for computing angle-domain Common Image Gather (CIG) using prestack reverse time migration. The proposed method is an extension of the method proposed by Rickett and Sava (2001) to compute CIGs by downward-continuation shotprofile migration. I demonstrate with a synthetic example the use of the CIG gathers for migration velocity updating. A challenge for imaging both overturned and prismatic reflections is the discrimination of the reflection generated on either side of interfaces. I show how the propagation direction of the reflections can be easily determined by evaluating the crosscorrelation of the source wavefield with the receiver wavefield at time lags different than zero. Reflections can be easily separated once their direction of propagation is determined. I demonstrate the method by imaging overturned events generated by a segment of dipping reflector immersed in a vertically layered medium. The example shows that for overturned reflections velocity errors cause asymmetric residual moveouts. This observation suggests that for updating the velocity from overturned reflections, the migrated CIGs should be scanned using a different family of residual moveouts than the standard parabolic moveouts used for non-overturned events. The migration of a synthetic example containing prismatic reflection shows that the CIGs computed using only horizontal subsurface offset are not useful, and that the vertical subsurface offset should be also used.