Prestack imaging of overturned reflections by reverse time migration

Abstract

We present a simple method for computing angle-domain Common Image Gathers (CIGs) 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 shot-profile migration. We 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. We show how the propagation direction of the reflections can be 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. We demonstrate the method by imaging overturned events generated by a segment of dipping reflector immersed in a vertically layered medium. We also applied the method to a North Sea data set with overturned events. The results of reverse time prestack migration are superior to the one obtained by a downward-continuation migration, and the CIGs obtained by applying the proposed method provide useful information for velocity updating.