Migration velocity analysis using residual diffraction moveout in the poststack depth domain Tiago A. Coimbra 1 , J. Jadsom S. de Figueiredo 2 , Jörg Schleicher 1 , Amélia Novais 1 , and Jessé C. Costa 2 ABSTRACT Diffraction events contain more direct information on the medium velocity than reflection events. We have developed a method for migration velocity improvement and diffraction localization based on a moveout analysis of over- or undermi- grated diffraction events in the depth domain. The method uses an initial velocity model as input. It provides an update to the velocity model and diffraction locations in the depth domain as a result. The algorithm is based on the focusing of remigration trajectories from incorrectly migrated diffraction curves. These trajectories are constructed by applying a ray-tracing-like ap- proach to the image-wave equation for velocity continuation. The starting points of the trajectories are obtained from fitting an ellipse or hyperbola to the picked uncollapsed diffraction events in the depth-migrated domain. Focusing of the remigra- tion trajectories points out the approximate location of the as- sociated diffractor, as well as local velocity attributes. Apart from the migration needed at each iteration, the method has a very low computational cost, but relies on the identification and picking of uncollapsed diffractions. We tested the feasibility of the method using synthetic data examples from three simple constant-gradient models and the Sigsbee2B data. Although we were able to build a complete velocity model in this example, we think of our technique as one for local velocity updating of a slightly incorrect model. Our tests showed that, within re- gions where the assumptions are satisfied, the method can be a powerful tool. INTRODUCTION The identification and processing of diffraction events in seismic data can be useful for a multitude of purposes, some of which are hydrocarbon trap indication, velocity analysis, and superresolution analysis. Therefore, great effort has been made in seismic processing to improve focusing and positioning of diffractor images. Disconti- nuities in the subsurface (deep or shallow) can cause anomalies in seismic reflection events. Away to detect their presence is by iden- tifying diffraction curves in the data because the latter are typical signatures of such discontinuities. More importantly, diffraction pat- terns may serve as indicators of hydrocarbon traps as well as of aban- doned buried targets near the surface (Zeng and McMechan, 1997). Because of these characteristics, diffractions have long been a subject of study in seismic methods (Krey, 1952; Kunz, 1960; Hu- bral, 1975). Using the focusing properties of incorrectly migrated diffractions, de Vries and Berkhout (1984) develop a technique to extract velocity information based on a minimum-entropy criterion. Several other methods to image diffractions have been proposed in the recent past (Khaidukov et al., 2004; Moser and Howard, 2008; de Figueiredo et al., 2011, 2012). Khaidukov et al. (2004) also in- vestigate the consequences of diffraction imaging on seismic reso- lution. Moser and Howard (2008) propose two approaches to diffraction imaging. One is based on reflection focusing followed by reflection filtering, and the other is reverse application of aperture restrictions to Kirchhoff migration to filter out the specular reflections and consequently to enhance diffractions. Both ap- proaches were applied in the depth domain. More recently, de Figueiredo et al. (2011, 2012) have developed a method for auto- matic detection of diffraction points based on a k-nearest-neighbor Manuscript received by the Editor 21 August 2012; revised manuscript received 18 January 2013; published online 11 April 2013. 1 University of Campinas — DMA/IMECC, Department of Applied Mathematics, Campinas, Brazil; National Institute of Petroleum Geophysics (INCT-GP), Brazil. E-mail: tgo.coimbra@gmail.com; amelia@ime.unicamp.br; js@ime.unicamp.br. 2 National Institute of Petroleum Geophysics (INCT-GP), Brazil; Federal University of Pará, Faculty of Geophysics, Belém, Brazil. E-mail: jadsomjose@gmail .com; jesse.ufpa@gmail.com. © 2013 Society of Exploration Geophysicists. All rights reserved. S125 GEOPHYSICS, VOL. 78, NO. 3 (MAY-JUNE 2013); P. S125–S135, 10 FIGS. 10.1190/GEO2012-0340.1