Geophysical Prospecting zyxwvuts 41,661-670, 1993 zyxwv MODEL-BASED STACK: A METHOD FOR CONSTRUCTING AN ACCURATE ZERO-OFFSET SECTION FOR COMPLEX OVERBURDENS' E. LANDA,2*3 P. THORE394 and M. RESHEF' ABSTRACT LANDA, E., THORE, P. and RESHEF, M. 1993. Model-Based Stack: a method for constructing an accurate zero-offset section for complex overburdens. Geophysical Prospecting 41,661-670. The interpretation of stacked time sections can produce a correct geological image of the earth in cases when the stack represents a true zero-offset section. This assumption is not valid in the presence of conflicting dips or strong lateral velocity variations. We present a method for constructing a relatively accurate zero-offset section. We refer to this method as model-based stack (MBS), and it is based on the idea of stacking traces within CMP gathers along actual traveltime curves, and not along hyperbolic trajectories as it is done in a conven- tional stacking process. These theoretical curves are calculated for each CMP gather by tracing rays through a velocity4epth model. The last can be obtained using one of the methods for macromodel estimation. In this study we use the coherence inversion method for the estimation of the macromodel since it has the advantage of not requiring prestack trav- eltime picking. The MBS represents an accurate zero-offset section in cases where the esti- mated macromodel is correct. Using the velocity-depth macromodel, the structural inversion can be completed by post-stack depth migration of the MBS. INTRODUCTION After-stack processing can produce a plausible geological image of the subsurface, provided the conventional CMP stack closely represents a zero-offset section. This is not the case in situations with either (a) conflicting events with different stacking velocities or (b) strong lateral velocity variations. Sometimes, these problems can be resolved by DMO, improved stacking techniques (De Bazelaire 1988) or prestack layer replacement (Berryhill 1984; Dent 1983). However, the optimal method of handling the above problems is by the relatively expensive procedure of prestack depth imaging. We propose a method to produce a more accurate zero-offset section which is Paper presented at the 53rd EAEG meeting, Florence, Italy, May 1991. Received January 1992, revision accepted January 1993. Institute for Petroleum Research and Geophysics, Hamashbir St 1, Holon, 58112 Israel. Elf Aquitaine, France. Elf UK Geoscience Research Centre, Emperors Gate, 114A Cromwell Road, London, U.K. zy 66 zy 1