Introduction

Although many migration algorithms are now capable of accurately imaging seismic data, imaging near salt structures remains a hard problem to solve. Complications in the imaging arise for several reasons. First, because of the high-velocity salt intrusion, velocity generally varies strongly, both in depth and laterally. Reflection events are therefore non-hyperbolic, and time migration does not produce satisfactory results. Instead, depth migration is needed, not only for focusing the data, but also for correctly locating salt boundaries (Larner, 1987). Second, data quality can often be poor in salt regions. The intrusive salt flow breaks up sediments above and alongside the structure, and these broken-up sediments do not generate clear reflections. Third, because of the high reflection coefficient of salt-sediment contrasts, little seismic energy penetrates the salt structure, and reflections from sediments below the salt are weak.

In previous reports (Van Trier, 1988; Van Trier, 1989a) I discussed a method to try to solve the first problem, the depth-migration and its related velocity-estimation problem - the second and third problem are mostly data acquisition problems. The velocity-estimation method determines structural velocities using migrated seismic data. After migration with an initial velocity model, structural boundaries are picked from the migrated image. The residual perturbations in the prestack migrated events that correspond to the picked reflectors are then used to optimize the velocity model. The optimization method is a gradient method, where the goal is to maximize semblance in the stacked migrated image. The gradient calculations in the optimization consist of two parts: a tomographic part that is calculated using ray tracing, and a data part that calculates semblance derivatives in the prestack migrated data.

In the last report I concentrated on the ray-tracing part; in this report I discuss the data gradient. I first show a data example that was recorded over a salt layer. Then I discuss some of the issues in applying the calculation to salt data, and, finally, I describe the data-gradient calculations in detail.