Estimating and interpreting velocity uncertainty in migrated images and AVO attributes

Estimating a suitable velocity field for use in prestack depth migration is inherently uncertain because of limitations on the available data and esti mation techniques. This uncertainty affects both the migrated depth of stru ctures and their amplitudes in the inverted images. These effects can be es timated by performing multiple migrations with a set of velocity fields and colocating features in the migrated images. This lets us examine the imagi ng procedure's sensitivity to changes in the velocity field so we can asses s both structural and amplitude uncertainties in migrated images. These two types of uncertainties affect interpretation in different ways. F or instance, with structural uncertainty interpretation we consider the cha nge in migrated location of structures when deciding on drilling locations, optimizing well trajectories, or computing uncertainty in volumetric calcu lations. With amplitude uncertainty or amplitude versus offset (AVO) uncert ainty interpretation, we consider (1) uncertainty in crossplots of pairs of AVO attributes at a point of interest or (2) uncertainty of the attribute values along identified structures. For any interpretation informing a deci sion, the uncertainty can help estimate risk. Our data processing approach is based on amplitude-preserving prestack dept h migration followed by AVO inversion, or AVO migration/inversion. It is va lid for estimating AVO attributes in simple to moderately complex structura l settings. Our methods of assessing the effect of velocity uncertainty can also be applied when obtaining structural uncertainties for a complex over burden geology or amplitude uncertainties in conventional NMO-based AVO ana lysis. They may also be applied straightforwardly to any poststack attribut e analysis. Key to the approach is the availability of multiple velocity fi elds to generate multiple migrated images. In our application, an automatic algorithm samples possible fields, but the set of fields to consider could be generated from another source, such as interpretation.