Seismic velocities in complex media

Abstract

Knowledge of seismic velocities in natural rock formations is needed for several purposes: converting travel-time to reflector depth, efficiently performing data processing such as common-midpoint ( cmp) stacking and reflector migration, and finally studying lithofacies. When dip and faulting conditions are mild, all these goals can be reached without too much difficulty. Complex structures can be defined from a geophysicist's standpoint as being those which make it difficult or even impossible to conduct conventional processing operations, such as cmp stacking and post-stack migration. The principal reason for the difficulties encountered lies in the presence of lateral velocity variations strong enough to preclude the use of any processing procedure which would require hyperbolic time- distance curves on cmp trace gathers. This does not necessarily mean the presence of steep dips, but rather intense faulting that puts blocks of very different velocities in contact. Medium velocity can be estimated by using the redundancy of possible migration results. These results must be coherent from shotpoint to shotpoint or from one offset to another. If the velocity-model is unsatisfactory, it can be modified in order to give better coherence to migrated events. Travel-time tomography is another method for obtaining velocity-distributions, but for a successful application it assumes that reflected events can be picked and timed. Because tomography is an inverse method, it lends itself very easily to the introduction of outside information or of a priori constraints. Velocity investigation in complex regions is still in its infancy. Attempting to derive detailed and accurate information for lithofacies studies must be put aside for the time being, all efforts being concentrated obtaining the velocity needed to improve reflector imaging and positioning. Much progress is still necessary before 3-D, P and S velocity surveys can be conducted in all regions with complex geological structures.