A Case Study of Pre-Salt Imaging

Conventional imaging techniques operating in the time domain are recognized to poorly resolve complex geologic environments. Complex subsurface geology with rapid spatial changes in interval velocity requires pre-stack imaging techniques operating in depth. However, these techniques rely on good definition of the velocity field. A common approach to defining this field involves multiple iterations of pre-stack depth migration through successively deeper velocity layers - an expensive and time-consuming process. In this paper, we describe a technique to cost and time effectively develop the required velocity model using travel-time inversion, of followed by replacement of the complex velocity field with a simplified field thereby removing nonhyperbolic moveout effects and allowing implementation of the most cost and time effective imaging solution. The technique is applied to data from the Pre-Caspian Basin of the Soviet Union where the ability to image below significant accumulations of Permian salt and overcome the false time pull-ups due to the fast saIt velocity is essential to accurate mapping of this basin. Following development of the velocity model using travel-time inversion, its use in conjunction with both pre and post-stack time and depth migration techniques is demonstrated in a set of seismic data comparisons. The conclusion is that the concept of Replacement Dynamics provides an attractive, cost-effective alternative to other computationally intensive techniques in unraveling many areas of complex subsurface geology.