Depth Imaging: A Practitioner's View
Authors: Leveille Jacques P. 
Published: October 2008

Abstract
Depth imaging has become extremely popular in recent years. We present our experience with this technology in 2D and 3D. Pre-stack depth migration in 2D is routinely used for a quick determination of a first pass velocity model, structural control and for AVO screening of prospects. 3D post-stack depth migrations are very efficient and provide a good first look atgeologically complex areas near salt or sub-salt. However, the inability to update the velocity model post-stack and the lack of confidence in the actual stack in these complex geological areas makes 3D pre-stack depth imaging our preferred technique. Algorithms have been designed to handle the inevitable compromises that must be made in order to migrate very large data volumes in time to meet real world exploration and production deadlines, Examples are shown of the superior imaging obtained with very large apertures for steep dips and longer offsets for velocity control. Finally, we conclude with our view on the many remaining problems, such as multiples, multi-arrivals, as well as the future directions such as superlong offsets and multi-mode imaging.

Introduction
This paper provides an overview of seismic imaging at Amerada Hess (AHC). The processing capabilities described in the sequel are an outgrowth of many years of work in this area, and the author is summarizing the work of many others (see Acknowledgments). In fact the first 2D post-stack depth migration was done at AHC approximately ten years ago, using a reverse time migration algorithm1. Algorithms used at AHC are proprietary, developed in-house, and run in AHC's proprietary UNIX based seismic processing system. The software runs on a wide array of computer platforms, from single workstations to supercomputers. For large computational demands we have two IBM SP2 systems (a 32 node and a 40 node machine), and a sixteen-processor SOI Power Challenge. All computer intensive computations are done in parallel, and use LINDA from Scientific Computing Associates (SCA) for the parallel functions. Less computationally intensive tasks such as 2D pre-stack migrations and full waveform modeling are also performed on single UNIX workstations (SUNs and SOIs) either as standalone or as distributed parallel systems2. In Table I we show the scale of the work that was performed in 1997 to give the reader a feeling for the capacity of the system, and of the experience of the processors using it. The numbers in that table are prior to the acquisition of the newer 32 node SP2 early in 1998. Before delving into detailed results of the various migrations, it is important to set the stage as to the processing philosophy at AHC. Seismic processing is performed within Exploration Technology for Amerada Hess Exploration and Production. The US operations account for approximately 70% of the work, mostly in the Oulf of Mexico, with the remaining 30% for Amerada Hess Limited, Amerada Hess Norge and Amerada Hess Denmark, in the UK, Norwegian and Danish sectors of the North Sea. This provides for a rich variety of geological settings, which is ideal to test various imaging techniques.