Demonstration of a full volume 3D Pre-Stack Depth Migration in the using Massively Parallel Processor (MPP) Technology. Garden Banks Area
Authors: Solano Marcek , Chang Herman , VanDyke John , McMechan George , Epili Duryodhan 
Published: October 2008

Abstract
This paper describes the implementation and results of portable, production-scale 3D Pre-stack Kirchhoff depth migration software. Full volume pre-stack imaging was applied to a six million trace (46.9 Gigabyte) data set horn a subsalt play in the Garden Banks area in the Gulf of Mexico. The velocity model building and updating, were derived using image depth gathers and an image-driven strategy. After three velocity iterations, depth migrated sections revealed drilling targets that were not visible in the conventional 3D post-stack time migrated data set. As expected from the implementation of the migrationalgorithm, it was found that amplitudes are well preserved and anomalies associated with known reservoirs, conform to petrophysical predictions.
Image gathers for velocity analysis and the final depth migrated volume, were generated on an 1824 node Intel Paragon at Sandia National Laboratories. The code has been successfullyported to a CRAY (T3D) and Unix workstation Parallel Virtual Machine environments (PVM).

Introduction
This project is the result of a unique partnership between Oryx Energy Company, Sandia National Laboratories, and theUniversity of Texas at Dallas, through a Cooperative Research And Development Agreement program from the federal government (CRADA).
The project developed out of a need identified at Oryx Energy Company in 1992 for a cost effective, yet high quality 3D Pre-Stack Depth Migration (PreSDM) program for imaging seismic data underneath salt structures in the Gulf of Mexico. Lacking resources for internal development of a PreSDM system, the CRADA program provided Oryx with parallel programming expertise and access to one of the fastest computers in the world at Sandia, and a prototype parallel code, geophysical and programming expertise from the University of Texas at Dallas. The entire project took 13 months from preliminary code porting, through the generation of a 3D synthetic data set for testing purposes, velocity field updating and, the final migration run generating the PreSDM volume.

Migration Algorithm
3D pre-stack depth migration has been conceptually available for decades (Schneider, 1978), it only required the advent of super computers to become viable (French, 1992). lmplementations of3D Pre-Stack Kirchhoff Depth Migration now exist for both vector and parallel computers (Charrette, 1991; Highnam andPieprzalG 1991; Kao, 1992; Liu, 1993) along with numerous field data examples (Cabrera et al, 1992; Wyatt et al, 1992; Lewis et al, 1994; Ratcliff et al, 1994 and 1995; Schleicher et al, 1995; Epili and McMechan, 1996).
The prototype 3D PreSDM algorithm used in this project was described in detail by Epili and McMechan (1996), so only a brief summary is given in this paper. The algorithm consist of three main steps. The first step is ray tracing to compute arrays of travel times flom each source and receiver location, to each point in a grid that corresponds spatially to the desired migrated image. For efficiency, the source and receiver locations are assigned to a grid defined by the binned line and receiver spacing.