1. Formerly Department of Petroleum and Geosystems Engineering, University of Texas at Austin, University Station, Mail Stop C0300, Austin, Texas 78712; presently Shell International E & P, 3737 Bellaire Boulevard, P. O. Box 481, Houston, Texas 77001..
2. Department of Petroleum and Geosystems Engineering, University of Texas at Austin, University Station, Mail Stop C0300, Austin, Texas 78712..
3. Schlumberger-Doll Research, Mathematics and Modeling Department, 36 Old Quarry Road, Ridgefield, Connecticut 06877..