1. Center for Wave Phenomena and Department of Geophysics, Colorado School of Mines, Golden, Colorado 80401;
2. Earth Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720
3. Geophysics Department, Sandia National Laboratories, Albuquerque, New Mexico 87185-0750
4. U.S. Geological Survey, Sacramento, California 95819;
5. Shell International Exploration and Production Inc., Rijswijk, The Netherlands;
6. CNRS-CEREGE, Department of Hydrogeophysics and Porous Media, 13545 Aix-en-Provence, Cedex 4, France;