The authors thank the National Science Foundation, the Office of Naval Research, the U. S. Department of Energy, and the sponsors of The Rice Inversion Project for support of this work. The authors performed part of the research reported here while guests of the Stanford Exploration Project; JLQ and WWS thank SEP members and director Prof. Jon Claerbout for their hospitality and many stimulating discussions. JLQ thanks Jim Berryman, Nizar Chemingui and James Rickett for proofreading the paper.