Acknowledgement

The author performed part of the work reported here while a guest of the Stanford Exploration Project. The author thanks the Director of SEP, Prof. Jon Claerbout, and the other project members for their hospitality and for the stimulating intellectual atmosphere of SEP.

This work was partially supported by the National Science Foundation under grant numbers DMS 9404283 and DMS 9627355; the Office of Naval Research under grant number N00014-96-1-0156; the U. S. Department of Energy under grant number DE-FG07-97 ER14827; and The Rice Inversion Project. TRIP Sponsors for 1998 are Amerada Hess, Amoco Production Co., ARCO Oil and Gas, Conoco Inc., Discovery Bay, Exxon Production Research Co., Mobil Research and Development Corp., and Shell International Research.