1. Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas, U.S.A.
2. Now at, Delft University of Technology, Delft, Netherlands
3. Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas, U.S.A.
4. Now at, Sandia National Laboratories, Albuquerque, New Mexico, U.S.A.