1. ION Geophysical, GXT Imaging Solutions, Denver, Colorado, U.S.A. .
2. ION Geophysical, GXT Imaging Solutions, Houston, Texas, U.S.A.
3. ION Geophysical, GXT Imaging Solutions, Surrey, U. K.
4. Princeton University, Department of Geosciences and Program in Applied & Computational Mathematics, Princeton, New Jersey, U.S.A.