1. Formerly University of Leeds, School of Earth and Environment, Leeds, U. K.; presently CGGVeritas, Calgary, Alberta, Canada. .
2. Formerly University of Leeds, School of Earth and Environment, Leeds, U. K.; presently University of Bristol, Department of Earth Sciences, Bristol, U.K. .
3. Formerly University of Leeds, School of Earth and Environment, Leeds, U. K.; presently University of Alberta, Department of Physics, Edmonton, Alberta, Canada. .