1. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada.
2. Institute for Geophysics, University of Texas at Austin, Austin, TX 78758, USA.
3. Department of Earth Sciences, Montana State University, Bozeman, MT 59717, USA.
4. Department of Geophysics, Stanford University, Stanford, CA 94305, USA.
5. Department of Electrical Engineering, Stanford University, Stanford, CA 94305, USA.
6. Scott Polar Research Institute, University of Cambridge, Cambridge, UK.