1. Center for Gravity, Electrical, and Magnetic Studies, Department of Geophysics, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401, USA.
2. Present address: UBC-Geophysical Inversion Facility, Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada.
3. Corresponding author.