1. Uppsala University, Department of Earth Sciences, Uppsala, Sweden..
2. Council for Scientific and Industrial Research (CSIR) and University of the Witwatersrand, Johannesburg, South Africa..
3. Geological Survey of Canada, Ottawa, Canada..
4. Curtin University, Department of Exploration Geophysics and Deep Exploration Technologies Corporative Research Centre (DETCRC), Perth, Australia..
5. University of Toronto, Toronto, Canada. E-mail: bm@physics.utoronto.ca.
6. GAP Geophysics, Johannesburg, South Africa..