1. Dept. of Physics, University of Alberta, Edmonton, Alta., Canada T6G 2J1. and Dept. de Fisica, Univ. de Evora, 7000 Evora, Portugal
2. Dept. of Physics, University of Alberta, Edmonton, Alta., Canada T6G 2J1
3. Atlantic Geoscience Centre, Geological Survey of Canada, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada B2Y 4A2