1. Department of Earth Science, University of Bergen, P.O. Box 7803, N-5020 Bergen, Norway
2. Department of Geoscience and Petroleum, Norwegian University of Science and Technology (NTNU), S.P. Andersens vei 15A, NO-7491 Trondheim, Norway
3. Department of Computational and Applied Mathematics, Rice University, 6100 Main MS-134, Houston, TX 77005-1892, USA
4. Department of Mathematics and Statistics, University of Jyväskylä, P.O. Box 35 (MaD), FI-40014 University of Jyväskylä, Finland