1. Centre for Mathematical Sciences, Lund University, Lund, Sweden
2. Seismic Apparition GmbH, Zurich, Switzerland
3. Institute of Geophysics, ETH-Zurich, Zurich, Switzerland. E-mail: johan.robertsson@erdw.ethz.ch
4. Statoil ASA, Trondheim, Norway
5. Department of Geoscience and Petroleum, Norwegian University of Science and Technology, Trondheim, Norway