1. Department of Earth Sciences; University of Bristol; Bristol BS8 1RJ UK
2. Now at Pinnacle, A Halliburton Service; St Agnes; Cornwall TR5 0RD UK
3. Now at Department of Earth Science and Engineering; Imperial College London; London SW7 2AZ UK
4. Schlumberger Gould Research; High Cross; Cambridge CB3 0EL UK
5. Schlumberger Australia; Brisbane QLD 4000 Australia