[1] Statoil Research Center, Arkitekt Ebbells vei 10, Trondheim,7005, Norway
[2] Norsk Hydro, P.O. Box 7190, Bergen,5020, Norway
[3] Norwegian Univerity of Science and Technology, Department of Mathematical Sciences, Trondheim,7491, Norway
[4] Stanford University, Department of Geophysics, Stanford Rock Physics Laboratory, Stanford,CA,94305, United States