[1] Lab. de Physique Mathématique, Université de Montpellier II, Cc050, Place Eugène Bataillon, 34095 Montpellier Cedex, France
[2] Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, United Kingdom
[3] Research Division, BRGM, BP 6009, F-45060 Orleans Cedex, France
[4] Dept. for Theor. Elec. Engineering, University of Erlangen-Nuremberg, Cauerstrasse 7, D-91058 Erlangen, Germany