Monitoring mountain permafrost evolution using electrical resistivity tomography : A 7-year study of seasonal, annual, and long-term variations at Schilthorn, Swiss Alps - Hilbich, C.; Hauck, C.; Hoelzle, M.; Scherler, M.; Schudel, L.; Völksch, I.; Vonder Mühll, D.; Mäusbacher, R. (Journal of Geophysical Research, 2009)