HR: 13:40h
AN: H23H-01 INVITED    [Abstracts]
TI: Assessing the Reliability of Geoelectric Imaging Results for Permafrost Investigations
AU: * Marescot, L
EM: laurent@aug.ig.erdw.ethz.ch
AF: ETH Zurich, Institute of Geophysics, HPP07, ETH Hoenggerberg, Zurich, 8093, Switzerland
AU: Loke, M
AF: Universiti Sains Malaysia, School of Physics, Penang, 11800, Malaysia
AU: Abbet, D
AF: Universite de Fribourg, Departement de Geosciences, Geographie, Perolles, Fribourg, 1700, Switzerland
AU: Delaloye, R
AF: Universite de Fribourg, Departement de Geosciences, Geographie, Perolles, Fribourg, 1700, Switzerland
AU: Hauck, C
AF: Institute for Meteorology and Climate Research, Forschungszentrum Karlsruhe, Postfach 3640, Karlsruhe, 76021, Germany
AU: Hilbich, C
AF: Institut fuer Geographie Friedrich-Schiller-Universitaet, Loebdergraben 32, Jena, 07743, Germany
AU: Lambiel, C
AF: University of Lausanne, Institut de Geographie, Quartier Dorigny, Batiment Humense, Lausanne, 1015, Switzerland
AU: Reynard, E
AF: University of Lausanne, Institut de Geographie, Quartier Dorigny, Batiment Humense, Lausanne, 1015, Switzerland
AB: The effects of global climate change on mountain permafrost are of increasing concern; warming thaws permafrost, thereby increasing the risk of slope instabilities. Consequently, knowledge of the extent and location of permafrost are important for construction and other geotechnical and land-management activities in mountainous areas. Geoelectric imaging is a useful tool for mapping and characterizing permafrost occurrences. To overcome the generally poor electrical contacts in the active layer, geoelectric surveys usually involve coupling the electrodes to the ground via sponges soaked in salt water. The data are processed and inverted in terms of resistivity models of the subsurface. To monitor the evolution of mountain permafrost, time-lapse geoelectric imaging may be employed. A challenging aspect in geoelectric imaging of permafrost is the very large resistivity contrast between frozen and unfrozen material. Such a contrast makes inversion and interpretation difficult. To assess whether features at depth are required by the data or are artifacts of the inversion process, the reliability of models needs to be evaluated. We use two different approaches to assess the reliability of resistivity images in permafrost investigations: (i) depth of investigation (DOI) and (ii) resolution matrix maps. To compute the DOI, two inversions of the same data set using quite different reference resistivity models are carried out. At locations where the resistivity is well constrained by the data, the inversions yield the same results. At other locations, the inversions yield different values that are controlled by the reference models. The resolution matrix, which is based on the sensitivity matrix calculated during the inversion, quantifies the degree to which each resistivity cell in the model can be resolved by the data. Application of these two approaches to field data acquired in the Swiss Alps and Jura Mountains suggests that it is very difficult to obtain dependable bedrock resistivity information beneath occurrences of massive ice. The reliability tests, which tell us to what depth the resistivity images are trustworthy, help us explain (i) erratic and non-geologic features in the inversion models and (ii) suspicious changes in geoelectric time-lapse imaging. The DOI and resolution matrix techniques do not provide exactly the same information about model reliability. Instead, a combination of the two approaches prevents over-interpretations or misinterpretations of inversion results.
DE: 0702 Permafrost (0475)
DE: 0722 Rock glaciers
DE: 0900 EXPLORATION GEOPHYSICS
SC: Hydrology [H]
MN: 2007 Fall Meeting