HR: 0800h
AN: NS11A-0160    [Abstracts]
TI: Time-lapse Electrical Resistivity Measurements for Investigating Temperature Changes in Extrazonal Ventilated Permafrost
AU: Delaloye, R
EM: reynald.delaloye@unifr.ch
AF: Dept. of Geosciences, Geography University of Fribourg, Ch. du Musée 4, Fribourg, 1700, Switzerland
AU: Morard, S
EM: sebastien.morard@unifr.ch
AF: Dept. of Geosciences, Geography University of Fribourg, Ch. du Musée 4, Fribourg, 1700, Switzerland
AU: * Marescot, L
EM: laurent@aug.ig.erdw.ethz.ch
AF: ETH-Swiss Federal Institute of Technology Institute of Geophysics, HPP O7 ETH Hoenggerberg, Zurich, 8093, Switzerland
AB: Field experiments have been carried out to evaluate the ability for time-lapse electrical resistivity tomography and/or vertical electrical sounding for estimating seasonal temperature changes at depth in a low-altitude permafrost terrain in the Swiss Jura mountains. The prospected site is a limestone blocky talus slope located at 1200 m a.s.l. (mean annual air temperature +5.5°C) far below the regional lower limit of discontinuous mountain permafrost. Due to a seasonally reversible air circulation mechanism throughout the whole porous medium, driven by the thermal contrast between the outside and inside air, a strong negative thermal anomaly is observed and permafrost in the lower and deeper parts of the ventilated terrain locally occurs. The predominance of advective heat fluxes causes significant seasonal temperature shifts until the base of debris accumulation at about 10-20 m depth. Indeed, the ascent of relatively warm light air during wintertime (the so-called "chimney effect") provokes the sucking of cold outside air deep inside the ground in the lower and intermediate parts of the slope, building up a "cold" reservoir. Consequently, a permanent gravity discharge of cold dense air occurs in summertime and prevents the ground temperature to increase above about +5°C in the lower section of the slope. Permafrost is thus likely to occur in both the lower and deeper parts of the talus slope. Evaporation during the winter phase of ventilation makes that the ice content of permafrost is probably low. Among other experiments, a 33-m tomography profile was repeated nine times between May 2001 and May 2002 along the lower part of the talus slope using permanently-installed electrodes (1 m electrode spacing) and a Wenner array. Only the uppermost talus layer beneath the organic soil was thus investigated. According to ground temperature measurements performed at 30 cm depth, results show that changes in electrical apparent resistivity collected with our tomography system are not identical for temperature shifts above or below freezing point: the temperature increase in the active layer from 0°C (in May 2001) to about +5°C (in October 2001) was accompanied by a decrease of the mean apparent resistivity of 23%, whereas a drop in temperature (from November to January 2001) down to about -5°C caused a multiplication of the mean apparent resistivity at least by a factor of 15. Theses observations are confirmed on the inversion results. This experiment has shown that a temperature change above freezing point induces a low linear resistivity variation, whereas a temperature shift below freezing point seems to provoke an strong (exponential ?) resistivity change. Vertical soundings were carried out repeatedly at the same place over the permafrost in the lower part of the slope. Close to the surface, the calculated resisitity and its seasonal changes were similar to those observed on the tomography profile. Moreover, interpretation of the data indicated a smaller but significant increase of the ground resistivity until the deepest layer (8 - 20 m depth) of the talus slope by mid-winter. Applying the relationships linking temperature and resistivity determined from the tomography data, it could be estimated that this change has corresponded to a temperature decrease from 0 down to about -0.7°C. This demonstrates the efficiency of the air circulation process to cool the deepest layers of a talus slope in wintertime.
DE: 0702 Permafrost (0475)
DE: 0710 Periglacial processes
DE: 0768 Thermal regime
DE: 5109 Magnetic and electrical properties (0925)
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting