Ninth International Conference on Permafrost ... - IARC Research

Ni n t h In t e r n at i o n a l Co n f e r e n c e o n Pe r m a f r o s tConclusionThe measurements show that the internal structure ofthe moraine is very heterogeneous. Massive ice lensesare probably present above 10 m depth. This assumptionis supported by the observation of different ice types(sedimentary, congelation) in an excavation. An unexpectedand strong increase in resistivities occurred in relativelydeep layers between August and October. Totally differentclimatic conditions before the acquisitions and the influenceof the first layer on the inversion process are probably theonly way to explain such differences. However, furtheracquisitions and modeling of the first layer influence arenecessary to validate this hypothesis.Figure 3. Two-dimensional resistivity profiles on 13 August and 23October 2007 on the Col des Gentianes moraine.Borehole data indicate warmer temperatures between 1and 13 m depth in October than in August (Fig. 2). At thesame time, we observe a strong increase in resistivitiesbetween the 2 acquisitions up to 6–10 m deep. Now, below0°C, many studies reported that decreasing temperaturesprovoke increasing resistivities (e.g., Hauck 2001). Thus, thechange in resistivities that we observe cannot be explainedby a change in ground temperatures.A hypothesis which could explain these differentresistivities is the existence of 2 completely different climaticconditions during the 2 weeks preceding the 2 acquisitions.In August, the weather was very rainy, whereas October wasdry and very cold at the acquisition time. As a consequence,the active layer was probably saturated with unfrozen waterin August, whereas it was dry and frozen in October (Fig.2). This led to higher resistivities at the subsurface. Thestrong increase in resistivities below the active layer is moredifficult to understand, but we can assess that the differencein resistivity contact due to difference in surface humidityplayed an important role in the inversion process at greaterdepth.According to the 2-D resistivity imaging, the groundstratigraphy is very heterogeneous. This is not surprising,insofar as a push moraine is often a complex landformincorporating, for instance, frozen sediments and sedimentaryice from the glacier. The presence of unfrozen lenses cannotthen be excluded. Thus, it is possible that water couldpenetrate the deep layers during the rainy episode of August,which could have provoked lower resistivities.Finally, the slight resistivity decrease at 6–10 m deep inthe center of the profile may be explained by slight warmingof the ground between the two acquisitions (Fig. 2).ReferencesDelaloye, R. 2004. Contribution à l’étude du pergélisol demontagne en zone marginale. Thèse. Fac. Sciences,Univ. Fribourg, Geofocus 10: 240 pp.Haeberli, W. 1979. Holocene push-moraines in alpinepermafrost. Geografiska Annaler 61A(1–2): 43-48.Hauck, C. 2001. Geophysical methods for detectingpermafrost in high mountains. Mitteilungen der VAW,ETH Zürich, 171.Kneisel, C. 2004. New insights into mountain permafrostoccurrence and characteristics in glacier forefields athigh altitude through the application of 2-D resistivityimaging. Permafrost and Periglacial Processes 15:221-227.Lambiel, C. 2006. Le pergélisol dans les terrainssédimentaires à forte déclivité: distribution, régimethermique et instabilités. Thèse, Université deLausanne, Institut de Géographie, coll. “Travaux etRecherches,” n° 33: 260 pp.Marescot, L., Loke, M.H., Chapellier, D., Delaloye, R.,Lambiel, C. & Reynard, E. 2003. Assessing reliabilityof 2D resistivity imaging in mountain permafroststudies using the depth of investigation index method.Near Surface Geophysics 1: 57-67.Reynard, E., Lambiel, C., Delaloye, R., Devaud, G., Baron,L., Chapellier, D., Marescot, L. & Monnet, R. 2003.Glacier/permafrost relationships in forefields of smallglaciers (Swiss Alps). Proceedings of the Eighth<strong>International</strong> <strong>Conference</strong> on Permafrost, Zürich,Switzerland, July 21–25, 2003: 947-952.162