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Internal Structure of Rock Glacier Murtèl Delineated by Electrical ResistivityTomography and Forward/Inverse ModelingChristin HilbichDepartment of Geography, University of Jena, GermanyIntroductionRock glacier Murtèl (Engadine, Swiss Alps) is one ofthe most intensely investigated rock glaciers worldwide.Borehole temperatures have been recorded since 1987(Vonder Mühll & Haeberli 1990, Haeberli et al. 1998, VonderMühll et al. 1998, Vonder Mühll et al. 2007), providing oneof the longest temperature records in Alpine permafrostFromthe broad range of investigations on this rock glacier, it hasbeen learned that:• The rock glacier Murtèl consists of a considerableamount of massive ice (up to 80–100% in a ca. 25 mthick layer below the active layer) (Vonder Mühll &Haeberli 1990).• The active layer depths vary only slightly throughoutthe years (by max. 0.5 m) (Vonder Mühll et al. 2007).• Rock glacier creeping is generally rather slow withvelocities between 5 and 15 cm/a (determined over theperiod 1987–1996) (Kääb et al. 1998).• A shear zone exists in 30 m depth, where most of thehorizontal movement is concentrated (Arenson et al.2002).Apart from the borehole observations, no directobservations of the interior exist at rock glacier Murtèl.However, indirect information could be inferred fromgeophysical measurements (Vonder Mühll 1993, Hauck &Vonder Mühll 2003, Maurer & Hauck 2007).In general, rock glacier Murtèl is considered an activerock glacier under stable conditions. The internal structure isassumed to be homogeneous without any distinct degradationphenomena.Geoelectrical MonitoringIn summer 2005 a longitudinal Electrical ResistivityTomography (ERT) monitoring profile was installed at rockglacier Murtèl. The horizontal distance of the profile is 235m (crossing the borehole and the tongue). Results from theERT monitoring give rise to an interesting inhomogeneity inthe central part of the rock glacier.The general resistivity pattern (Fig. 1) clearly confirms thestratigraphy derived from borehole measurements: the activelayer is represented by relatively low resistivities between 15and 30 kΩm in the upper 3 m. Beneath the active layer thereis a sharp increase in resistivities to values between 500 kΩmand 1.9 MΩm, indicating the presence of massive ice. In thecentral part of the rock glacier, this pattern is interrupted bya vertical anomaly with smaller resistivity values (300–400kΩm). The monitoring results indicate that this feature ismuch more pronounced in summer than in winter.Figure 1. ERT tomogram from rock glacier Murtèl, measured onAugust 17, 2006.To assess whether this anomaly is an artefact of theinversion process (cf. Hauck & Vonder Mühll 2003) orproduced by a real structural inhomogeneity the resistivitydistribution of the rock glacier was analysed by means ofsynthetic datasets and forward/inverse modeling using thesoftware Res2DMod and Res2DInv (Loke & Barker 1995,Loke 2002). In forward/inverse modeling approaches,apparent resistivities are calculated from a synthetic model ofthe assumed resistivity distribution. The resulting inversionresult can then be compared to the corresponding inversionresult of the measured data. If the results are similar in termsof both resistivity distribution and total values, the syntheticmodel can be evaluated as a realistic model of the subsurfaceresistivity distribution.This procedure was performed for a variety ofgeomorphological situations that may explain theinhomogeneity within rock glacier Murtèl.Best results were obtained for a simulated crevasse inthe massive ice body of the rock glacier, which is, at leastpartly, filled with unfrozen water. This result is surprising, asthe rock glacier does not show indications of disintegrationat the surface. An interpretation of this feature in termsof a degradation phenomenon seems likely, but cannot besupported by other data from the rock glacier.ConclusionsThe forward/inverse modeling showed that the resistivityanomaly in the central part of the rock glacier is verylikely due to a crevasse-like structure within the ice core.These findings are of interest in the context of the observedspeed-up of many rock glaciers in the Alps, which isoften associated with disintegration of the surface and theformation of crevasses (Roer et al. 2005, Kääb et al. 2007).Since comparable developments (speed-up, formation ofcrevasses at the surface) are not known for rock glacierMurtèl so far, further studies are necessary to verify theassumptions resulting from forward/inverse modeling.101
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 tReferencesArenson, L., Hoelzle, M. & Springman, S. 2002. Boreholedeformation measurements and internal structure ofsome rock glaciers in Switzerland. Permafrost andPeriglacial Processes 13: 117-135.Barsch, D. 1996. Rock Glaciers – Indicators for thePresent and Former Geoecology in High MountainEnvironments. Berlin: Springer.Burger, K.C., Degenhardt, J.J. & Giardino, J.R. 1999.Engineering geomorphology of rock glaciers.Geomorphology 31(1–4): 93-132.Haeberli, W., Hoelzle, M., Kääb, A., Keller, F., VonderMühll, D. & Wagner, S. 1998. Ten years after drillingthrough the permafrost of the active rock glacierMurtèl, eastern Swiss Alps: Answered questionsand new perspectives. Proceedings of the Seventh<strong>International</strong> <strong>Conference</strong> on Permafrost, Yellowknife,Canada.Hauck, C. & Vonder Mühll, D. 2003. Inversion andinterpretation of two-dimensional geoelectricalmeasurements for detecting permafrost in mountainousregions. Permafrost and Periglacial Processes 14:305–318.Kääb, A., Gudmundsson, G.H. & Hoelzle, M. 1998. Surfacedeformation of creeping mountain permafrost.Photogrammetric investigations on rock glacierMurtél, Swiss Alps. Proceedings of the Seventh<strong>International</strong> <strong>Conference</strong> on Permafrost, Yellowknife,Canada: 531-537.Kääb, A., Frauenfelder, R. & Roer, I. 2007. On the responseof rock glacier creep to surface temperature increase.Global and Planetary Change 56(1–2): 172-187.Loke, M.H. & Barker, R.D. 1995. Least-squaresdeconvolution of apparent resistivity. Geophysics 60:1682-1690.Loke, M.H. 2002. RES2DMOD, Ver. 3.01: Rapid 2Dresistivity forward modelling using the finitedifferenceand finite-element methods.Maurer, H. & Hauck, C. 2007. Geophysical imaging ofalpine rock glaciers. Journal of Glaciology 53(180):110-120.Roer, I., Kääb, A. & Dikau, R. 2005. Rock glacieracceleration in the Turtmann valley (Swiss Alps):Probable controls. Norwegian Journal of Geography59: 157-163.Vonder Mühll & Haeberli 1990Vonder Mühll, D.S. 1993. Geophysikalische Untersuchungenim Permafrost des Oberengadins. ETH Zürich, Diss.ETH 10107: 222.Vonder Mühll, D., Stucki, T. & Haeberli, W. 1998. Boreholetemperatures in alpine permafrost: a ten years series.Proceedings of the Seventh <strong>International</strong> <strong>Conference</strong>on Permafrost, Yellowknife, Canada.Vonder Mühll, D., Noetzli, J., Roer, I., Makowski, K.& Delaloye, R. 2007. Permafrost in Switzerland2002/2003 and 2003/2004. Glaciological Report(Permafrost) No. 4/5 of the Cryospheric Commission(CC) of the Swiss Academy of Sciences (SCNAT)and the Department of Geography, University ofZurich: 106.102
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ContentsPreface ...................
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Mapping and Modeling the Distributi
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Satellite Observations of Frozen Gr
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xiiIce Wedge Thermal Regime in Nort
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xivPermafrost Response to Dynamics
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NICOP SponsorsUniversitiesUniversit
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Deep Permafrost Studies at the Lupi
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Effect of Fire on Pond Dynamics in
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Cryological Status of Russian Soils
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Acoustical Surveys of Methane Plume
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Permafrost Delineation Near Fairban
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Preparatory Work for a Permanent Ge
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A Provisional Soil Map of the Trans
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Martian Permafrost Depths from Orbi
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Time Series Analyses of Active Micr
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Impact of Permafrost Degradation on
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DC Resistivity Soundings Across a P
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Modeling Thermal and Moisture Regim
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A Provisional Permafrost Map of the
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Alpine Permafrost Distribution at M
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Cryogenic Formations of the Caucasu
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Modeling Potential Climatic Change
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A Hypothesis: A Condition of Growth
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Modeled Continual Surface Water Sto
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Freeze/Thaw Properties of Tundra So
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Discontinuous Permafrost Distributi
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Thermal Regime Within an Arctic Was
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Seasonal and Interannual Variabilit
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Twelve-Year Thaw Progression Data f
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Continued Permafrost Warming in Nor
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Landsliding Following Forest Fire o
Page 72 and 73: A Permafrost Model Incorporating Dy
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Page 76 and 77: Greenland Permafrost Temperature Si
Page 78 and 79: The Importance of Snow Cover Evolut
Page 80 and 81: The Account of Long-Term Air Temper
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Page 84 and 85: Adaptating and Managing Nunavik’s
Page 86 and 87: Human Experience of Cryospheric Cha
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Page 92 and 93: Mapping and Modeling the Distributi
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