Ninth International Conference on Permafrost ... - IARC Research

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