Ninth International Conference on Permafrost ... - IARC Research

Ground Truth Observations of the Interior of a Rock Glacier as Validation forGeophysical Monitoring DatasetsChristin HilbichDepartment of Geography, University of Jena, GermanyIsabelle RoerDepartment of Physical Geography, University of Zurich, SwitzerlandChristian HauckInstitute for Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), GermanyIntroductionMonitoring the permafrost evolution in mountain regionsis currently one of the important tasks in cryospheric studies,as little data on past and present changes of the ground thermalregime and its material properties are available. In additionto existing borehole temperature monitoring networks,techniques to determine and monitor the ground ice contenthave to be developed. A reliable quantification of ground iceis especially important for modeling the thermal evolution offrozen ground and for assessing the hazard potential due tothawing permafrost- induced slope instability.Near-surface geophysical methods are increasingly appliedto detect and monitor ground ice occurrences in permafrostareas. Commonly, characteristic values of electricalresistivity and seismic velocity are used as indicators for thepresence of frozen material. However, validation of the interpretationof geophysical parameters can only be obtainedthrough boreholes, limited to vertical temperature profiles.Ground truth of the internal structure and the ice content isusually not available.During the construction of a ski track in summer 2007 we hadthe unique opportunity to conduct geophysical measurements(Electrical Resistivity Tomography [ERT] and Refraction SeismicTomography [RST]) on a partly excavated rock glacier nearZermatt (Valais, Swiss Alps) and to calibrate the data with directobservations. (For further description of the study site, impacts,and investigations, see Maag et al. 2008 and Wild et al. 2008).Rock Glacier CompositionThe general stratigraphy of rock glaciers is described byseveral authors (e.g., Barsch 1996, Burger et al. 1999) as asequence of 3 main layers: (1) The uppermost 1–5 m consistof big boulders riding on (2) an ice-rich permafrost layer(with 50–70% ice and ca. 30% finer-grained material (Barsch1996), which is creeping downslope. (3) The lowermost layerconsists of larger rocks, which were deposited at the rockglacier front and subsequently overrun by the other layers.Despite numerous investigations of rock glaciers worldwide,direct observations of the internal structure of rock glaciersare rare. More detailed studies on stratigraphy result fromdirect observations in boreholes (e.g., Haeberli et al. 1998,Arenson et al. 2002) and indirect geophysical data (e.g.,Vonder Mühll 1993, Hauck 2001, Maurer & Hauck 2007).Observed composition of the Gornergrat rock glacierTrenches cut during the construction of the ski track (upto 12 m) provided insight into the composition of the blockyTable 1. Estimated fractions (f) of rock, ice, air, and water fordifferent layers observed during the final construction stage.f rockf airf waterf iceActive layer (2-3 m)Coarse blocks 0.6-0.7 0.3-0.4 0 0Mix of blocks and fine 0.8 ? ? 0sedimentIntermediate layer (3-5 m)Blocks with interstitial ice 0.5 0 0 0.5Lowest observed layerMassive ice with few blocks 0.2-0.4 0 0 0.6-0.8Figure 1. Inversion results for electrical resistivities and P-wavevelocities obtained at the survey line.active layer, a frozen mix of blocks, fine sediment and ice underneath,followed by a massive ice core. To validate the geophysicaldata, the respective fractions of rock, ice, water, andair were roughly estimated by visual inspection (Table 1).Geophysical DataIn the beginning of the construction of the ski track, whileonly little material had been removed so far, we conductedERT and RST measurements on a longitudinal profile (70 m)in the upper part of the rock glacier (Fig. 1).High resistivity zones coincide with high velocity zones,indicating a high ice content within the rock glacier, especiallyin the upper (right) part of the profile. The active layer isclearly delineated by smaller resistivities and velocities in theuppermost 2–3 m of the profile. In the central part, the activelayer had already been disturbed by the construction work.99