Jeannette Noetzli1, Christin Hilbich2, Christian Hauck3, Martin Hoelzle1, and Stephan Gruber1

Comparison of 2D Temperature Profiles with Time-lapse Electrical Resistivity Data at the Schilthorn Crest, Switzerland

1 Glaciology, Geomorphodynamics & Geochronology, Department of Geography, University of Zurich, Switzerland2 Department of Geography, University of Jena, Germany3 Institute for Meteorology and Climate Research, University of Karlsruhe / Forschungszentrum Karlsruhe, GermanyContact: jeannette.noetzli@geo.unizh.ch

IntroductionThe Schilthorn Crest, Switzerland, is an intensively investigated permafrost site in the European Alps. Three boreholes were drilled within the PACE-project between 1998 and 2001, which provide the basis for monitoring and quantification of changes in the permafrost thermal regime.

In mountains, the interpretation of T(z) profiles from boreholes with respect to climate signals is complicated by steep topography. Even though such profiles enable an initial assessment of topo-graphical and transient effects, they are only representative of isolated spots. A comprehensive analysis of permafrost conditions can be achieved by integrating additional subsurface data. Here, we combine measurements of ground temperatures and electric resistivity tomography (ERT) with numerical modeling for a detailed investigation of permafrost below the Schilthorn crest.

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Field Site and Field Data

Acknowledgements

Discussion and Joint Interpretation

Conclusions and Outlook

Results

Modeling

Figure 1. Overview of the field site Schilthorn Crest showing the locations of the near-surface temperature loggers, the boreholes, the measured ERT profile, and the modeled north-south cross section. Map: Swisstopo.

Figure 2. Modeled subsurface temperature field for a north-south cross section of the Schilthorn crest. The 0 °C isotherm is depicted in black, and the dashed red lines indicate the two 101 m deep boreholes.

Figure 3. T(z) profiles for the 101 m vertical (vert, blue) and oblique (obl, red) boreholes on the Schilthorn for spring and autumn 2006 (left). In addition, T(z) profiles were extracted from the modeled temperature field in Figure 2 at the locations of the two boreholes (right).

Figure 4. ERT monitoring data illustrated as individual resistivity tomograms for subsequent measurement dates (a), and as calculated change in resistivity based on the reference profile from August 10, 2006 over one, four, and 13 months (b). Measured resistivities are low compared to other permafrost sites (i.e., <4000 Ωm). This is mainly due to the thick fine-grained debris layer covering the summit region. Outcrops of the underlying bedrock also indicate strongly weathered conditions of the micaceous shales with crevices, where water can percolate. In addition to the comparably conductive host material, the low ice content is in accordance with the low resistivity values

Figure 5. The 3 features addressed in the discussion are highlighted by red circles: (A) A homogeneous permafrost zone in the lower northern slope with a low ice-content, (B) a cold zone in the north slope with a high ice-content, and (C) no permafrost near the surface on the southern slope. Additionally, red dashed lines indicate the boreholes, and the grey dashed line the extent of the ERT profile.

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The Schilthorn is located in the Bernese Alps, CH. The 3 Boreholes (14 m and 2x101 m deep, drilled in 2001) are located on the north-facing slope. One of the deep boreholes is drilled vertical to the surface (i.e., at an angle of 60° to the vertical) in order to address topography-related influences.

Near-Surface Temperatures were measured on both sides of the crest from summer 2005 to 2007 and provide the upper boundary condition for the numerical heat transfer model. In addition, they can be used to constrain the interpretation of the geophysical results.

A semi-automatic ERT Monitoring System was installed in 1999 on a 60 m line near the bore-holes. In 2005, a 2nd ERT line (188 m) was installed across the crest, complemented by a quasi-3D ERT survey along 4 transects in 2006. The measured signal is sensitive to temporally variable proper-ties (temperature, via the unfrozen water and ice content) as well as unchanging material charac-teristics (lithology, porosity). Besides a qualitative comparison of individual tomograms, a time-lapse inversion of time series of ERT data allows for a quantitative assessment of the resistivity changes.

Elevation: 2970 m a.s.l.MAAT: –2.8 °C (1999–2007)Precipitation: 2700 mmAverage snow depth: 80 cmUnderground: dark micaceous shales, low ice content

The resulting temperature pattern is expected to be similar for any cross crest profile, hence, simu-lations are conducted for a 2D section across the crest and the borehole site on the northern slope. Comparison with field data allows for a qualitative vallidation of modeling results.

Model Setup purely conductive subsurface temperature field; transient and two-dimensional; homogeneous; isotropic, no seasonal variations considered (e.g., no active layer); latent heat effects via apparent heat capacity; UBC: measured mean annual near-surface temperatures; LBC: constant heat flux of 0.08 W m-2; Initialization with varying UBC: starting in 1850, GST changes based on air temperature variations from the Jungfraujoch; FE mesh: 10 m resolution at the surface, then coarsened, 1500 ele-ments; time steps of 10 days; software COMSOL Multiphysics.

Subsurface Properties Thermal conductivity 2.5 W K-1 m-1; heat capacity 2.0 x 106 J m-3 K; ice content 5% (saturated).

Three zones in the investigated cross section through the subsurface thermal field of the Schilt-horn Crest can be distinguished that are particularly interesting. Cross validation of the results from the two complementary approaches (i.e., numerical modeling vs. geophysical monitoring) enables an interpretation as follows:

(A) Homogeneous Temperatures and Resistivities The small temp. variations may be due to the fact that values are little below 0 °C and the energy input of recent warming is consumed by latent heat. ERT results also suggest high amounts of unfrozen water and a small ice content.

(B) Cold Temperatures The high resistivity in the ERT profile is probably caused by higher ice con-tent rather than by geological characteristics. This is also supported by the larger seasonal resistiv-ity changes pointing to higher contents of ice and unfrozen water than in the lower part.

(C) Permafrost Boundary at Shallow Depth This is mainly attributed to surface warming of the past century. Seasonal resistivity changes support the hypothesis that there is no permafrost.

Our thanks go to Michael Krauer for making us available the ERT data from his MSc thesis, to motivated students

for help in the field, and to the Schilthornbahnen AG for logistic support. Part of this study was financed by the

Swiss National Science Foundation (NF 20-10796./1), and geophysical field work was partly supported by PERMOS.

The results of the comparison of a modeled subsurface temperature field with measurements demonstrate that the general pattern of the subsurface temperature field in bedrock permafrost in high-mountains can be modeled using a 2D or 3D transient heat conduction scheme. Based on such an approach, temperature fields at depths that cannot be reached by geophysical measure-ments or direct measurements in boreholes, as well as for future scenarios, can be simulated.

Permafrost monitoring on Schilthorn is continued in the scope of PERMOS. The combination of thermal modeling, temperature measurements in boreholes and geophysical surveys bears poten-tial to further improve modeling and validation strategies.

Figure 6. Modeled temperature distribution in the Schilthorn Crest in 100 years based on the assumption of a uniform warming of +3 °C. In addition, the 0 °C isotherms are displayed for today (black line), in 50 years (dashed line), and in 100 years (dotted line). Dashed red lines indicate the location of the two 100 m boreholes.

The subsurface thermal regime of the Schilthorn Crest is mainly influenced by topography and transient effects.

The thermal regime can be characterized by a cold zone below the upper part of the north-ern slope, permafrost occurrence at shallow depth below the southern slope and in the lowest part of the northern slope, and rather homogeneous conditions at and below the area of the boreholes.

The modeled temperature field agrees with the results from ERT monitoring.