Future instabilities develop subsurface e.g. by gradual thawing, weathering, hydraulic changes, fracture propagation. Here, “modelling techniques have advanced beyond the capability to confidently constrain the necessary data (Eberhardt et al., 2004).” We presently model slope instability up to 100 m deep and more - based on surface input data e.g. on fracture patterns. Enhanced subsurface insights into unstable slopes are a key challenge for slope instability research in the next years.
- Accurate subsurface monitoring of water levels i.e. hydrostatic pressure in soil and rock landslides: Especially electrical methods are sensitive to water saturation of soil and quantitative geophysics could allow quantification and monitoring of hydrostatic pressure and the spatial distribution of water levels.
- Accurate subsurface description of fractures (geometry, joint spacing, joint material): seismic, electrical and radar methods need to be modified to reveal subsurface properties and geometries of fractures, induced polarisation is sensitive to fracture infill.
- Accurate subsurface monitoring of permafrost changes in the state of freezing: In high alpine systems daily, seasonal and multiannual freezing cycles and permafrost have a significant impact on stability. Electrical, seismic and radar methods need to be developed to accurately monitor these in remote conditions.
Sensitivity of rock permafrost to climate change and implications for rock instability (SORP, DFG, 2008-2011):
In this project we developed two geophysical monitoring techniques, electrical resistivity tomography and seismic refraction tomography, that can monitor the spatial extend of frozen rock (i.e. permafrost) in rock faces in meters to tens of meters depth. Thawing permafrost rapidly decreases rock and ice-mechanical strength of rock material and presently heavily affects dozen of cables cars and infrastructure facilities in the Alps and other high mountain systems. In the example above, permafrost degradation spatially and temporally corresponds with highest rates of rock displacement (extensometers) along the crestline. With the developed methods we can investigate permafrost close to infrastructure within days up to a few decameters depth and can monitor changes daily.
ISPR Influences of snow cover on thermal and mechanical processes in steep permafrost rock walls:
In this project in cooperation with the SLF (Snow and Avalanche Research, Davos) we use regular LiDAR, borehole and crackmeter data, piezometers in fractures, extensometers and real (electrical and seismical) 3D-geophysics, nanoseismics and infrared thermal photography to understand the influence of snow accumulations on thermal and mechanical processes in steep permafrost rockwalls. This data feed into the development of an UDEC (Universal Distinct Element Code) mechanical and hydraulic underground model of fractured rock walls. Permafrost and snow melt water infiltration causes enhanced cryostatic and hydrostatic pressure regimes that may control mechanical processes and also rapid melting events along fractures. Here we try to combine thermal, mechanical and geophysical expertise from four different research facilities (also Universities of Zurich and Fribourg) for an enhanced understanding and prediction of rock slope failure in high mountains.
Electrical resistivity tomography monitoring of permafrost in solid rock walls
This article describes the first attempt to conduct ERT (electrical resistivity tomography) in solid permafrost-affected rock faces. Electrode design, instrument settings, and processing routines capable of measuring under relevant conditions were developed. Four transects, with NW, NE, E and S aspects, were installed in solid rock faces between Matter Valley and Turtmann Valley, Switzerland at 3070-3150 m a.s.l.. DC-resistivity in the transects was measured repeatedly during the summer and compared by applying a time-lapse inversion routine. Resistivity values were calibrated using observed rock surface conditions of thawed, damp rocks (1-8 kΩm), deeply frozen rocks (18-80 kΩm) and the transition from thawed to frozen rocks (8-18 kΩm).
Rock layers at depths of 2-6 m display a general trend of resistivity decrease in summer, corresponding to a persistent thawing process. Only the E, NE and NW transects display persistent, high resistivity permafrost bodies (> 50 kΩm) mostly at depths of 6-10 m. The maximum thaw depth of a continuous thawing front above permafrost is 6 m. However, the ERT results emphasize the role of heat transfer by deep-reaching cleft water systems. Thus, permafrost occurs in lenses rather than layers. ERT provides rapid detection of ice and water distribution in permafrost-affected bedrock.
Temperature-calibrated Electrical Resistvity Tomographies along the Zugspitze North Face
Changes of rock and ice temperature inside permafrost rock walls crucially affect their stability. Permafrost rocks at the Zugspitze were involved in a 0.3-0.4 km³ rockfall at 3.7 ka B.P. whose deposits are now inhabited by several thousands of people. A 107-year climate record at the summit (2962 m a.s.l.) shows a sharp temperature increase in 1991-2007. This article applies electrical resistivity tomography (ERT) to gain insight into spatial thaw and refreezing behavior of permafrost rocks and presents the first approach to calibrating ERT with frozen rock temperature. High-resolution ERT was conducted in the north face adjacent to the Zugspitze rockfall scarp in February and monthly from May to October 2007. A smoothness-constrained inversion is employed with an incorporated error model, calibrated on the basis of normal-reciprocal measurement discrepancy. Laboratory analysis of Zugspitze limestone indicates a bilinear temperature-resistivity relationship divided by a 0.5 ±0.1 °C and 30 ±3 kΩm equilibrium freezing point and a 20-fold increase of the frozen temperature-resistivity gradient 19.3 ± 2.1 kΩm/°C). Temperature dominates resistivity changes in rock below -0.5 °C, while in this case geological parameters are less important. ERT shows recession and readvance of frozen conditions in rock correspondingly to temperature data. Maximum resistivity changes in depths up to 27 m coincide with maximum measured water flow in fractures in May. Here we show, that laboratory-calibrated ERT does not only identify frozen and unfrozen rock but provides quantitative information on frozen rock temperature relevant for stability considerations.
Krautblatter, M., Verleysdonk, S., Flores-Orozco, A. and Kemna, A. (2010). Temperature-calibrated imaging of seasonal changes in permafrost rock walls by quantitative electrical resistivity tomography (Zugspitze, German/Austrian Alps). Journal of Geophysical Research - Earth Surface. (i.-f. 3.3) → dx.doi.org/10.1029/2008JF001209
Dräbing, D. and Krautblatter M. (2012). P-wave velocity changes in freezing hard low-porosity rocks: a laboratory-based time-average model. The Cryosphere, 6, 793–819, 2012. (i.-f. 3.6) → link
Krautblatter, M. (2010): Patterns of multiannual aggradation of permafrost in rock walls with and without hydraulic interconnectivity (Steintälli, Valley of Zermatt, Swiss Alps). Lecture Notes in Earth Sciences. Vol. 115: 199-214. → www.springerlink.com/content/e6680441tx048417/
Krautblatter, M. (2008): Rock Permafrost Geophysics and its Explanatory Power for Permafrost-Induced Rockfalls and Rock Creep: a Perspective. In: Proc. of the 9th Int. Conf. on Permafrost, Fairbanks, US: 999-1004. → www.nicop.org (Vol 1, IV).
Sass, O., Krautblatter, M. and Morche, D. (2007): Rapid lake infill following major rockfall (bergsturz) events revealed by ground penetrating radar (GPR) measurements (Reintal, German Alps). Holocene 17(7): 965-976. (i.-f. 2.8) → link