Abstract
Abstract. The ice content of the subsurface is a major factor controlling the natural hazard potential of permafrost degradation in alpine terrain. Monitoring of changes in ice content is therefore similarly important as temperature monitoring in mountain permafrost. Although electrical resistivity tomography monitoring (ERTM) proved to be a valuable tool for the observation of ice degradation, results are often ambiguous or contaminated by inversion artefacts. In theory, the sensitivity of P-wave velocity of seismic waves to phase changes between unfrozen water and ice is similar to the sensitivity of electric resistivity. Provided that the general conditions (lithology, stratigraphy, state of weathering, pore space) remain unchanged over the observation period, temporal changes in the observed travel times of repeated seismic measurements should indicate changes in the ice and water content within the pores and fractures of the subsurface material. In this paper, a time-lapse refraction seismic tomography (TLST) approach is applied as an independent method to ERTM at two test sites in the Swiss Alps. The approach was tested and validated based on a) the comparison of time-lapse seismograms and analysis of reproducibility of the seismic signal, b) the analysis of time-lapse travel time curves with respect to shifts in travel times and changes in P-wave velocities, and c) the comparison of inverted tomograms including the quantification of velocity changes. Results show a high potential of the TLST approach concerning the detection of altered subsurface conditions caused by freezing and thawing processes. For velocity changes on the order of 3000 m/s even an unambiguous identification of significant ice loss is possible.
Highlights
According to the theoretical suitability of repeated seismic measurements to permafrost related research, a time-lapse refraction seismic tomography (TLST) approach and its potential to observe temporal changes in ice and water content in alpine permafrost will be evaluated in this paper
Apart from a vast research related to reflection seismic monitoring of deep reservoirs in exploration geophysics, similar efforts to investigate the potential of a time-lapse refraction seismic tomography approach for the observation of shallow targets have not been reported so far
The novel time-lapse refraction seismic tomography (TLST) application presented in this paper is based on the assumption that P-wave velocities within the subsurface are affected by seasonal or inter-annual freezing or thawing processes, and that repeated refraction seismic measurements allow the assessment of such temporal changes
Summary
Monitoring of permafrost in polar and mountainous regions becomes more and more important in the context of ongoing global warming. Apart from ERTM, repeated refraction seismic measurements theoretically have a considerable potential to observe permafrost evolution, since the seismic P-wave velocity (vp) is highly sensitive to variations in the ice or water content (by changes in vp between frozen and unfrozen water of up to 2000 m/s). Seismic refraction is often considered to be a valuable additional method to verify subsurface structures identified by ERT (e.g. Hauck and Vonder Muhll, 2003; Kneisel et al, 2008). It is generally capable of discriminating unfrozen and frozen sediments or massive ice, and is a common method to determine active layer thickness. P-wave velocities range from ca. 2500–4200 m/s for Published by Copernicus Publications on behalf of the European Geosciences Union
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