Improved ice quantification at alpine permafrost sites based on electrical and electromagnetic measurements of spectral induced polarization

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Mountain permafrost is currently undergoing substantial changes due to climate change, and consistent warming and thawing has been observed at many sites worldwide. As permafrost and especially its ground ice content can neither be assessed visually from the surface nor by standard remote sensing techniques, geophysical methods are increasingly being used to investigate the spatio-temporal dynamics of permafrost. In particular electrical resistivity tomography (ERT) has been proven a suitable tool to characterize and monitor permafrost occurrences. However, since the resistivity of rocks depends not only on ice/water content but also on porosity, pore connectivity, mineralogy, water chemistry and temperature, it is hardly possible to reliably quantify ice content from ERT measurements.Utilizing the fact that ice exhibits a characteristic electrical polarization signature, we propose a new geophysical methodology for the imaging and quantification of subsurface ice content at alpine permafrost sites based on the spatially resolved measurement of spectral induced polarization (SIP). Corresponding to the frequency range in which ice shows a maximum polarization (100 Hz to 100 kHz), we plan to complement direct SIP measurements by indirect SIP measurements by means of the time-domain electromagnetic induction method (TEM), in order to compensate a possible lack of information in the direct SIP measurements at higher frequencies. The SIP spectrum will be described as a superposition of the background signal and the characteristic ice response, and ice content will be determined on the basis of a newly developed petrophysical model of the SIP properties of (partially) frozen rocks.The proposed new methodology for the quantitative, spatially resolved determination of ice content will be tested at selected sites in the Alps with typical alpine permafrost characteristics, where independent information for calibration and validation is available from previous geophysical investigations and existing monitoring data. The involved petrophysical model will be validated by means of SIP laboratory measurements in the course of controlled freeze-thaw experiments on rock samples from the considered field sites, which will be independently analyzed for relevant model parameters.To our knowledge the project represents the first comprehensive study on the quantitative imaging of subsurface ice content in alpine permafrost terrain based on combined SIP/TEM measurements. While the laboratory studies and the petrophysical model development contribute to an improved fundamental understanding of the electrical properties of (partially) frozen porous media, by considering various types of alpine permafrost occurrences the field studies are highly relevant with a view to an effective permafrost monitoring in the context of global warming.