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

Mountain permafrost is currently undergoing substantial changes due to climate change, and consistent warming and thawing has been observed at many permafrost sites worldwide. Geophysical methods are widely used to investigate the spatio-temporal permafrost evolution. Based on the known fact that ice exhibits a characteristic induced electrical polarization signature, we here propose a novel geophysical methodology for the improved imaging and quantification of ice content at alpine permafrost sites based on the measurement of both electrical conduction (resistivity) and induced polarization (IP).

Within this project, we test this method at selected sites in the Alps with typical alpine permafrost characteristics, where independent information for calibration and validation is available from previous geophysical investigations (ERT, refraction seismics) and temperature monitoring data (surface, borehole). At one of the sites, a SIP monitoring profile is installed and year-round measurements should help to understand the seasonal variation in the IP response.

The involved petrophysical model will be validated by means of SIP laboratory measurements (conducted by the project partners at University of Bonn, Germany) collected during controlled freeze-thaw cycles on soil/rock samples from the considered field sites, which will be independently analyzed for relevant model parameters (pore/grain size distribution, mineralogy, pore water chemistry).

Aims :

• Can electrical and electromagnetic measurements of SIP be used to reliably quantify ice content changes in mountain permafrost regions?

• Can the SIP response of partially frozen soils/rocks be predicted from a petrophysical model, and can the model be applied to real permafrost field cases?

• To what extent does the new methodology represent an improvement over the standardly used electrical resistivity and refraction seismic tomography approach in mountain permafrost research?
  

Methods: Spectral Induced Polarization (SIP) (field-scale and in the laboratory), Electrical Resistivity Tomography (ERT), Transient Electromagnetic, Refraction Seismic Tomography (RST), Electromagnetic Induction (EMI), automatic weather station, borehole temperature.

Study area: A variety of different permafrost sites in the Swiss, Italian, Austrian and German Alps

• Illustrations
  

  Induced Polarization measurements at selected study areas in the Swiss, Austrian and Italian Alps.

  The study sites are established long-term permafrost monitoring sites and provide comprehensive geophysical data and ground temperatures for validation. The morphology of the sites ranges from rock glaciers and talus slopes with high ice content to bedrock permafrost with lower ice contents. First SIP measurements have been conducted in summer 2018 at 8 different permafrost sites, in 2019 measurements have been repeated at 6 permafrost sites.

  

  Temperature evolution for modelled (red) and measured (blue) values over 16 years for different depths at the Murtèl rock glacier.

  Polarization values for the frequency range between 0.5 Hz – 225 Hz plotted in logarithmic scale. The SIP data were collected in a representative permafrost site in Lapires, Valais Alps. We observe different spectral responses for ice-rich and ice-poor parts of the profile, indicated as blue and red lines, respectively.