A systematic approach was used for the interpretation of the electrical resistivity tomography carried out on two permafrost mounds at Umiujaq in Nunavik, Canada, to assess their internal structure and conditions. Prior information under the form of a geocryologic model of the permafrost mounds was integrated in the inversion of the pseudo-section of apparent electrical resistivity. The geocryologic model was developed from the synthesis of previous field investigations, including shallow and deep sampling, temperature and electrical resistivity logging, and cone penetration tests performed in the permafrost mounds. Values of electrical resistivity were ascribed to the different layers making of the geocryologic model to define a synthetic resistivity model of the permafrost mounds used as a reference model to constrain the inversion. The constrained resistivity model clearly show the presence of ice-rich cores in the permafrost mounds underscored by high resistivity values in excess of 30 000 Ωm, while the unfrozen zones surrounding the permafrost mounds are characterized by resistivity values lower than 1000 Ωm. The spatial distribution of unfrozen water and ice contents in the permafrost mounds were also assessed according to empirical relationships between the electrical resistivity and water contents. The ice content is highly variable and can be as high as 80% in the ice-rich cores, while the unfrozen water content varies between 2% and 5%. The integration of prior information in the inversion process leads to a more realistic constrained resistivity model showing sharp resistivity contrasts expected at the boundaries such as the permafrost table and base.
A finite-element, one-dimensional, heat conduction model, which takes thaw settlement into account following drainage of excess water produced by the melt of ice lenses at the permafrost boundaries, is used to assess the thermal response of a permafrost mound in Northern Québec to different scenarios of climate warming. In addition to the cryostratigraphy of the mound, the unfrozen water content, thermal conductivity and volumetric heat capacity of the marine sediments comprising the mound were integrated in the simulation.Warming rates from 0.03 to 0.01 C/year are predicted in the mound for a gradual linear increase in mean annual air temperature of 0.05 C/year over a 100 year period. Downward thawing occurs at the permafrost table at rates of 1-13 cm/year but there is also upward thawing from the permafrost base at rates of 2.4-5.8 cm/year. The thermal response of permafrost is not linear with time. At the end of the 100 year period, thaw settlement predicted was 1.4 m and the active layer was 3.22 m thick over a talik of 1.46 m thick. This is in comparison to an active layer of 2.14 m thick over a talik of 1.86 m if thaw settlement is not considered in the simulation. Thaw settlement, the direct result of ice melting in permafrost, brings permafrost nearer to the surface and accelerates its thawing. It should be included in any numerical simulation.
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