Estimating Permafrost Ice Content from Independent Frequency Inversion of High-Frequency IP Data: A Case Study from Heliport Mire, Abisko, Sweden

Abstract High-frequency induced polarisation (HFIP) measurements enable quantification of ground ice content in frozen media by capturing ice relaxation within the frequency range of 1 to 100 kHz. Existing parameterised inversion approaches may bias results by imposing an ice relaxation signature where none exists, assuming a Cole-Cole-type response that may not reflect the true dielectric behaviour of ice, and neglecting low-frequency polarisation. These limitations can lead to high data misfits and ambiguities in interpretation. This study presents an alternative approach that applies independent frequency inversion to directly derive complex resistivity spectra from field measurements, avoiding reliance on predefined models. The resulting inverted spectra provide a representation that more closely captures the true subsurface response. A second, petrophysical, inversion is then performed by fitting a two-component mixture model to the inverted spectra, weighted by the volumetric fractions of its components. One of these components is ice, allowing for the estimation of the volumetric ice content. The approach was applied at Heliport Mire (Abisko, Sweden), a permafrost peatland site, using two complementary profiles: a 50-m 2D profile that captured broad lateral variations of frozen to unfrozen conditions, and an 8-m high-resolution 2D profile that resolved the vertical transition between the upper unfrozen and underlying frozen layers. Independent frequency inversion, across 1 Hz to 57 kHz, successfully produced smooth, coherent spectral responses of true resistivity and phase shift across both profiles. Petrophysical inversion results show diverse conditions along the profile, identifying three distinct zones: ice-rich frozen peat (40-77% ice content), a thawed or degraded peat region (<10% ice content), and unfrozen forest (<5% ice content, effectively representing ice-free conditions). HFIP-derived ice content values were consistent with those derived from laboratory measurements on a permafrost core extracted along the profile. The high-resolution profile distinctly identified the boundary between unfrozen and frozen ground, as confirmed by direct probing measurements. Additionally, the petrophysical model resolves parameters such as shape factor and matrix permittivity, offering further insight into subsurface properties. This methodology advances ground ice characterisation by providing robust quantitative estimates of ice content while retaining spectral information with broader interpretative potential.