Advancing Permafrost Ice-Content Estimation: Independent Frequency Inversion of High-Frequency Induced Polarisation Data

This thesis presents methodological advances in the application of high-frequency induced polarisation (HFIP) to characterise frozen ground in permafrost environments. By developing an alternative independent frequency inversion approach combined with petrophysical modelling, this research establishes a robust framework for quantifying ice content distribution, with results validated through comparison with extracted permafrost cores. Field investigations were conducted at multiple sites representing contrasting permafrost conditions: three peat mires in northern Sweden (Heliport, Stordalen, and Storflaket) and a mountain permafrost site in the Italian Alps (Cervinia). The methodology combined broad-scale profiling to map lateral heterogeneity with highresolution surveys that served two purposes: resolving vertical transitions between frozen and unfrozen layers and providing datasets at scales comparable to extracted permafrost cores. The first detailed examination of reciprocal errors above 1 kHz was performed using the Chameleon II HFIP measurement system, applying established data quality assessment protocols to these high-frequency measurements. The 2D inversion results demonstrate smooth spectral variation of complex resistivity, with phase shift peaks in the 1-10 kHz frequency range, characteristic of ice polarisation. Petrophysical modelling of these inversion results enables quantification of 2D ice content distribution. High-resolution profiles resolved unfrozen layer thickness and revealed ice contents exceeding 50% in peat mires (up to 90%), while the mountain permafrost site showed predominantly ice-poor conditions (20%). This study demonstrates the reliability of HFIP through successive refinements in measurement and data processing protocols. The results collected across different seasonal conditions over multiple years contribute valuable insights to both permafrost research and practical applications, establishing HFIP as an effective tool for monitoring permafrost.