Resolving ice content heterogeneity within permafrost peatlands using high-frequency induced polarisation.

Permafrost peatlands are highly vulnerable ecosystems in a warming climate; their thaw greatly impacts carbon storage capacity and endangers existing landscape morphology. Due to their remoteness and, in some cases, protected status, it is difficult to characterise and monitor the subsurface using invasive methods. Geophysical investigations are useful in such cases allowing relatively rapid and extensive subsurface mapping. We focus here on the emerging high-frequency induced polarisation (HFIP) method, which can be effective in permafrost hydrology research as the geoelectrical properties of frozen water display a characteristic frequency-dependence between ranges of 100 Hz and 100 kHz.

HFIP field measurements were conducted using the Chameleon-II equipment (Radic Research) on two peat permafrost sites located in Abisko, Northern Sweden: Storflaket mire and Heliport mire. The sites have been subject to routine permafrost monitoring since 1978 and are known to have an upper peat layer underlain by a silt-rich subsoil. We present the results of 2D surveys measuring frequencies ranging from 1 Hz to 57 kHz, which capture a high-frequency phase shift peak. Field data are inverted for each measured frequency separately with ResIPy, using an appropriate data error quantification model. The spectral data analysis captures heterogeneity within the subsurface, i.e., layered medium, permafrost mire boundary and ice-rich versus ice-poor regions. Identification of spectrally distinct regions allows the application of an appropriate relaxation model. For this study, we apply a two-component mixture model for ice-content estimation. Our results extend the existing knowledge at this site by quantifying ice content in a 2D plane, thus improving the foundation for further modelling studies.