Pore Structure and the Low Frequency Permittivity of Sea Ice

Field and laboratory measurements of the dielectric permittivity of first-year sea ice both show that below a frequency of about 10 Hz the real part of the relative permittivity (ɛ') increases with decreasing frequency. Field measurements in Barrow, Alaska and McMurdo Sound suggest that this rise in low frequency ɛ' steepens as the ice warms, and is confined primarily to the upper 0.50m of the ice cover as it approaches maximum thickness. We propose that this behaviour may be related to membrane polarization occurring in the pore structure within the ice. With ice-liquid interfaces carrying a net charge, an electric double layer forms within the brine filled pores. Polarization occurs at grain boundaries, intragranular films and "necks" in the pore structure where the effective thickness of the double layer approaches the width of the pore resulting in differential transport of ions. This process is dependent on both the characteristic lengths and radii of pores relative to the length and radii of the "necks" or the geometry of inter/intragranular brine layers. By representing the measured dielectric permittivity in terms of a Cole-Cole model it is possible to show that the distribution of pore sizes evolves with temperature. Derived values of complex conductivity are also examined in relationship to the temporal evolution of pore geometry including smoothness of the pore-ice interface.