Mapping the thickness of slush on sea ice with multi-frequency EM induction sounding

Slush from flooding of sea ice contributes significantly to the sea ice mass balance in the Arctic and Antarctic and poses significant hazards for Arctic communities, affecting the safe use of sea ice for travel, hunting, and other activities. This study demonstrates the effectiveness of multi-frequency electromagnetic (EM) induction sounding for the joint retrieval of slush and ice thicknesses. For the multi-frequency GEM-2 instrument, we identified optimal frequency combinations, for example 5, 10, 20, 30, and 93 kHz, through inversion of synthetic data with realistic noise to achieve minimal mean absolute errors (MAE) of less than 5 cm for slush as thick as 60 cm.Field EM surveys, validated with coincident drill hole data, demonstrated reliable performance of the method under practical field conditions for slush layers up to 20 cm thick. Instrument calibration was robust but faced challenges at sites where snow and ice conditions deviated from the ideal one-layer model for snow and ice. The inclusion of varying sea ice conductivities in the calibration process enhanced reliability, and we show that a single instrument calibration remains stable for over a week for this instrument of the newest GEM-2 generation.The method’s transferability to airborne applications, such as drone-mounted surveys, offers the potential to eliminate operator risks associated with ground-based measurements on thin ice with thick slush. Overall, multi-frequency EM induction sounding provides a time-efficient and accurate tool for mapping the separate thicknesses of slush and of snow-plus-ice thicknesses.