Seasonal evolution and spatial variability of sea ice properties from multi-frequency electromagnetic induction sounding

This thesis advances electromagnetic (EM) induction sounding methods for measuring sea ice thickness and internal layers, focusing on the sub-ice platelet layer (SIPL) and slush. The SIPL is a layer of loosely aggregated ice crystals beneath Antarctic sea ice, characterized by exceptionally high biological activity, substantial regional contributions to sea ice mass, and its role as an indicator of ice shelf--ocean interactions. In the first study, we analyzed approximately 1000 km of multi-frequency EM survey data collected within a single season on fast ice in Atka Bay, eastern Weddell Sea, with a GEM-2 towed by a snowmobile. This unprecedented dataset allowed us to resolve both SIPL thickness and ice plus snow thicknesses. Using an open-source inversion framework, detailed spatial maps of SIPL thickness and conductivity were produced. SIPL solid fractions were estimated from SIPL conductivities and ranged from 0.11 to 0.28. Calibration in a zero-conductivity environment minimized sensor drift and offsets. Validation against drill hole measurements showed that both EM-derived SIPL thickness and ice plus snow thickness were accurate within a few decimeters. Results revealed regional patterns of platelet ice accumulation and provided the first bay-wide SIPL map including ice shelf fringes, with modal thicknesses around 5 m, local minima around 2 m and previously uninvestigated local maxima reaching up to 9 m in the south-eastern bay. In the second study, we use the GEM-2 to investigate slush from sea water flooding snow-covered sea ice. In Antarctica, this promotes snow-ice formation, contributing up to 25 % of ice thickness in young first-year ice in the Weddell Sea, while in the Arctic, it is rarer but may become more common under climate change, additionally posing travel safety concerns for Arctic communities. We showed that multi-frequency EM sounding can resolve slush and ice plus snow thickness jointly, with the optimal frequency combination achieving mean absolute errors below 5 cm for slush up to 60 cm in inversions of modelled data. Field surveys in Qikiqtarjuaq, Nunavut, Canada, confirmed robustness under variable conditions for slush up to 20 cm. A reliable height-step calibration routine using a wooden ladder was established and calibration parameters remained stable over two weeks for a GEM-2 sensor of the latest generation. In Antarctica, the combination of platelet ice and surface slush measurements could enable more accurate assessments of sea ice mass balance changes. In the Arctic, this work represents an initial step towards developing an operational system that can support local communities in mapping hazardous areas. Because access to potentially hazardous ice, particularly thin or slush-covered areas, is restricted when using an EM sensor mounted on a sled pulled by a snowmobile, we tested a drone-based system to extend its operational reach. A GEM-2 combined with custom altitude and attitude monitoring was flown as suspended load or sled-towed by a drone over landfast sea ice near Qikiqtarjuaq, Nunavut, Canada. Drone hover tests caused high EM noise at a 4 m distance, decreasing below thresholds required for 5 cm thickness resolution at 7 m. An in-flight calibration was tested, and the resulting calibrated EM signals differed only slightly from those obtained with ladder-based calibration, confirming effective calibration without the use of a ladder. When the GEM-2 was used with the drone, the thickness profiles agreed well with drill hole measurements, with mean thickness differences below 10 cm and closely matching thickness distributions. Flight stability was high, with sensor roll and pitch standard deviations below 3° and maxima around ±10°, ensuring reliable sensor orientation. These results confirm that the drone-borne EM sensor can provide accurate ice thickness measurements while minimizing operator risk and enabling surveys over thin or hazardous ice. Together, these studies advance EM methods for mapping sub-ice platelet and slush layers and introduce innovative approaches for airborne thickness surveys. By providing high-resolution, field-validated tools, this work enhances our ability to monitor and understand the mass balance and structural complexity of polar sea ice and improve safety on sea ice.