Year-round assessment of sea ice pressure ridges by multi-frequency electromagnetic induction sounding

Abstract. The thickness and consolidation state of pressure ridges are variables relevant for sea ice mass balance, melt processes and ecosystem habitat. We show how both variables can be detected by the multi-frequency electromagnetic induction (EMI) sounding, based on data collected during Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) in the central Arctic Ocean between November 2019 and July 2020. We validate the EMI results by collocating them to sea ice topography from the airborne laser scanner, underwater topography from multi-beam sonar, and sea ice thickness and structure from drill holes. Selected channels of low frequency give good estimates of total thickness, while selected channels of high frequency give good estimates of consolidated layer thickness. The MOSAiC EMI dataset was collected over a large number of ridge systems formed between freeze-up and break-up. Nine individual ridge transects can be used to track the seasonal development. The footprint size and sensitivity of the method make the EMI sounding appropriate for the detection of ridges with up to 10 m total thickness. Where available, the ridge structure data from other methods are consistent with our results. The superior temporal and spatial coverage of the MOSAiC EMI data permits further analysis that indicates a slow reduction of the total thickness towards the consolidated layer thickness. Interestingly, the consolidated layer thickness exceeds the level ice thickness by a factor of 1.6 to 2 and also shows, at least for individual ridges, a seasonal decrease. This may be a feature of the thin snow cover at MOSAiC. Multi-frequency EMI is a promising method for non-intrusive pressure ridge surveying with a large potential for pressure ridge dataset extension.