| 23 Dec 2025
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.
This manuscript presents a valuable and timely study by utilizing multi-frequency Electromagnetic Induction (EMI) data from the MOSAiC expedition to assess pressure ridge properties. A particular strength is comprehensive validation against drill-hole, airborne laser scanner (ALS), and multibeam (MB) sonar data. The work usefully explores the potential of EMI to estimate both total ridge thickness and consolidated-layer (CL) thickness. The manuscript requires some revisions for clarity. Below are my specific comments.
Abstract:
Please highlight the core innovation of multi-frequency EMI method in the abstract.
The conclusion section could be more concise, focusing on the advantages and limitations of the EMI method. The EMI method performs well in detecting ridges with a thickness of ≤ 10 m, but systematic biases occur in areas with high porosity or steep topography.”
Main text:
The entire paragraph in Line 62 regarding “ridge melting rate” has weak relevance to the main theme of the study and is suggested to be removed.
For the paragraph in Line 73, a smoother transition from “commonly used ridge detection methods” to the “EMI method” is advised. It is recommended to add little sentences explaining the limitations of conventional techniques. This will naturally lead to highlighting the advantages of the EMI method.
- Line 110: The choice of the five specific frequencies (1.5, 5.3, 18, 63, 93 kHz) needs clearer physical justification. Has a sensitivity analysis been conducted to clarify the response characteristics of each frequency to ridges with different porosities and thicknesses? This is particularly important because, the figures in the appendix, the thicknesses inverted by different frequencies vary.
The EMI method detects overall electrical conductivity, which the manuscript correlates with "macro-porosity". For the same porosity, will concentrate large porosity or scattered small porosity lead to different EMI responses? Moreover, based on drilling data from six ridges, the manuscript suggests that the porosity threshold of the consolidated layer detected by the quadrature of high-frequency EMI is approximately 20–30%. Is this threshold universal? In reality, the porosity of ridges will be lower than 20% after reconsolidation.
Section 3.1 :The matching methods between EMI data and drilling, sonar, and etc. should be clearly stated at the beginning of Section 3.1. This will prevent readers from having to wait until the results section to understand how the data fusion was performed. Additionally, while the manuscript compares EMI data with positioned drilling, ALS, and MB sonar data, there are differences in spatial resolution among these data. How were these differences evaluated? What causes the local discrepancies between EMI, the thermodynamic model, and the thermistor chain data?
After defining CL in Line 38, the definition is repeated in Line 46.
The terms “macro-porosity” and “rubble macro-porosity” are used interchangeably throughout the manuscript. What is the difference between these two terms?
Fig. 3 & Fig. 8:The continuous color bar for snow freeboard uses similar hues to the discrete colors overlain for ridge age groups.
A sentence regarding suggestions for the future application of the EMI method could be added at the end of the conclusion section. This will extend the implications of the study and provide guidance for subsequent research in this field.