the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Oct 2024
4D imaging of a near-terminus glacier collapse feature through high-density GPR acquisitions
Abstract. Recent advancements in drone technology have introduced new possibilities for high-density 3D and 4D ground-penetrating radar (GPR) data acquisition over alpine glaciers. In this study, we present a 4D dataset acquired over a near-terminus collapse feature at the Rhône Glacier in Switzerland. The survey covers an area of approximately 100 m x 150 m, consists of over 100 parallel GPR lines with a lateral spacing of 1 m, and was repeated four times between July and October 2022. The glacier’s rough surface made such high-resolution and high-density surveying impossible with conventional acquisition methods, highlighting the advantages of the drone-based GPR system. The GPR data provide insights into the formation of the collapse feature as well as the evolution of associated glaciological structures. Our analysis suggests that the collapse initiated where the main subglacial water channel meanders and merges with a smaller secondary channel, coinciding with a small step in bedrock topography. After initiation, the subglacial cavity expanded through a combination of ice melting and mechanical failure, with ice lamellas detaching from the cavity roof. This process led to a progressive thinning of the roof, contributing to further instability. At the surface, these subsurface processes manifested as concentric circular crevasses, ultimately culminating in the collapse of the cavity roof. The GPR measurements also reveal the rapid temporal evolution of the main subglacial channel downstream of the cavity. During the observed summer, the channel underwent significant changes in both shape and size, which we attribute to the advection of warm air from the glacier’s large portal and the resulting increase in melt at the channel walls.
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RC1: 'Comment on egusphere-2024-3074', Anonymous Referee #1, 01 Dec 2024
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General Comment:
The paper by Ruols et al. presents an innovative application of drone-based ground-penetrating radar (GPR) technology to monitor the evolution of a near-terminus glacier collapse feature on the Rhône Glacier, Switzerland. The authors study a high-spatial-density 4D GPR dataset collected during four surveys in summer-autumn 2022. It focuses on the formation and temporal evolution of a subglacial air cavity and associated drainage systems. The authors demonstrate how the interplay of subglacial hydrology, mechanical failure, and inflow of warm air led to the development of circular surface crevasses and eventual roof collapse.
Building on methods developed in earlier work by a similar author team, this study achieves a full 4D reconstruction of a near-terminus glacier collapse feature. Considering the rarity of 4D GPR studies in the literature, particularly using drones over glaciers, this work is undoubtedly valuable and within the scope of The Cryosphere. Moreover, the focus on glacier snout collapse—a process increasing due to global warming—places the study in a timely and broader context.
By leveraging drone-based GPR, the authors overcome limitations of traditional GPR acquisition in rough terrain, with advantages for dataset resolution, acquisition speed, and safety. Drone-based ground-penetrating radar (GPR) technology has emerged as a transformative tool for subsurface exploration across various fields, including glaciology. Importantly, the results have implications for understanding glacier instabilities and their responses to external drivers, such as climate change.
Overall, the study is well-structured and clearly written, with figures and supplementary material that effectively support the narrative. However, there are areas where minor improvements could enhance the manuscript, particularly in expanding the discussion of uncertainties, and further contextualizing the study within a broader glaciological-climatological framework. I hope that my comments help the authors improve the manuscript.
Specific comments:
Abstract
I suggest slightly reducing the detailed results in the abstract and focusing on a concise summary of the findings. This would better emphasize the innovation of the applied methodology as a key tool, enabling new insights into glaciological processes, strengthening the connection between the method and the results.
Introduction
The introduction effectively sets the stage, but a broader explanation of the advantages and limits of 4D drone/helicopter-based GPR compared to other ground-based methods would strengthen the impact. For example, discussing how non-ground-based systems improve safety, enable data collection in inaccessible or hazardous areas, and allow for high-resolution, repeated surveys over short timescales could highlight their importance for advancing glaciological research. At the same time, acknowledging potential limitations, such as challenges in vertical positioning accuracy due to glacier surface changes or lateral obstacles as well as the interaction between the 3D GPR signal lobes with the topography (e.g. see Forte et al., 2019), would provide a balanced perspective and enhance the methodological transparency of the paper.
l51: Consider adding “and in proximity of strong lateral reflectors (Forte et al. 2019).”
- Forte, E., Basso Bondini, M., Bortoletto, A. et al.Pros and Cons in Helicopter-Borne GPR Data Acquisition on Rugged Mountainous Areas: Critical Analysis and Practical Guidelines. Pure Appl. Geophys. 176, 4533–4554 (2019). https://doi.org/10.1007/s00024-019-02196-2
ll54-61: There is a slight imbalance in the details provided for Ruols et al. (2023) compared to the other works (Jenssen et al., 2020; Tan et al., 2021; Valence et al., 2022). Rephrasing this section and including recent works such as Tjoelker et al. (2024) and SelbesoĞlu et al. (2023) would create a more balanced discussion.
- Tjoelker, A. R., Baraër, M., Valence, E., Charonnat, B., Masse-Dufresne, J., Mark, B. G., & McKenzie, J. M. (2024). Drone-Based Ground-Penetrating Radar with Manual Transects for Improved Field Surveys of Buried Ice. Remote Sensing, 16(13), 2461. https://doi.org/10.3390/rs16132461
- SelbesoĞlu, M. O., Karabulut, M. F., Oktar, Ö., Akpinar, B., Vassilev, O., Arkali, M., ... & Özsoy, B. (2023). Accuracy assessment of glacier depth monitoring based on UAV-GPR on Horseshoe Island, Antarctica.Turkish Journal of Earth Sciences, 32(8), 999-1012.
l92 Please, be more specific about “recently” as lakes formed during 1990s and in 2005 according to Tsutaki et al. (2013).
I also suggest providing some more context concerning glacier changes (including collapse) in relation to the ongoing climate change.
Methods (acquisition and processing)
While the authors often refer to a previous paper (Ruols et al. 2023) for methodological details, adding a few more information directly in this manuscript would improve clarity and accessibility. Unless I missed it, I suggest adding some information about:
- the drone set-up
- l112: is it a shielded antenna?
- A zoomed picture of the drone-GPR system could be added as a box to Fig. 3
- the development of the flight plan
- I did not understand if/how cross-profile were acquired or interpolated, as in Fig. 2 the acquisition seems to be done along parallel profiles only, but in Fig.6 both inline and crossline profile are shown).
- How the 3D GPR signal lobes interact with the topography in particular in relation with the height above the ground and the angle between the GPR and the surface?
Fig 2 and Table 1 - I might be wrong, but I think photogrammetry was never mentioned before (or after) in the text. Even if the acquisition of the orthophotos was carried out by ETH Zürich’s VAW Glaciology group, you should mention in the main text how you use this dataset and provide some details about it.
Please, when possible, substitute general statements with quantitative ones:
- L118: “with a high level of repeatability for the horizontal positioning (Fig. 2e).” Can you quantify this?
- l119: “However, differences in vertical positioning between acquisitions were present due to glacier melting” Do you mean that the altitude above sea level has changed, but the altitude above the surface is always 5 m since it is controlled by the True Terrain Following? Can you quantify this change?
- Ll120-122 – “Advantages of a drone- based GPR acquisition are clear, as high-density data could not have been acquired on the glacier surface because of the large crevasses.“ What are these advantages? E.g., safety, time, difficulty in pulling a ground-based GPR over rough terrain…
Ll191-192: Considering the highly heterogeneous case study with ice, air and water, what is the associated error of using a single velocity? Can it be estimated, perhaps using bedrock depth from the four acquisitions?
Results and Discussions
ll274-275: The picking process for the air cavity should be introduced earlier in the methods.
Table 2: Would it be possible to provide an error for the measurements?
ll302-304 - “Regarding the two subglacial channels, the main one, originating from the northeast, is likely to drain the majority of the glacier’s subglacial water system, whereas the second one, originating from the southeast, likely drains a constrained hydrological basin on the orographic left-hand side of the glacier.“ Could you provide some information to explain why you think this?
Broader implications
While the paper provides a thorough examination of a specific glacier collapse, it could enhance its impact by more explicitly contextualizing this phenomenon within the broader framework of global warming. Currently, the connection between the findings and global warming is only briefly mentioned through the reference (Egli et al., 2021b). While it is clear that a single collapse event cannot be directly attributed to the ongoing climate change, the increasing frequency of such events is linked to rising temperatures. Adding one or two sentences to address this point would help draw attention to the broader relevance of glacier snout collapses, which are not only indicative of cryospheric changes but can also have significant implications for human safety in mountain environments. This discussion could be incorporated into the Discussion or Conclusions sections, highlighting the importance of monitoring these phenomena in the context of climate-driven hazards.
Technical comments:
ll62-64: I suggest moving this paragraph after the discussion on terminal collapses (L75) to consolidate all relevant content in one section.
L95: The reference to "boxes b-c" and "d-e" in Figure 1 could be clarified by separating these into distinct references for each sentence.
Fig1: Consider making box (a) as wide as boxes (b+c) and highlighting the crevasses and collapse features in boxes (b), (c), and (d).
Ll146-149: The first 3 sentences fit more into the acquisition section. I suggest moving them.
l82 changes in changes of
L189: Wasn’t the height 5 m above the surface?
L93 tongue --> terminus
L231 sentences --> paragraph
L238 Please define DOP (it was defined in the label of Fig. 2, but should be defined also in the main text).
Ll262-263: This sentence fits more in the methods section than in the results.
Fig 9.: “Elevation” in the y-axis label could be repeated only once per side.
Citation: https://doi.org/10.5194/egusphere-2024-3074-RC1
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