the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Mar 2025
Multichannel Analysis of Surface Waves (MASW) for the internal characterisation of the Flüela rock glacier: overcoming the limitations of seismic refraction tomography
Abstract. A multi-method geophysical campaign was carried out to characterize the subsurface of the Flüela rock glacier, Grisons, Switzerland, using electrical resistivity tomography (ERT), seismic refraction tomography (SRT) and multichannel analysis of surface waves (MASW). Surface wave analysis is not commonly used in mountain permafrost environments, although it could be applied to any dataset acquired for conventional SRT analysis if collected with low-frequency geophones. Here, we show that the use of the MASW method can be efficiently applied to highlight the presence of an ice-bearing layer, thus overcoming potential limitations of the common SRT analysis in these environments, such as tackling velocity inversions at depth or identifying layers which are invisible due to the lack of head wave arrivals. Our results are corroborated by synthetic models that simulate the propagation of seismic waves in a mountain permafrost environment with changing ice and water contents.
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RC1: 'Comment on egusphere-2025-962', Wojciech Dobiński, 19 May 2025
The work submitted for evaluation presents interesting and original research material obtained from field studies conducted using complementary ERT and seismic methods used for decades in studies on the occurrence of permafrost.
The area selected for the study is already very well known from similar permafrost studies, of which a great many have been conducted in this area since the 1970s. Neither the choice of methods nor the choice of the area is therefore particularly original and rather fits into the trend of research conducted for many years.
The article can be divided quite clearly into a part concerning permafrost and a part concerning methods. The authors focus strongly on the latter, because its specific application brings the most interesting scientific result. However, I will start with the issue related to permafrost.
Here, a very sensible approach to permafrost in general is worth noting, in which the authors avoid terms such as ‘permafrost creep’, ‘ice-rich’ or ‘ice-poor permafrost’, ‘permafrost hydrology’, etc. This is a big advantage for the work, because these very simplified and in fact incorrect terms are still and quite often used in permafrost research. It should be emphasized here that for many years there has been a general agreement regarding the definition of permafrost, which describes it as a state of the ground. Therefore, since permafrost is a thermal state, it is impossible to assign a material expression to it. The authors seem to understand this well by avoiding incorrect terms, but they do not do it consistently and unfortunately use some incorrect terms interchangeably. I have noted some cases of such use in the reviewed work, which I am sending as an attachment and which is part of the review.
The introductory part also lacks at least a short critical review of geophysical studies of permafrost in the studied region and a short review of the application of the MASW method in the study of permafrost in mountain and Arctic environments. Such a text would allow for better highlighting the achievement that the authors describe in the work. See for example:
Kula D, Olszewska D, Dobiński W, Glazer M, 2018. Horizontal-to-vertical spectral ratio variability in the presence of permafrost . Geophysical Journal International 214, 1, 219-231
The proposal to determine the presence of permafrost based on the results of original studies is very interesting, because the lack of agreement between ERT and seismic results is very well filled by MASW and this is an original and important result of these studies, most worthy of publication and testing also in other conditions and by other researchers.
However, I have the impression that the article focuses too much on methodological issues, which makes the article more engineering than scientific in nature. While characterizing the methodology and the results of empirical research well, it leaves the proposed models of permafrost occurrence without further discussion. As I noted at the beginning, we know that many similar research works have been carried out in this area since the 1970s. Therefore, in my opinion, it is also important to compare the obtained results with those that are already in scientific circulation. Against this background, the empirical model of permafrost occurrence constructed by the authors will be more credible, more universal and ready to be applied also in other permafrost occurrence environments. This may cause the work to become more universal and more widely cited in the scientific community.
- AC1: 'Reply on RC1', Ilaria Barone, 01 Aug 2025
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RC2: 'Comment on egusphere-2025-962', Anonymous Referee #2, 12 Jun 2025
This paper presents a multi-method geophysical campaign utilizing Electrical Resistivity Tomography (ERT), Seismic Refraction Tomography (SRT), and Multichannel Analysis of Surface Waves (MASW) to characterize the subsurface of the Flüela rock glacier. The study effectively demonstrates the complementary strengths of MASW in permafrost environments, particularly in overcoming some limitations of conventional SRT, such as issues with velocity inversions. The comparison of synthetic seismic models with field data is a valuable aspect of the work, corroborating the authors' interpretations.
The manuscript is generally well-structured and clear. The methodology is adequately described, and the figures are informative. The application of MASW in this challenging high-mountain environment is a significant contribution to permafrost research.
Main Comment:
My primary concern revolves around the interpretation of the ERT results concerning the hypothesized thin, water-saturated layer. While it is acknowledged that ERT sensitivity decreases with depth, it is generally expected that a 1-meter thick layer near the surface should be resolvable with a 3-meter electrode spacing. Furthermore, since you are considering a more conductive layer (interpreted as water-saturated sediment), the ERT method should exhibit heightened sensitivity to its presence.
It would be highly valuable to include synthetic ERT modeling to illustrate the expected response to such a thin, low-resistivity layer at the proposed depth (around 4m, based on Figure 4). A synthetic model would help clarify whether the observed field ERT data aligns with the theoretical detectability of such a feature, given the acquisition parameters and the assumed resistivity contrast. This would strengthen the argument for why the ERT model does not clearly resolve this layer despite its potential conductivity.
Overall, this is a well-executed study that provides important insights into the internal characteristics of rock glaciers. Addressing the main comment with additional synthetic modeling would significantly enhance the clarity and robustness of the ERT interpretation.
- AC2: 'Reply on RC2', Ilaria Barone, 01 Aug 2025
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RC3: 'Comment on egusphere-2025-962', Anonymous Referee #3, 07 Jul 2025
The submitted work proposes the use of Multichannel Analysis of Surface Waves (MASW) in conjunction with Seismic Refraction Tomography (SRT) and Electrical Resistivity Tomography (ERT) for permafrost characterization within a rock glacier. It effectively demonstrates the benefits of designing seismic surveys that can be processed not only for SRT but also for MASW. The complementary information provided by MASW is particularly valuable in the context of mountain permafrost studies, where it helps address the limitations of SRT and thus resolves structural discrepancies with ERT, ultimately leading to a more accurate understanding of subsurface composition.
The paper is generally well-structured and presents the methodology and materials in a clear and concise manner. I have just a couple of minor comments that I would like to raise.
A somewhat misleading statement in the manuscript is that the open-source library pyGIMLi was used for the processing of the SRT data. As pyGIMLi does not currently support first-arrival picking, it can be assumed that a different software package or custom code was used for this step. It would be helpful if the authors clarified which tool was employed for picking the travel times.
Furthermore, the use of 0 m/s as the lower bound in the colorbar of the SRT results shown in Figure 2(b) is questionable, as no real material exhibits a P-wave velocity of zero. It would be more appropriate to set a lower limit that reflects the minimum physically meaningful or measured velocity in the dataset.
Regarding the SRT imaging result, both the synthetic and field data experiments suggest the presence of a thin, water-bearing layer above the ice lens, with no critical P-wave refractions observed beneath it. In this context, it may be more appropriate to display the actual ray coverage instead of what appears to be a convex hull surrounding the resolved model domain. This would provide a clearer indication of which regions are sensitive to the data. In particular, I would assume that the area at and below the interpreted ice body has poor/no coverage and therefore, variations in the model in these regions likely only show the starting model. These areas should perhaps not be emphasized in the comparison and interpretation of the results.
Another comment relates to the reliability of the MASW results, particularly given the narrow frequency range in the presumed ice-rich area, the low velocities observed in the extracted dispersion curves, and the apparent variability —and thus uncertainty — of the S-wave velocity models below 10-15 meters. As the authors note themselves, model variations beneath this depth should be treated with caution as they are likely not constrained by the data anymore. Hence, I wonder why the fourth layer was considered for the inversion at all. I think it would also be helpful to add further information on the initial model parameter space definition, i.e., what lead to the choice of number of layers, thickness distributions (e.g., was this guided by site-specific information based on the other geophysical methods or prior investigations, etc.) and whether different initial set-ups were tested that lead to similar results.
Additionally, the final misfit values and error bounds used for the MASW inversion should be added in the text. For matters of consistency, the authors could also consider adding the staring conditions for the deterministic SRT and ERT inversions.
Due to some redundancies in the text, the authors could also consider merging the interpretation of the results in sections 4.1, 4.3 and 6.1 to provide a qualitative only description and comparison of the ERT/SRT and MASW results in 4.1 and 4.3 and the joint interpretation in terms of subsurface materials in 6.1.
A minor suggestion to improve the comparability of the geophysical results would also be to superimpose the final S-wave velocity models from MASW onto either Figure 2 or Figure 7. Alternatively, an additional figure comparing the final results across methods could be included instead. I think this would aid the reader in identifying and comparing the (structural) similarities between the three methods.
All in all I enjoyed reading this manuscript and believe it presents an original and relevant approach to permafrost investigations in a mountainous setting.
Citation: https://doi.org/10.5194/egusphere-2025-962-RC3 - AC3: 'Reply on RC3', Ilaria Barone, 01 Aug 2025
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