Contrasting geophysical signatures of a relict and an intact Andean rock glacier

de Pasquale, Giulia; Valois, Rémi; Schaffer, Nicole; MacDonell, Shelley

doi:https://doi.org/10.5194/tc-16-1579-2022

Articles | Volume 16, issue 5

https://doi.org/10.5194/tc-16-1579-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/tc-16-1579-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 16, issue 5

Research article

|

04 May 2022

Research article |

| 04 May 2022

Contrasting geophysical signatures of a relict and an intact Andean rock glacier

Giulia de Pasquale, Rémi Valois, Nicole Schaffer, and Shelley MacDonell

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Final revised paper (published on 04 May 2022)
Preprint (discussion started on 12 Nov 2020)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'comments to the Authors and Editor', Anonymous Referee #1, 11 Dec 2020
- AC1: 'Reply on RC1', Giulia De Pasquale, 22 Jan 2021
RC2: 'Review on tc-2020-306', Anonymous Referee #2, 21 Dec 2020
- AC2: 'Reply on RC2', Giulia De Pasquale, 22 Jan 2021
RC3: 'Review Comments', Lukas U. Arenson, 28 Dec 2020
- AC3: 'Reply on RC3', Giulia De Pasquale, 22 Jan 2021

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (further review by editor and referees) (05 Feb 2021) by Christian Hauck

AR by Giulia De Pasquale on behalf of the Authors (08 Feb 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (09 Feb 2021) by Christian Hauck

RR by Anonymous Referee #3 (01 Mar 2021)

Suggestions for revision or reasons for rejection

Thank you for revising he manuscript. The new version has improved and reads better than the initial version. I also would like to thank the authors for their responses to the reviewer’s comments. However, I feel that the paper is not ready for publication at this point and I wish that the authors would take more time when writing and revising the text. As an authors and co-author you must read every sentence carefully, which means more than once, and make sure that it follows the logic of the text and that you are not using terminology that does not fit, or that you suddenly switch to speculations in the middle of a factual argumentation. I also encourage the authors to carefully read the original (!) references that are cited and make sure that what you are saying in the manuscript was actually referenced in that document. For example, on line 31 you attribute the term stagnant for a rock glacier to Barsch (1996). However, such a statement is not made in that book, which makes me question, if you carefully checked all your references and made sure that what you write is actually cited in the original reference.

The annotated version attached does have several comments and suggestions that I encourage the authors to have a close look. Below are some examples where the authors should pay more attention:
- Rock. The manuscript uses “rock” synonym to solid particle. This is incorrect and leads to mistakes such as consolidated rock material (Line 320). In that line (320) the manuscript also talks about consolidated ice-rich material, which again is not correct in the absence of associated consolidation pressure.
- The authors also need to pay attention on how the porosity is defined. Phase change doesn’t affect porosity, unless you have an increase in the pore volume in response to the expansion of the ice during freezing.
- The authors claim that the acquisition of data and their quality were limited because of the environment. This is not accurate and the fact that the group had limit resources can’t be blamed on the environment . If the group had spent another 4 weeks and had better equipment, it would have been different.
- Throughout the manuscript the authors use the term reserve (or reservoir) as well as resource. However, these terms have not been defined and since these definitions are not clear it would be good if the authors were to define what it means in the context of the research.
- At various places in the manuscript it would be good if the authors were to define if they are using volumetric or gravimetric percentages. There is a significant difference.
- Line 35: “which suggest that rock glaciers in the European Alps have experienced increased melt rates in recent decades”. However, a rock glacier cannot melt. Melting is the physical process of phase change from solid to liquid. This is a scientific publication, and the authors must use precise terminology.
- Line 127: “…even though their employment in mountainous environments demands specialised techniques for sensor coupling, data acquisition and inversion routines.” I disagree as it ignores arctic conditions where course materials also do exist.
- There is no need to say “ice (which is assumed to be an electrical insulator)” (line 144). Please be more concise and just say: “low conductivity”. There is no need to artificially add more wording. This is a scientific publication, which should be factual and to the point.

Finally, Section 5.5 seems to be out of context, i.e. not related to the actual work. The research is about the characterization of the two rock glaciers and not the assessment of the hydrological contribution. The authors have not presented any assessment or calculations that would support Section 5.5. For example, on line 331 the authors write: “Each have a distinct and important hydrological role.” Based on the findings from the paper, one can argue that this might be true, and I understand that the authors follow this statement with some wording and thoughts. However, these statements do not follow a scientific line of arguments and assessments and should only be formulated in form of a working hypothesis that requires additional investigations or assessments.

Referee Report: PDF

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RR by Anonymous Referee #1 (02 May 2021)

Suggestions for revision or reasons for rejection

Dear Editor, dear authors,

The revised version of the manuscript tc-2020-306 shows a significant work by the authors to address all the comments from the reviewers. This shows a real interest in improving the quality of the manuscript. However, in my opinion, the manuscript was interesting due to the discussion of the geophysical algorithms using different regularization approaches. Such discussion permitted to address different interpretation of the results obtained for the same data. The authors have now simplified the geophysical component of their study and only present the results obtained by the petrophysical joint inversion scheme presented by Wagner et al. (2019). In the current version of the manuscript, the results are presented and interpreted without addressing any concern regarding the uncertainty inherent to the inversion, for instance the non-uniqueness or the influence of poor data into the final inverted model. The lack of any ground truth or complementary information hinders any discussion of the inversion results. Likewise, the interpretation of the results is rather speculative due to the lack of ground truth and the limited amount of geophysical data.
In my last revision, I recommended the conduction of numerical investigations to enhance the selection of parameters for the inversion schemes presented before. The authors decided to avoid that and just present the qualitative interpretation of the inverted images. In my opinion, the presentation of inversion results of two disconnected profiles cos not constitutes a case study. I am sorry for the effort of the research team, as I am sure that the collection of the two data sets is not an easy task. However, in my opinion, the two profiles are not enough to understand the hydrogeological dynamics of the site, and the discussion provided does not provide any news insights on the application of geophysical methods for the characterization of rock glaciers. Hence, my recommendation for the current version of the manuscript is to be rejected.
The manuscript shows a clear lack of balance. There is lengthy introduction, where different hydrogeological aspects are discussed. However, there is no real incorporation of the geophysical data into hydrogeological model. I am not sure whether the scarce geophysical information could be of any help in the groundwater management of the site. Maybe an alternative is to present further data aiming at quantifying the total ice content in one (or each) of the rock glaciers? – The presentation of two or three profiles collected over the area may also permit to evaluate the uncertainty of the inversion results, for instance comparing the values resolved for the same location using data collected in perpendicular profiles. Maybe also the authors could present a more careful analysis of the different parameters used within the inversion, for instance based on the discussion of Mollaret et al. (2020) regarding different regularization parameters, Archie’s constants, or including even a poor estimation of surface conductivity. In this regard, my opinion is that the previous version of the manuscript provided a much more scientific discussion and could easily address the relevance of including structural information to reduce the uncertainty in the inversion and improve the estimation of hydrogeological parameters. As I understand from the long introduction, the adequate estimation of hydrogeological information is the main interest of the manuscript.
I also think that the authors could provide more information regarding the quality of their data, especially the seismic data presented in Figure 6 show ray paths that evidence a poor processing of the data. Are these trajectories really reproduced in the inversion? Maybe this is the reason that the inversion does not converge. In this regard, I would recommend the authors to provide a more careful analysis of the data. It is not adequate to interpret results with no convergence and a chi-square value of two is clearly an evidence of the inversion not being able to explain the data, even with the relative high error parameters mentioned by the authors. This example clearly demonstrates that there is a big issue with the data presented, thus, with the inverted images. However, such issue is never addressed in the manuscript and the hydrogeological parameters are interpreted as valid.
I am also attaching a marked PDF (I really apologize for spelling and grammatical errors), which may be of use for the authors to revise sections of their manuscript, in case the study is accepted by the recommendation of the other reviewers, or if they decide to submit it to another journal.

Best regards

Referee Report: PDF

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ED: Reconsider after major revisions (further review by editor and referees) (05 May 2021) by Christian Hauck

Dear authors,

first of all I have to apologise that it took so long for this second reviewing round. I appreciated your efforts in responding to the three reviewers of the first review round and the revised manuscript. Two of the original reviewers provided now re-reviews, which you find below.

As you can see, both reviewers still think that the paper is not yet ready for publication, although they acknowledge that the new version has improved and reads better than the initial version. Reviewer 1 recommends minor revisions with a focus on revising the text. The reviewer points out quite a large number of details though, which are currently not written correctly or where the logical flow of the text is not correct. Suggestions for improvements are given in the review text and an annotated manuscript.

Reviewer 2 is more critical. By taking out most of the methodological novelty of the study (the comparison of the two different joint inversion algorithms) in the current version of the manuscript, the results are now presented and interpreted without addressing any uncertainty inherent to the inversion (e.g. non-uniqueness or the influence of poor data into the final inverted model). According to the reviewer, the lack of any ground truth or complementary information hinders any discussion of the inversion results.
The reviewer states also, that two profiles are not enough to understand the hydrogeological dynamics of the two sites, and that the discussion provided does not provide any new insights on the application of geophysical methods for the characterization of rock glaciers. Further, the quality of the seismic data should be revisited (Figure 6), as ray paths are shown that apparently evidence a poor processing of the data and concerns regarding convergence and the obtained chi-square value has been raised. More points are addressed in the evaluation of reviewer 2 as well as in the annotated manuscript.

Taking all reviews (and their recommendations) into consideration I have to admit that I also feel that the novelty of the study has actually been reduced during the revision. One reviewer rates it as "fair", the other as "poor". Two very different rock glaciers with different ice contents (or activity status/kinematics) are not so difficult to distinguish with the help of geophysical measurements. That has been presented many times before in other studies, although of course rarely with a petrophysical joint inversion scheme. A generalisation and/or detailed analysis of the results as case study is difficult in the present case due to the lack of additional data and ground truth. But I have the impression that the authors removed all parts that were criticised during the first review round from their manuscript, instead of improving the criticised parts as suggested by the three reviewers (which would be the aim of a scientific review process).

On the other hand, in my opinion the study has still a high potential to contribute significantly to the scientific knowledge in this field and deserves publication, especially as field studies from permafrost in the Central Andes are rare. But for this, the many points raised by the reviewers have to be addressed adequately, i.e. by adding analyses regarding the hydrogeological setting/significance and especially regarding the data and inversion quality and their uncertainties.

I hope these comments will help you to address the reviewer concerns adequately.

Kind regards,
Christian Hauck
Editor

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AR by Giulia De Pasquale on behalf of the Authors (15 Oct 2021) Author's response Author's tracked changes Manuscript

ED: Reconsider after major revisions (further review by editor and referees) (05 Nov 2021) by Christian Hauck

ED: Referee Nomination & Report Request started (09 Nov 2021) by Christian Hauck

RR by Lukas U. Arenson (04 Jan 2022)

Suggestions for revision or reasons for rejection

Thank you for revising the manuscript. In general, the manuscript has improved, and I appreciate the replies provided to various comments. However, the overall lack of data and challenges with data quality still exists. Unfortunately, the authors follow a general tendency of presenting a few data as soon as they become available instead of investing in acquiring sufficient good data. I do understand the significant limitations and challenges related to collecting data in the environment they are working in. But that is reality and if it takes 10 campaigns and 10 years, with associated research budgets, then that is the reality the researchers must face. At this point I think that the authors present what is possible with the data available and I leave it to the editor to decide if it warrants publication or not. Independent on the editor’s decision I would really encourage the authors that they make sure that they have sufficient resources in future work so that they can collect sufficient data and prepare a manuscript where a research hypothesis can be presented that is well supported by field data (or numerical modelling, if the authors decide to present a modelling paper in the future).

With respect to the revised version, I do have some minor suggestions and comments that should be addressed if the editor decides that the manuscript is worth to be published in this Journal.

Title: As the study only presents data from one relict and one intact rock glacier, the title should be adjusted to: “Contrasting geophysical signature of a relict and an intact Andean rock glacier”
Line 1: Rock glaciers are not special types of glaciers as such it is scientifically wrong to say “other glacier types”
Line 19: Herrington et al. (2018) did not write that the flow is coming from the rock glacier as the current wording implies. They have shown that water is passing through the rock glacier, which had an impact on water temperature and in consequence on the aquatic life. Please have a careful read at what Herrington et al. (2018) concluded.
Line 32: What do you mean by “often unfrozen pore water”?
Line 33: Cogley et al., 2011 is a really bad reference for that, as they only say: "Rock glacier: A mass of rock fragments and finer material in a matrix of ice, showing evidence of past or present flow."
Line 40: I encourage the authors to cite the IPA Rock Glacier Action Group documents: https://www.unifr.ch/geo/geomorphology/en/research/ipa-action-group-rock-glacier/
Line 90: why is this unpublished? The inventory is publicly available.
Line 95: I suggest deleting “mining” since the mining industry actually doesn't need as much water as other industries. I therefore suggest to simply combine all industries and state “industry”.
Line 148: The inclusion to Table 1 isn’t needed. Those are values cited many times in other references and as such a cross reference will just do.
Line 164: Delete the reference to Table 1
Line 246: Make sure you use sediments and not rocks: “unconsolidated sediments”. This not only applies in this line, i.e., check the whole manuscript.
Line 249: It should be “debris”, not “debrids”. This typo also appears at other places, so please check the manuscript.
Line 269: Based on the electrode spacing I assume that error in the estimation of the active layer thickness is at least 25%. Is that correct, and if so, please add the error to the estimate.
Line 341: “Dos Lenguas rock glacier”
Line 342: “volumetric ice content”
Line 342: Delete the % sign from the text. A unit sign (keep in mind % is a unit) only appears behind a number. If you want to use it in the text, write “percentage”. Check the whole manuscript as you can probably delete the symbol in most places anyway.
Line 373: This Section 5.5 does not provide value and should be deleted. The authors should focus on the objective of the manuscript, which is the geophysical investigations. The discussion on hydrogeology, in fact it is a discussion on hydrology only, is completely insufficient. Many key references are missing, and such a discussion would need to be expanded significantly. If this is an important topic for the authors, I encourage them to partner with a hydrologist and a hydrogeologist and write an independent paper that is dedicated to that topic. In its current form it is inadequate and should not be accepted. It’s telling that this aspect isn’t reflected in the conclusion or outlook section. Not even the key sentence from Section 5.5 (Line 387-388).
Line 406: replace “complicated” with “challenging”.
Figure 1: Elevations in masl.
Figure 9: Middle image should be b).

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RR by Anonymous Referee #1 (18 Jan 2022)

Suggestions for revision or reasons for rejection

Dear authors,

I am sorry for the long time used to review the manuscript. In general, the English formulations in the text has been improved. However, I fail to understand the technical improvements of this new version. I still think that the authors removed the interesting components of the study to avoid technical revisions, yet now the manuscript is only presenting data collected along two independent profiles with no evaluation of data error, model uncertainty or validation of the results. Hence, at the best this provides only a speculative interpretation of geophysical results.
It took me some time to come along with the revision as I do not want to sound too negative and I am interested to provide a constructive criticism. I understand that collecting further geophysical or direct data is impossible. Hence, after long thinking, my suggestion is still that the authors provide a numerical study where they investigate the actual uncertainty in their results. Here they can provide 3 to 5 scenarios with varying ice/rock/air/water content and maybe lateral variations. I would recommend to use exactly the same settings (quadrupoles and gather shots) as they use for the inversion of their results (i.e., after removal of outliers and geophones with poor coupling). Considering that the authors will have a real value (i.e., the numerical scenario), they can prove how good or bad their measuring setting can resolve different subsurface conditions, regarding both the geophysical heterogeneity and the air/water/ice/rock contents. The authors should also consider adding different noise levels.
The numerical study could investigate the ability of the geometry used to collect the data to recover for instance the geometry of the rockglacier bottom (i.e., the contact to the bedrock) and the depth of the active layer. This is mentioned by the authors in line 27-28 as critical parts of the investigation of rock glaciers. Hence, the authors could establish a numerical study (with lateral variations in both the depth of the permafrost and the rock glacier) where ice content are defined as well defined anomalies and investigate how well the given geometry could recover such values. This may address the uncertainty in the ice and water content estimates. Maybe also to address components regarding the regularization that were mentioned during the very first version of the manuscript.
The authors could also address the uncertainty associated to defining incorrect values for the Archie parameters (i.e., a, m n, and conductivity of the pore water).

Line 56 – 57 are incorrect and need to be addressed. The inversion is an ill-posed problem, not only affected by non-uniqueness. The presence of small uncertainty in the data generates large uncertainties in the inversion results. This is poorly addressed by the authors. The authors only discuss non-uniqueness. However, the influence of poor data quality has not been mentioned as a further source of error. Thus, I still believe that a numerical error could provide clear insights on the limitations of the results presented, especially the estimation of rock, air, water and ice content.
Line 73: The joint inversion does not reduce the uncertainty. There is no computation of the uncertainty within the inversion algorithm. It helps to solve for a consistent model for the resistivity and the seismic data. Moreover, the solution converges to values that provide plausible estimations of rock, ice and air water content (as mentioned in the line 73 of the manuscript)..However, there is no computation of uncertainties within the approach; this is still a deterministic solution. Hence, I believe that a numerical investigation based on ther actual configuration (the measurements collected after removal of outliers) and simulating different conditions of the subsurface could really provide a estimation of the uncertainty in the results presented.
Line 140 – 145 need to be carefully written. I provided some suggestions.
Line 151 – what is the contact resistivity? This needs to be corrected
Line 175 – repeating and stacking sound redundant in the sentence. Maybe the authors could rephrase it?
Line 183 contains technical errors that need to be addressed
Line 203: Please provide clear description of the methods used to estimate the error parameters? Also about the parameters used to detect and remove outliers?
Line 225: which petrophysical constants values were used in the inversion of the data (i.e., a, m n, and conductivity of the pore water)? – was there any analysis on the dependence of the proposed results to those values?
Line 241: if there are effects due to surface conductivity, could the authors discuss the applicability of the petrophysical joint inversion? – in the joint inversion deployed, the underlying model is only related to electrolytic conduction through Archie’s model; thus, neglecting surface conductivity. If surface conductivity is dominating at the site, the estimates of rock, ice, water and air are incorrect . For instances, high surface conductivity are then interpreted as high porosity and water content. Yet this could be also related to lower porosity (i.e., due to the presence of fine grains), or higher ice content and the polarization of the EDL at the ice-water contact as mentioned in the line 242. This is an interesting point that the authors need to develop. However, it is only mentioned here to interpret the results and not addressed in the manuscript (i.e., lines 243 – 253).
Line 284: what the authors imply with the marked sentence in the PDF? The P-wave velocity is defined by the ray paths within the inversion, not by other data. Maybe the authors mean that there are poorly resolved areas? – in such case, this needs to be rephrased. Is the poor coverage due to poor contact geophone-ground or due to the filtering within the processing of the data? – Maybe the numerical investigation can address for the particular measuring setting in this study (after removal of noisy measurements) and the ability of the data to detect anomalies within the poorly covered area are resolved?
Line 347: I think the authors start an interesting topic by mentioning that ”in many cases the implementation of petrophysical joint inversion can be limited by lack of proper petrophysical models”. Could the authors develop further in this? - Do the authors consider that the 4-phase model implemented is not adequate? What petrophysical model needs to be implemented?
Line 349-350: in which cases the proposed petrophysical inversion is not working? The authors should provide clear information. Address this directly with their data.
Figure 6: the raypaths in the RST data could be plotted in a better y-axis to permit a better visualization of the data.

Referee Report: PDF

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ED: Publish subject to minor revisions (review by editor) (02 Feb 2022) by Christian Hauck

Dear authors,

first of all let me apologize for the long time it took for this decision to arrive; the study is not an easy case to evaluate, as both reviewers pointed out in their reviews. Both fundamentally criticized (already during the first review round) the lack of sufficient data to reach the conclusions stated in the manuscript. On the other hand, both also mentioned the significance of (a) having more data published from permafrost areas in the Andes and (b) investigating geophysical means to quantify ice and water content within e.g. rock glaciers. Regarding the two latter points, the study provides useful results, especially as only few previous studies exist in this field. However it must be clearly stated again that without further field or numerical investigations, some of the results presented remain speculative.

In their replies, the reviewers also mentioned again that although it is understandable if additional field surveys cannot be conducted due to logistic or financial constraints, these aspects should be considered in the beginning of a study or a manuscript writing, so authors should take the time and resources necessary to reach the goals of a study and the resulting publication. It is therefore puzzling if such arguments are used to not add data or numerical investigations to a study.

The reviewer made again constructive propositions how the paper can be revised in order that the data can be published, without containing unconfirmed statements or too speculative claims. And without reducing it to the very minimum by deleting all criticized statements.

Both reviewers left it now to the editor to decide whether the manuscript can be published or not. I indicated minor revisions, because it means that it will not undergo another external review round. As before, I would like you to respond with a point-to-point response to the reviewer comments and hope that by this the paper will reach a state where it can be accepted.

Kind regards,
Christian Hauck
Editor

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AR by Giulia De Pasquale on behalf of the Authors (07 Mar 2022) Author's response Author's tracked changes

ED: Publish subject to technical corrections (21 Mar 2022) by Christian Hauck

Dear authors,

thank you very much for your revisions and the author comments. I am happy to tell that the paper can now be accepted for publication in TC. I have added a few technical corrections below, which can be incorporated before uploading the final manuscript.

Thank you very much for your patience in this review process, and thank you again for having considered TC to publish your results.

With my best regards,
Christian Hauck
Editor
-----------------------
Technical corrections:
line 33/34 and throughout the text:consistent spelling of "downslope" throughout the manuscript.

line 61: parameter-sets

line 85: is characterized

line 287-288: ...delimited by the lowermost ray path, but there are poorly
resolved areas in the P-wave velocity models presented due to the limited ray-coverage within the displayed area...

line 447: For both rock glaciers...

line 451: We run several numerical simulations...

line 451: ...level of uncertainty on the...

line 453: ...same sensor locations, shot gatherings and quadrupole settings

line 455: ...inversion model results calculated for the presence of data noise, which equal to the one...

line 457: ...whereas the resistivity and velocity data are ...

line 459: In Figure A1, we present the model...

line 461: model error (ME) for each parameter...

line 468: low ray coverage in (or "for") this area and the lack of a sufficiently large number of quadrupoles within the field measurements.

line 471: ...(while keeping the same noise level in the synthetic data), the model uncertainties increase...

line 471/472: by ~30% when...by ~40% when... --> change also line 480

line 473: In case of El Ternero (Figure A2), we focus...

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AR by Giulia De Pasquale on behalf of the Authors (29 Mar 2022) Manuscript

Short summary

We presented a geophysical study of one intact and one relict rock glacier in semi-arid Chile. The interpretation of the collected data through different methods identifies geophysical signature differences between the two rock glaciers and characterizes their subsurface structure and composition. This is of great importance because of rock glaciers' relevant role in freshwater production, transfer and storage, especially in this area of increasing human pressure and high rainfall variability.