Ice Thickness Estimation from Geophysical Investigations on the Terminal Lobes of Belvedere Glacier (NW Italian Alps)
Round 1
Reviewer 1 Report
Dear Authors,
I revised your manuscript entitled “Ice Bottom Characterization with Ground-Penetrating Radar and Single-Station Passive Seismic Measurements at the Belvedere Glacier (NW Italian Alps)”. It is overall interesting, properly organized, and well-written but it suffers some detrimental problems which should be properly addressed before a possible publication.
The most important issues are:
1) From the title a reader would expect a detailed characterization of the ice bottom. In the paper you make some interesting working hypotheses about the low continuity of the ice bottom reflector and about the overall low quality of the data, but this is not a real data-supported characterization. I therefore suggest to modify the title in accordance.
2) The role of the Passive Seismic data in the proposed case study is marginal and as you honestly point out in the conclusions “Without the reference subsurface model provided by GPR profiles, the interpretation of single-station passive seismic measurements would not have been possible.” Which is the actual applicability of passive seismic for alpine glaciers characterization? Which is the minimum station density advisable for a realistic (if possible) ice bottom and deepest layers reconstruction?
3) The GPR data quality is very low when considering the typical S/N ratio of data acquired on glaciers. I think that the possible explanation you give (i.e. that this is due to the debris above the glacier) is too simplistic. The (few) GPR data you provide (Fig. 4) show high scattering (and or noise) for the entire trace, which cannot be related just to the shallower part of the glacier.
In addition, I have several specific issues about Fig. 4 (I reported all of them in the commented copy of the manuscript and I copy also here for your convenience):
- You should explain some artifacts clearly visible on the figure like: residual background noise especially in the first 200 ns in (b); an horizontal event at 650 ns in (b) and at 700 ns in (c); some repeated traces especially in the third panel in (d).
- 2) Why you have a so high scattering? Why a so strong attenuation? Why a so relevant lateral variability? At least some hypotheses should be provided.
- I don’t agree that in (d) reflections are much more clear than in (a) and (b). For instance in the third panel in (d) the reflectors (if any) are totally elusive and the interpretation in Fig. 5d is quite speculative.
- The trace to trace amplitude variability is not always constant for each trace, so it cannot be just the affect of variable antenna coupling.
- Are the sections shown migrated or not? Could you provide at least one example of comparison between un-migrated vs. migrated profile? What about the migration algorithm and its parameters?
4) Some figures can be improved. Please see my comments about this point in the commented manuscript. In particular: some interpreted horizons in Fig. 5 are highly speculative; Fig. 6b and c are not informative at all; Fig. 7b is obtained with too few profiles and has a lot of artifacts; in Fig. 8 the 3D components at least for one station should be shown; Fig. 12 and 13 can be improved.
5) I think that you should address your efforts to enlarge the overall interest of the paper trying to get more general conclusions, better supported by the data so enlarging the possible audience as well as the applicability of the proposed methodology.
Please refer to the commented manuscript for further details and suggestions.
Comments for author File: 
Comments.pdf
Author Response
We provide detailed answers to the comments of the reviewer in the attached file.
Author Response File: 
Author Response.docx
Reviewer 2 Report
This is interesting investigation demonstrating novel results obtained under difficult environments. I believe that this MS can be accepted for publication in the present form.
Author Response
We are grateful to the reviewer for its positive feedback. The manuscript has however undergone a deep review according to the comment and suggestions of other two reviewers. We hope that the overall quality of the manuscript has profitably increased after this review round.
Reviewer 3 Report
Using non-destructive geophysical techniques (ground penetrating radar and single-station passive seismic) the authors would characterize ice bottom depth and properties on the terminal lobes of Belvedere Glacier (NW Italian Alps).
They found that the glacier lies on a thick sequence of subglacial deposits, rather than on stiff bedrock. Week deeper reflectors were identified only in the frontal portion of the northern lobe. These interfaces may indicate the bedrock presence at a depth of around 80 m, rapidly deepening upstream. Single-station passive seismic measurements, processed with the horizontal-to-vertical spectral ratio (HVSR) method, globally confirmed the three-layer (ice-glacial sediments-bedrock) configuration coming from GPR measurements, with similar depths of the identified interfaces.
The topic is intriguing and certainly suitable to be published, but these results were obtained from a not carefully interpretation of the seismic data.
According to me the HVSR curves showing in figure 8 can not be modelled with a three layer configuration as the authors have implemented (see Table 1). In their model configuration the glacial deposits should produce an inversion in the HVSR curves, due to a Vs value lower (1350 m/s) than the ice (1900 m/s). An inversion of velocity involves heavily the horizontal spectral components of the noise whereas it does not change the vertical component (Castellaro and Mulargia, 2009). Consequently, the velocity inversion produces HVSR values lower than 1 in a definite frequency range.
In all the graphics of figure 8, the HVSR values are greater than 1 in the investigated range (1-10 Hz) and therefore the model proposed by the authors can not reflect the measured data. Therefore it is unlikely that the glacier lies on a thick sequence of subglacial deposits.
The graphics of Fig. 8 should be shown better. It is necessary to produce, together to HVSR curves, the single spectral components (NS, EW, Z). In order to study the pattern of the peaks of the HVSR curves, the frequency range should be increase towards the high frequency (0.1-60 Hz, I suggest). In fact, in particular at site A and C the single peaks f1, f2, f3 have low amplitude and probably belong to a peak which develops in a wide frequency indicating a gradual increase of the velocity.
In the site D it seems that a highest peak is at 1 Hz. I think the choice of the peaks made by the authors is very arbitrarily. In the site C the f3 peak is low and it can not put into evidence a change in the impedance. In the site B f2 peak not has amplitude higher than the first peak of the site D which is not considered. According to me this interpretation of the HVSR method is too speculative in consideration also to the technique resolution.
In the site C, in the investigated frequency range, the HVSR curve does not show any type of amplification pointing out the lack of impedance contrast and so the bedrock has similar Vs value of the ice (fractured mass rock??). In alpine zone metamorphic rocks can normally have this Vs values.
In conclusion, I suggest controlling carefully the assumed shear wave velocities. The authors should know the lithology of the geological formations (igneous, metamorphic, sedimentary…) in the study area and discuss it report in the paper. I suggest to the authors to model another layer configuration for the investigated area. It is probable that the glacier lies on weak bedrock, which shows low shear wave velocity not much different from ice ones.
I think that the paper should not be published in this form.
The paper needs to be improved above all in the interpretation of the seismic data and consequently of the obtained results. It could be acceptable for publication only after its global reorganization.
Author Response
We provide detailed answers to the reviewer's comments in the attached file.
Author Response File: Author Response.docx
Reviewer 4 Report
Summary:
It is important to get the detailed reconstruction of the ice bottom depth and morphology to estimate the total volume and mass change for these mountain glaciers. With 3-year campaigns of GPR profiles and single-station passive seismic measurements, the authors reconstructed the ice thickness map for the Belvedere Glacier, and also compared and validated their results with previous investigations.
Major comments:
1. Figure 7a,b, I am not sure this is the best way to represent the 3D glacial bed topography. Maybe a bed elevation map could be better
2. I do not understand the meaning of the comparison between two Radar measurements with GPR profiles, which are not overlapped with each other (Figure 13).
Especially, the Radar measurements did in 1985, why the GPR campaigns not try to follow the radar tracks for easier comparison?
Minor Line-by-line comments:
Line 124. a.s.l., should fully spell like ‘above sea level’ for the first time. Similarly, to NE (Line 117), N (Line 125), S (Line 127) etc.
Line 196. On December… à in December
Author Response
We provide detailed answers to the reviewer's comments in the attached file.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
Dear authors, you have implemented most of my suggestions and this is generally fine with me, although there are still some residual issues hereafter detailed.
1) Thank you for specifying that the GPR surveys were done with air-couples antennas due to the presence of wide blocks on the surface of the Glacier. However the photographs provided in Fig. 3 are somehow misleading because they show that at least some of the GPR profiles were collected with the antenna directly on the snow that covers at least some parts of the glacier during the geophysical survey. Please be more precise and clarify this point.
In particular on pag. 7, rows 231-232 (revision tracked version of the manuscript) you say that “it was not possible to direct drag the instruments on the glacier surface to maximize the coupling”, while in Fig. 3 you show something different.
2) You say that, following the suggestions of another reviewer, you shortened the section about passive seismic. You further conclude that “Future perspectives of the method should be addressed to glacier monitoring. Multi-station long-term passive seismic measurements can potentially be used for the investigation of the ongoing glacial processes (hydrogeological modifications, meltwater flow, seepage and accumulation) and of the glacier movements and stability conditions (e.g. icequakes, opening of crevasses, basal movements, serac falls and stability of the frontal compartments).”
However, I think that in the present form there is nothing about monitoring with passive seismic, and, in general, the role of such a technique is insignificant as compared to the GPR. Do you think that passive seismic contribution is actually essential within this paper?
I have no additional relevant issues about the figures which have been certainly improved. I just suggest the following minor issues:
- Please check the readability of red labels on Fig. 1.
- On Fig. 4b and c I think that a simple trace mean subtraction could quite easily remove the coherent noise at 650-700 ns.
- The details inserted in Fig. 10b and c about ray paths are very interesting and possibly could deserve additional comments also in the text.
- On pag. 7 row 216 of the revision tracked version of the manuscript, it would be better to use the word “above” instead of “over”.
- Please notice that on Pag. 20 row 570 of the revision tracked version of the manuscript Fig. 9c is wrongly referred as Fig. 19c.
Author Response
We are grateful to the reviewer for having re-examined our manuscript. We provide answers to all its comments in the attached file.
Author Response File: Author Response.pdf
Reviewer 3 Report
The authors are carefully considering all my comments and suggestions. The manuscript has been global reorganized, in particular, taking into account the limitations of HVSR processing and interpretation for glacier characterization. I believe that the paper may be published in this form.
Author Response
We are grateful to the reviewer for the work done and the positive feedback. We will slighlty refine the manuscript according to the final comments of the reviewer 1.
