Peer review of: Distributed acoustic sensing of microseismic sources and wave propagation in glaciated terrain (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
This paper presents a pioneering glacial seismology study using DAS fiber optic strain 
instrumentation. As such it is presently unique and undoubtedly points the way towards a new era 
of glacial seismology featuring tractable essentially massive multichannel data acquisition. I'd expect 
that this work would be highly cited in this context. The system performed well, and, as hoped, 
recorded a plethora of diverse signals from the glacier and its surroundings. The paper is very well 
written, contains substantially compete supplementary material, and reasonably comprehensive 
regarding the signals observed and some of the attendant analysis. It is an impressive proof of 
concept, but does not report a revolutionary new observation or insight. The paper is essentially 
publishable in this context as written (there are a few minor typos that could easily be corrected in 
production). 
 
A criticism -- the response of the DAS system is impressive, but I would have appreciated a 
quantitative assessment of its noise level (e.g., what is the relative signal to noise and response of 
the fiber when compared as best as is possible to the co-sited velocimeters?) and an evaluation 
spatial averaging effects of the relatively large (10 m) gauge length (I realize that this is a standard 
gauge length for DAS systems) when studying a wave field at this scale. 
 
 
Reviewer #2 (Remarks to the Author): 
 
This manuscript describes data recorded by seismometers and by a distributed acoustic sensing 
(DAS) on a glacier. 
As shown in this work, DAS are easy to install and provide a large number of "sensors" with a wide 
frequency band. 
But they are not as sensitive as seismometers and can be noisy. 
The DAS allows for more accurate icequake locations than using a small network of 3 seismometers. 
It also allows identifying seismic phases (distinguishing between direct and refracted waves, 
detecting doubly reflected waveforms). 
 
This study is interesting at it shows how DAS can be used for monitoring glaciers or gravitational 
instabilities. 
It appears as a very promising and powerful tool. 
The manuscript is also very well written and illustrated. 
However, the work presented in this paper does not (yet) provide new information on glacier 
structure or on glacier dynamics. 
This study could be improved by a more rigorous and complex analysis of data (more accurate 
estimation of seismic wave velocities, 3D model of bedrock topography). 
 
- Velocity model. 
It is not clear to me how VP and VS are chosen. 
The values of VP and VS given on line 200 (VP=3800 m/s, VS=1900 m/s) are larger than the values 
given on line 352 (VP=3750 m/s, VS=1875 m/s). Why? 

Moreover, Dalban et al. (Journal of Glaciology 2016) measured much smaller values of VP=3330 +/- 
80 m/s for the same glacier! 
The seismic velocities used in this study (VP=3800 m/s, VS=1900 m/s, line 200) are significantly larger 
than values I obtained for another temperate glacier in the Alps. Using a network of 100 
seismometers, I have measured VP=3590 m/s and VS=1830 m/s by minimizing time residuals (using 
P and S direct waves) for 38 basal icequakes on Argentière glacier (unpublished work). The icequake 
depths are in very good agreement with bedrock topography estimated from radar profiles. 
 
I suspect that overestimating VP and VS may explain why the estimated icequake depth (165 m) is 
smaller than the glacier thickness (H=200 m, line 206). 
Also, the seismograms for the explosion shown in Fig S5 suggest that the VP value is slightly over-
estimated. 
Why didn't you use the explosions in order to estimate VP and VS? Or used values estimated 
previously on the same glacier by Dalban et al (2016)? 
 
Then the authors assume a 1D model in order to interpret seismic phases (Fig 4). 
The bedrock topography is known from radar measurements. Although it is not very accurate (error 
of at least 10%, l362), using a 3D model of the ice-bed interface would be much more realistic than a 
simple 1D model. 
Using a 3D model to identify seismic phases would provide more reliable arrival times and would be 
a good test to estimate the accuracy of the bedrock topography. 
 
- Icequake locations 
I am surprised by the very large location uncertainties obtained using the on-ice seismometers (l167 
and Fig 4B). 
Are all the red dots in Fig 4B consistent with the measured P and S arrival times within a picking 
error of 4 ms? 
 
- Rockfalls 
The authors show that the DAS can be used to detect and locate (source azimuth) a small rockfall. 
The results are nice, but not surprising. Many previous studies have used beam-forming methods to 
locate rockfalls, and even follow the rockfall propagation (e.g., Lacroix and Helmstetter BSSA 2011). 
 
- Discussion 
Line 320 : "DAS technology could in the near future significantly lower detection thresholds and 
increase warning capabilities for destructive mass movements." 
I agree that DAS could be used to detect many small events everywhere, but there are technical 
difficulties that should be discussed. 
How to store and process automatically so much information? 
While earthquakes can be detected and located automatically with a correct accuracy, detecting and 
locating mass movements is much more challenging. Indeed, seismic signals produced by mass 
movements are much more emergent than earthquake signals, and identifying (automatically) 
seismic phases is much more difficult. 
 

Reviewer #3 (Remarks to the Author) 
 

Walter and co-authors report results of a distributed acoustic sensing (DAS) survey on a glacier in the
swiss Alps. The authors show that DAS is able to recover signals from a variety of glacier-related
processes such as a surface icequake and a stick-slip basal event, as well as noise from a rock fall
occurring nearby. They are also able to recover signals from an active experiment using explosives
drilled into the ice of the glacier at several hundreds of Hz. They successfully identify a variety of
phases in the DAS recordings and show the added value of using the densely spaced DAS data to
recover some phases that remain hidden in classic sensor recordings. To my knowledge, this is the
first study focusing on using DAS to investigate glaciers. Results are very promising and could help
advance the field of cryo/environmental seismology. The paper is well written and, in general, well
presented. 

I suggest publication after small changes and a few clarifications. In my opinion, the paper will benefit
from a bit more description on the effects of the snow cover/ice-snow interface on the surface DAS
signal and to what degree this affects the results. As I point out later in my revision, the waveforms
from the DAS channels (deployed on the surface,on snow) are somewhat different to those recorded
by   the   seismometers   (deployed   in   boreholes   in   the   ice).   Expanding   on   the   analysis   of   these
differences would be helpful. I would also add a bit more discussion on the directionality of the DAS
cable, and how this could affect locating surface icequakes and stick-slip events. 

Comments on main text 

1. Line 107.  The authors clearly describe the layout of the experiment forming an equilateral triangle
with 220 m side lengths. However, they do not give any reason why they decided on this geometry as
opposed to other configurations such as one long profile, crossing transects, etc. Could you please
elaborate on your choice of survey layout?

2. Lines 107 to 112. I suppose the seismometers are three-component sensors? Please specify as
this becomes important when comparing the signals with the DAS recordings.

3. Lines 113-121. Given that this is the first reported DAS survey on a glacier, it might be useful to
include some more comments on the deployment of the interrogator, such as: where was it located?
How   much   power   does   it   need   in   comparison   with   a   seismometer   (important   for   long-term
deployments in this kind of environment,where power is not easily accessible)? Is it important to
isolate the interrogator from temperature variations? This brief explanation would give the readers an
idea of how much easier or challenging it is to perform DAS surveys in glaciers with respect to using
classic sensors. This information could be added here or to the Discussion section.

4. Line 124. The authors make reference to “explosives at ca. 30 cm depth in the ice were set off”. It
would be informative to briefly comment on aspects such as what the goal of the experiment was, how
many explosions were set off, whether they were always set off at the same location or the source
position was changed, what was the specific source used (i.e. size,expected frequency). Would it be
possible to indicate the location of the explosions in Figure 1? 

5. Line 137. Could you briefly describe the expected characteristics of a basal stick-slip event, here?
6.   Lines   141-142.  The   authors   state   that   they   “compare   the   signal   quality   between   DAS   and
seismometer”. Which component of the seismometer are you comparing with the DAS signal? Can
you  comment on the differences you would expect between the two signals given the directionality of
DAS?

7.  Lines  141-142.  In   relation   to   the   previous   comment   –   waveforms   shown   in   Figure   2   for   the
seismometers and the DAS channel are quite different in some cases. What could be the origin of
these differences? Coupling? Please elaborate on that.

8. Line 151. In relation to the previous question -  the authors explain the high-frequency limit of the
DAS data “ as the result of the damping snow layer”. Would this snow layer cause other effects on the
signal recorded by DAS, besides damping the frequency content and amplitude of the signal? Are
there any previous studies (using conventional sensors) reporting  the effects of snow cover in seismic
studies of glaciers?

9. Lines 151-152. The authors state that ‘Damping within the glacier ice is less severe as confirmed
by the explosion signals”. This statement is somewhat confusing, could you elaborate a bit more?
Signals from both the basal stick-slip event and the explosion need to go through glacier ice and
thorough the snow layer to hit the DAS cable. How do you discern between damping caused by ice as
opposed to that caused by the snow layer from looking at these signals?

10. Line 171-172. The authors state that the stick-slip event has a “source mechanism consistent with
bed-parallel slip along the glacier flow line”. Have you calculated the source mechanism using the
seismometers/DAS/both? Please clarify.

11. Line 200. Velocities are reported as “3800 and 1900 m/s for P and S-waves” here, but they are
slightly different on the Materials and Methods section (lines 352 and 358-359). Although the values
are very close, please be consistent in reporting the true values used in the modeling exercise.

12. Line 229. The authors suggest that the rock fall occurred at an “unstable moraine to the West of
the glacier”. How far away is this moraine from the survey?

Comments on Materials and Methods

13. General comment on arrival time estimation and location. The velocity model used for these
exercises only considers the ice of the glacier, but it neglects the ~ 3 m thick snow layer. Have you
tried including that layer in your model? If not, do you expect it to have a significant effect on the
arrival times estimated for the DAS recordings? Please comment on that. 

14. Line 341.  The authors report that they picked “as many P-wave arrivals as possible, mainly from
the southern cable section”. Given the directionality of the DAS cable, P-waves are expected to be
very weak on surface deployments. This could be a limitation for this type of exercise. Can you
comment on the amplitudes of the P-waves observed in your records and how they compare with the
seismometer records? 

15. Line 411.  The authors describe that they “spectrally whiten the signal”. Is this part of a pre-
processing scheme? Please describe/mention any pre-processing applied to the data before applying
MFP.

Comments on Figures

Figure 1, caption. Is the location of the stick-slip event on panel B based on DAS data analysis?

Figure 2, panel A.  Based on the spectrogram shown in Figure S1, the surface icequake contains
significant energy up to 50 Hz. Why do you choose an upper corner frequency of 30 Hz?

Figure 2, caption.  “Note that the time axis between two seismograms of one panel were slightly
shifted”.  I  guess  you  mean  “for  each  panel”,   or is  it   for  one  of  the  panels  only? This  phrase  is
confusing. 

Figure 3 caption. The reference needs to be changed to number format.

Figure 4, panel A. This image can be confusing for readers who are not used to see DAS seismic
profiles with co-located sections and symmetric signals. It would be useful to mark the location of
channel 278 (mentioned in the caption) and the position of the triangle corners. 

Figure 4, panel B.  What’s the color grid? It is not mentioned in the caption, and a colorscale is
needed. Also, maybe a different color scheme would help since the colors are very similar to those of
the different sections of the cable, which are very difficult to see.  

Figure 4, panel D. The colors for the different phases are very similar and sometimes very difficult to
see. It would also be useful to list the different phases in the legend in order of appearance from 0
time, when possible.  

Figure 4, panels A and D.   It would be better to display both panels in the same units of distance
(channels or distance), or indicate in panel A which section of the profile is shown in panel D. As it is
now, it is hard to relate the two figures.

Figure 4, caption. “ Green triangle and black lines” - did you mean colored lines?

Figure 5. It would be useful to add a colorscale for the normalized beam power values.

Comments on Supplementary material

Figures S1-S4. Are these the same recordings shown in Figure 2, or are they the same events but
recorded on a different corner of the cable layout? In the caption of Figure 2, you state that you are
showing events from the southern corner of the cable, whereas in the captions of Figures S1 to S4
you refer to the northwestern corner of the cable. If you are indeed showing different recordings, it
would be more useful if the spectrograms shown here were those corresponding to the events in
Figure 2. Please clarify. 

Figure S3. It is slightly misleading as the y axis is different to that of all other spectrograms shown
here. I recommend using the same y axis in all figures S1 to S4. 

Figure S5. Figure label is missing.