The role of Interferometric Synthetic Aperture Radar in Detecting, Mapping, Monitoring, and Modelling the Volcanic Activity of Piton de la Fournaise, La Réunion: A Review
Round 1
Reviewer 1 Report
A well written review of work that has come out of InSAR studies at Piton de la Fournaise. I am not extensively familiar with work on PdF, so can’t speak to how comprehensive the literature and deformation review is. I’m more familiar with InSAR technique and the parts of the manuscript describing satellites, their configurations, and methods are solid. There are no glaring omissions in the review of PdF deformation; the general categories are well covered.
Overall the paper would be stronger with more figures that directly reference the literature being cited/reviewed and provide more of an overview of the variety of deformation patterns through time. The existing figures seemed more geared towards illustrating techniques than illustrating deformation. It would be nice to see more examples of the specific interferograms that were used for the results mentioned in the text. It would also be nice to see an example of a time series for an interesting point from multi-temporal InSAR results.
The conclusions section would be improved with some mention of how InSAR at PdF has moved volcano science forward in a broad sense. That is, what are some “big picture” ways in which we better understand how volcanoes work, that came from InSAR work on PdF?
There are a number of English grammar mistakes that could use review. Too many to go through one-by-one, but not so many that it was difficult to read the paper. There were also some stray characters in the figure captions that should be deleted.
Line by line comments:
Figure 1: It doesn’t seem necessary to plot the seismic station locations, since they are never referred to. The color for the topography is distracting, it would be easier to see the colors in the seismicity if you used the same greyscale shaded relief that is behind most of the interferograms.
Figure 2: Likewise, since very little GPS data is shown, it doesn’t seem necessary to plot all the station locations. It would be more useful to plot SAR frame outlines or something related to the spatial coverage of the rest of the images. Potentially this figure could be removed altogether.
Figure 4: While it is somewhat interesting to see the number of pairs available for each satellite via the baseline graphs, that information could be easily given via text, freeing up space for figures related to deformation patterns.
Line 259 “day to months”: This seems like it should be “weeks to months“. If deformation happened over a couple days, I wouldn’t think of that as long-term.
Line 281 “short-term”: Did you mean “long-term”? That would make more sense to me.
Line 288 “Short wavelength signals in InSAR data”: While this is a commonly used phrase among people who work with InSAR data. I’ve found that it isn’t easily understood by non-specialists. “Small spatial scale“ might be a better phrase.
Line 314: Would it be possible within the journal’s style to write something like “published in 1999 by Sigmundsson et al. [23]“? It would be much easier to read the literature review portion of the paper if the names of the authors could be included. There are many places in the next couple paragraphs that would benefit from being able to name the reference.
Line 314-348: There is some switching between past and present tense in the next couple paragraphs. It should be consistent throughout. I haven’t marked all instances.
Line 329: Using only the paper number is particularly strange when it comes at the beginning of a sentence. It would be much more readable if this sentence was: “Froger et al. [40] used ENVISAT…“
Figure 6: The abbreviations “ASC”, “DSC”, and “Bn” should be defined before being used. I don’t see a red polygon in any of the frames, is it missing? What is the difference between "relative" and "original". Perhaps only one should be plotted, if they aren't both important.
It would be useful (if it can be done cleanly) to also see the time spans of the two interferograms plotted with the GPS results.
The figure could be edited down quite a bit, I’d rather see more variety of interferograms than multiple views of just two. I don’t think it’s necessary to show the coherence image and both the wrapped and unwrapped interferograms, for instance.
Section 3.2.2
It would be helpful to have an explanation for why GPS wasn’t useful in these situations. Was the deformation extent too small and not captured by GPS or was there a lack of instruments?
Line 384: Figure 6 doesn’t show the uplift and then subsidence, only the pre and co- eruptive, right? It would be nice to see the interferograms that show this observation.
Section 3.5 I would be very nice to have a figure with an interferogram showing the flank slip to illustrate this important use of InSAR.
Line 486 “upon the 2007 eruption”: I don’t understand this part of the sentence. Do you mean “on lava erupted in 2007”?
Line 543 “schoolbook”: “textbook” is the more likely word to be used here, in English.
Author Response
Dear Reviewer 1,
Thank you very much for your thoughts, comments, and suggestions that greatly improved our manuscript.
With kind regards,
Nicole Richter
Author Response File: 
Author Response.docx
Reviewer 2 Report
Dear Authors,
after careful reading, I consider your manuscript stands up to the standards of the journal and I recommend its publication after some minor points mentioned below be revisited. The overall work is interesting and provides several insights on active volcano monitoring.
### Please look again the following points.
l.19
PdF is correctly defined as an appropriate abbreviation, however it is not followed consistently throughout the text. Several instances move back and forth between the full and the abbreviated name of Piton de la Fournaise. Please fix it. Also, the definition PdF re-appears in Fig.1 caption. Please fix it.
l. 21
“inSAR is” —> “inSAR has been”
l. 69 and elsewhere
please put references in ascending order inside brackets eg. [16,1] —> [1,16], also lines 257, 262 etc. Use the proper property in the LaTeX command provided by the style file.
l.144
please put the URL in a separate citation and refer to it in the text, instead of listing the complete address along the text.
l.167
not sure what the “,-“ in “200,-€ and 900,-€” means
l.181 (and pages 6,7)
please distribute subfigures 3a and 3b in separates figures 3 and 4 (and renumber the trailing ones). They are large, single-page figures that can be self-standing with their own dedicated
captions. Also, I would merit mentioning the references from the where the data illustrated are drawn from. Citing repositories would be sufficient.
l.201
You refer to “coarse pixel spacing”. You have never mentioned what the spatial resolution of inSAR is and what possible limitations exist? Is there a range of resolution? This is an important point and needs to be taken care in the revised manuscript.
l.208
you mention that data quality of SAR data is generally high? what is the criterion of quality in the context of this paragraph?
l.283
enlarge figure
l.289
“large wavelength” —> “large-wavelength”
l.292
“such a large wavelength” —> “such large-wavelength displacements”
l.310
“intrusion related” —> “intrusion-related”
Fig. 6
page 13 has no caption under the figures nor number of the figure. I would recommend you split it in two groups and take care of this formatting inconsistence. Also, when you mention “red polygon” I assume you mean the ones highlighted in sub figs (i) and (j). This is not clear, as it appears in captions of (a) and (b) and one might start looking for that polygon where it does not exist unfortunately. Please fix it.
l.393
“was investigated by [45]” —> “was investigated in [45]”
l.399
“also high-resolution InSAR has proven” —> “high-resolution InSAR has also proven”
l.400
“Hawai’i” —> “Hawaii”
l.521
in this paragraph, the dense temporal sampling is discussed in relation with near-real-time operation of the system. I would ask you to include a short comment or note on what an appropriate time scale would be for that case. The reason is that if an eruption occurs, changes in the land structures, edifices etc can be done very fast, even in seconds. How a near-real-time operation in the hourly limit for instance could be any practical use. This is very important for the geohazards assessment point of view in such eruptive instances. I consider the 6 day cycle very nice, but I consider it is still too far away from any live monitoring efficiency, at least compared to other, perhaps more traditional methods. Any reference to support your comment would be welcome.
l.535
Table 1 needs some corrections, i.e.
fix column 4 width to include “microsatellites” in a single line
The table header (planned) Launch Date is out of context, as some most of the dates are past. If you need to mark some, do it individually.
Also, update the status of RCM (Canada) as it refers to start of planning in 2018-2019
In References
Some citations do not list the relevant DOI, while others do. Please add all those that exist but not currently listed.
Author Response
Dear Reviewer 2,
Thank you very much for your thoughts, comments, and suggestions that greatly improved our manuscript.
With kind regards,
Nicole Richter
Author Response File: Author Response.docx
Reviewer 3 Report
The paper proposes a comprehensive overview of the ground displacements of the Piton Island revealed by InSAR techniques. A reader can have a clear understanding of the potential of InSAR methodologies. I would have spent a few (more) words about the potential of obtaining information on the different 3-D contributions of the deformation as revealed by the SAR instruments. The discussion is well supported and the paper deserves surely a publication.
Author Response
Dear Reviewer 3,
Thank you very much for your positive thoughts and your suggestions that improved our manuscript.
With kind regards,
Nicole Richter
Author Response File: Author Response.docx
