Scanning Inside Volcanoes with Synthetic Aperture Radar Echography Tomographic Doppler Imaging
Round 1
Reviewer 1 Report
This paper proposed an imaging method base on the analysis of micro-motions present on volcanoes and generated by the underground earth's heat. The work is novel and the experimental results are sufficient.
1. It is suggested to quote more sufficient and recent references.
2. Can you supplement the comparison of relevant work of other researchers.
Compared with the last time, the schematic diagram of this version has made some changes, which I think is an improvement. There are no other suggestions here.
Author Response
Please consider the attached file.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Dear Author,
Thank you for your interesting paper aiming at the subsurface imaging of the Vesuvius volcano. Although the methodology you suggested here would be an important contribution to the literature, there are some problems that should be fixed for the publication. In my opinion, the main problem is the language of the manuscript which has various grammar mistakes, inappropriate sentences, and narrative. I did my best in the annotated pdf to revise some of them, but someone with an English mother tongue certainly needed to better accomplish the text. Another important problem is related to the Introduction and Discussion parts. Please see my comments regarding this issue in the annotated pdf. In addition, it would be great if you can discuss your results with other geophysical models (e.g., magnetotelluric) available for this volcano. Considering all, I suggest that your manuscript can be ready for publication after a major revision.
Best regards
Comments for author File: Comments.pdf
Author Response
Please consider the attached file.
Author Response File: Author Response.pdf
Reviewer 3 Report
Dear Author,
please see inline comments in the provided PDF file. I find the approach interesting and somewhat original in its development, however I have strong objections on the level of success the method has, as stated in the manuscript. Neither do I share the enthusiasm on the game-changing character the approach may have.
My main reservation is against the validation. The mathematical model is oversimplified, completely stripped of any non-linearities and oversimplified. To this end, the validation fails to support the model in a convincing way.
I am sorry to say that I can not recommend the paper for publication.
Comments for author File: Comments.pdf
Author Response
Please consider the attached file.
Author Response File: Author Response.pdf
Reviewer 4 Report
Please consider attached document
Comments for author File: Comments.pdf
Author Response
Please consider the attached file.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Dear Author,
Thank you for the revised version of the manuscript that has been improved compared to the previous version. However, there are still problems in the narrative that makes some parts inappropriate for a scientific journal. This is especially valid for the Introduction where there is a problem with the content integrity. In addition, there is a problematic issue regarding the authorship, as there was only one author in the first submission. Therefore, adding a new author to the revised version is not much appropriate. Please check the journal requirements and rules regarding this issue. After an editorial approvement and a minor revision, I think your manuscript should be accepted for publication.
Best regards
Author Response
Dear reviewer, thank you for your latest remarks. These are very helpful in providing greater strength and harmony to the writing of the manuscript. In this context, I have made the introduction more fluent and provided greater smoothness to the treatment of the bibliographical research.
As for author number two, in agreement with him, we have decided to delete his name as co-author. I therefore confirm that I continue to be the only author of this work. (which I hope will make an effective contribution in the field of volcanology).
I would like to take this opportunity to thank everyone, the Editor, Associate Editors and Reviewers, for their contribution, which has been crucial in making this a high-quality article.
Sincere regards
Filippo Biondi
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
The manuscript describes a very interesting new idea, i.e. to monitor volcanic surface deformation on very short time scales from space. However, the proposed tomography methodology is in my opinion oversimplified and not applicable to the over-complex subsurface structure of volcanoes. The manuscript is well written in parts, and the author seems to be an expert in SAR processing. However, to be understood by the target audience (volcanologists, geophysicists), some more explanation of the SAR processing must be given (some symbols in equations and steps are not explained). But even more important is the usage of correct scientific terms: a volcano has a conduit (no chimney; lava tubes are (sub-)horizontal) and a vent (no mouth). Furthermore, the correct term for "sonic waves" in this context would be acoustic seismic wave, as "matter waves" within Earth are called seismic waves. In fact, the simple vertical harmonic oscillators used here as a model, do not account for anything that has been achieved in geophysical monitoring and seismic tomography at volcanoes in the last 20 years. If the author insists to focus on seismic tomography at volcanoes from space-derived seismic traces (the time series of ground vibrations of SAR-pixels), this manuscript must include a sound discussion of recent literature on this topic. What in my opinion is new (and I am a geophysicist, specialized in volcanology and Doppler radar processing, not in SAR, though), is the retrieval of the seismic ground motion from space, i.e. the moving target SAR processing. This could be interesting for the volcanology (and the seismological) community. Many volcanoes worldwide are equipped with seismic sensors and often this is the only instrument for monitoring volcanic activity. Ground based seismic sensors give a (hopefully gap-less) time series of ground motion (one or three components) at one spot. SAR-derived ground motion would contain a lot of gaps in time but have an intriguing spacial coverage.
Some more specific notes: seismic velocities are usually given in m/s and much higher than the values given here. Water (in the ocean) has a seismic velocity of 1500m/s and all velocities in rock and solids are usually higher (unless sediments are water saturated). The typical frequencies of earth vibrations (earthquakes, eigen-oscillations or ambient noise) are below 50Hz and magnitudes (ground deformation) are typically in the millimeter to centimeter range (unless surface waves during earthquakes pass or explicit fracturing occurs).
Seismic waves occur as body waves (longitudinal pressure and/or transversal shear waves) and surface (Rayleigh and Love) waves. Seismic energy is scattered at discontinuities in the underground (and there are a lot of them at volcanoes) and it is not produced by Earth's heat but by any flow and fracturing in the subsurface. In fact, there is still a debate on how some of the harmonic and in-harmonic tremor at volcanoes is actually generated.
Given the above comments, I reccomend to reject the manuscript in the present form. But I recomment the author to team up with a geophysicist and volcanologist to further explore the possibilites and maybe focus only on the remote detection of ground motions using SAR.
Author Response
See Attached File.
Author Response File: Author Response.pdf
Reviewer 2 Report
This paper presents an imaging method based on the analysis of micro-motions present on volcanoes to form series of tomographic maps representing the internal echography of volcanoes. It is a quite interesting idea applied on the single-look-complex configuration. Generally, this paper is well organized. Some minor suggestions are: 1. The title might be misleading using two sentences. 2. The limitation of the proposed method should be clearly stated in the conclusion. 3. The figures should be well organized. 4. Some state-of-art literature are suggested to be included.Author Response
See Attached File.
Author Response File: Author Response.pdf
Reviewer 3 Report
- This paper introduces a preliminary research on an approach that is able to scan inside volcanoes to a depth of 3 km by using Tomographic Doppler Imaging. The paper provides good information for further studies in this field and the results are promising.
- The paper is written well. Some minor corrections are recommended. Some examples are given here for author’s reference: in Page 4, above Equation 2, change “nozero” to “nonzero”; in Page 6, change “the entire row of 4, and 5” to “the entire row of equations 4, and 5”, etc.
- Please reword the title of the paper and makes it into one sentence.
- Please Define “DFT” and “DFT2” in equation 3.
- Define r for equation 13.
Author Response
See Attached File.
Author Response File: Author Response.pdf
Reviewer 4 Report
The results shown in this paper are certainly very interesting and relevant. My concern is that the results seem to push the limits of what should be possible considering fundamental physical principles. I think that the author should at least clarify some points before this paper could be considered for publication.
My main objection is this: What amplitude of vibrations do we actually see on the surface of the Earth due to the sound waves present beneath the Earth's surface? Surely they must be very small, probably less than a millimeter. How then is it possible for a radar to be sensitive to these tiny motions of the surface? A spaceborne radar has many error sources, such as thermal noise, atmospheric delay, etc., that limit its accuracy. Has the author done any analysis on the accuracy of the method presented here?
Some other issues to consider:
The geometry shown in Figure 1 is highly idealized. For example, the spacecraft velocity is considered to be exactly constant, and the Earth surface considered perfectly flat. Has the author accounted for the actual geometry in the data processing?
At first glance, it would seem that with a PRF of 2 kHz (as shown in Table 1), it would be problematic to measure a Doppler bandwith of 24 kHz, as claimed in the text. I suppose that the antenna steering done for the spotlight SAR acquisition makes this possible. Is it valid to simply assume that there are no Doppler aliasing effects to be concerned about? Could there be any artifacts due to data at the wrong Doppler ambiguity?
Author Response
See Attached File.
Author Response File: Author Response.pdf
Reviewer 5 Report
It is an interesting work but I have three issues to point out:
- The work is based on just one case study. I understand that to add more case studies is almost impossible in the time of a revision.
- There are no quantitative evaluation of the advantages of the proposed method, just visual comparisons. It would be interesting to show some kind of numerical resullts
- On page 3, please be consistent: use micro-motion or micromotion, but not both
- 13 out of 20 (this is, 65%) of the references are 5 or more years old
Author Response
See Attached File.
Author Response File: Author Response.pdf
Reviewer 6 Report
This paper proposed a imaging method base on the analysis of micro-motions present on volcanoes, and generated by the underground earth heat.The work is novel and the experimental results are sufficient, but there are the following questions:
1. In section 3.1, the instantaneous shift is estimated by the pixel tracking technique in literature [14,15]. Can we think that the main innovation of this paper is to use a series of physical knowledge to construct the vibrational model of the earth.
2. It is suggested to use the flow chart to show the theory and the proposed imaging method more clearly.
3.It is suggested to show the experimental results more clearly. For example, in Figure 8, when the tomographic result is shown in the right figure, the direction of range is from "W" to "E" or "E" to "W".
4.In Chapter 3, there is only Section 3.1, but there is no section 3.2, and the notes of Figure 1, Figure 3 and Figure 4 are the same, it is suggested to distinguish them.
5.What is the applicability of the proposed imaging algorithm and whether it can effectively predict the occurrence of natural disasters as in the literature [14,15]. If the method is feasible , I think it will be a very meaningful work.
Author Response
See Attached File.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
since my time is very limited, I have to step back from reviewing this manuscript.
Reviewer 4 Report
I appreciate that the author has addressed my concerns in his response. I still think that the results shown here should be considered preliminary until there is much more extensive validation against in-situ measurements. Note that there still seem to be some English spelling and grammatical problems, so I recommend proof-reading before publication.
Reviewer 6 Report
My questions have been answered and there are no other questions.
