Manifestation of Gas Seepage from Bottom Sediments on the Sea Surface: Theoretical Model and Experimental Observations

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper is aimed to develop a method for remote sensing gas seepage at the sea surface. The purpose is sound and scientifically valuable. The text is written in rather good English, except for too long and complex sentences. The proposed model does fit observational data. Yet, the major conclusion is that “the use of such tools may be economically impractical at the moment”. The major problem is that high-resolution remote sensing data is needed, and rather long-time sensing is needed to increase the signal-to-noise ratio. Thus, this work can be only considered as a study of possibilities for remote sensing of gas seepage at the sea surface. If so, a more extensive analysis of other publications in this area is needed. Instead, the authors suggest 14 of 34 references of their previous publications addressed to anything but the subject of this paper. As the result, for example, the introduction does not introduce the major issue of this paper but some details on methane seepage in the Arctic Ocean.

Comments on the Quality of English Language

English is rather good. 

Author Response

The authors are grateful to the Reviewer for valuable comments. Indeed, the main subject of the article is the development of a method for radar sensing of underwater sources of gas emerging on the sea surface. However, when discussing the article with co-authors, it was decided to increase its interest for readers by introducing a description of the problem of intensifying the output of natural methane in the Eastern Arctic, as one of the possible and promising directions for applying the proposed method. Moreover, the results of the detection of such underwater seeps were first obtained by the authors in the East Siberian Sea and are presented in this article. This suggests that the proposed method is not only useful in research terms but can also be used for the practical detection of new, unknown gas seeps reaching the sea surface. Processing of a large array of radar data collected on the indicated expedition showed their presence, but these results are not presented in this article. At the same time, as the reviewer correctly notes, such a low-contrast object on the sea surface as a manifestation of gas seep obviously imposes requirements on the characteristics of radar detection equipment. The results of this article can help formulate the requirements for future satellite or aircraft radar equipment for detecting seeps, if such a task arises. A discussion of this issue has been added to the article.

If we talk about a review of the literature on this problem, then almost all sources are listed in the References. However, the authors additionally searched the literature and added additional references.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors, together with other studies carried out by the same or other groups of researchers, bring to attention a truly crucial question: the ESAS contains the largest and arguably most vulnerable stores of subsea CH4 and consequently must be included as a source in global climate models.

The paper aims on a comprehensive theoretical and experimental study of the characteristic features of the gas seep in the structure of surface waves, and the manifestation of these features in sensing radar signals.

To achieve this goal, the paper starts with a theoretical model of the wind wave transformation in the current field created by a buoyant gas jet on the sea surface. After that, the theoretical results are approved by authors with the results of a full-scale experiment on remote sensing of an artificial gas seep, modeled from a stationary oceanographic platform in the Black Sea in 2021. Finally, the results of remote sensing of natural methane seeps obtained in the East Siberian Sea during a research vessel cruise are presented.

The paper is well structured by comparing theoretical data, models and in-situ measurements. However, in addition to some notes and suggestions for small variations, it should be noted that the discussion and conclusions part is a bit sparse and should be well argued and expanded.

Below are some notes in detail:

- Abstract: This seems to be a bit too broad. Consider whether to remove some details (even if present in the introduction);

- line 127: please define the acronym IAP RAS;

- line 162: typo  vector along𝑟 ---> along 𝑟;

- line 347- 360: It seems like a repetition of the topics covered in the introduction. Please move or merge the text in the introduction section;

- line 414: Why is the satellite issue introduced? it is not reported in the introduction. In order to make the reader understand the problem better, without necessarily reading the paper reported in reference [26], please discuss the issue better;

- line 426: what kind of carrier? a plane or an oceanographic vessel used for a scientific cruise? Can I use a properly equipped ship of opportunity? If I can't use this methodology with the satellite, why should I use a research vessel? In this last case I could use scientific echo sounders and also have a quantification of CH4. In order to make the most of the results and the proposed methodology, please try to answer the questions above and better argue the issue;

 

Author Response

The authors express their gratitude to the Reviewer for his attentive attitude to the manuscript and valuable comments!

1. The abstract has been shortened, lines 14 – 19 have been removed.
2. Abbreviation defined: Institute of Applied Physics of the Russian Academy of Sciences
3. Fixed
4. lines 347-360 rewriting and moved to the Introduction.
5. A discussion of the problem with satellites has been added to the Discussion.
6. Thank you very much for your important note. The carrier can be any: ship, helicopter, airplane, drone, satellite. But for each of these carriers, the conditions for the radar equipment installed on board must be met. This is an opportunity to obtain information from a given area of the sea surface several times during the survey. That is, the radar must be scanning. And the scanning speed is determined by the speed of the equipment carrier. Sequential shooting from several satellites is possible, like TanDem-X, but, presumably, two images will not be enough. A discussion of this issue added in Discussions and in Conclusion

Reviewer 3 Report

Comments and Suggestions for Authors

This paper shows the theoretical model and experimental observations of the characteristic features of the gas seep in the structure of surface waves. The model is based on the kinetic equation for the wave action spectral density. And then, the theoretical results were approved with the results of a full-scale experiment on remote sensing of an artificial gas seep. Finally, the results of remote sensing of natural methane seeps obtained in the East Siberian Sea are presented. Due to the detection of weak gas seep is difficult, this paper is innovative and acceptable after minor modifications.

1. The abstract is lengthy and needs further improvement. Lines 13-20 is can be compressed or even deleted, because this study can not quantify the venting of CH₄ to atmosphere in ESAS.

2. The deficiency of the technologies described in the manuscript need be discussed. If surface waves are huge, or gas bubbles are tiny, can the seepage be distinguishable? What’s the threshold of gas seep can be identified using the microwave circular radar?

3. Figure 5 is not pretty and needs further modification. The length horizontal axis of Figure 5(a) is not equal to other subfigures? I can not see obvious relationship between radar contrasts and current velocity, current direction, wind velocity, wind direction, and wave period. Are those subplots necessary?

4. Line 410 “formation of wind waves generated by gas seep on the sea surface” is ambiguity. Wind waves are not generated by gas seep.

5. The conclusions need improvements and be separated from discussions.

Author Response

The authors are grateful to the reviewer for his attentive attitude to the article and valuable comments.

1. The abstract has been shortened, lines 14 – 19 have been removed.
2. Thank you for your important note. A discussion of this issue added in Conclusion.
3. With Figure 5, the authors wanted to demonstrate the completeness of the data set of the model experiment. However, as the reviewer correctly notes, the connection between the studied radar contrasts and the characteristics of hydrometeorological conditions is poorly visible from this figure. The drawing has been rebuilt and some panels have been removed.
4. Thank you very much for flagging the error. The proposal has been rewritten.
5. Conclusions and discussion are divided and expanded.

Reviewer 4 Report

Comments and Suggestions for Authors

The manuscript presents in detail the theoretical and experimental features of the formation of waves generated by gas seep on the sea surface. The practical application of the obtained results for remote sensing of natural gas seep using existing equipment installed on satellites can be performed. The manuscript solves the problem of determining the main features of the manifestation of gas seep to the sea surface in the area of wind waves and reflected radar signal. The results of this work can be employed in conducting research in potential gas seepage. However, the method and data presented in the manuscript are not enough. Detailed comments are listed below:

1. The manuscript shows only the radar signal in HH polarization. In fact, the signals of wave and gas are quite different for HH and VV polarization at different incidence angles, which can help to separate the signals of wave and gas. It’s better to include the remote detection of gas using radar signals from different polarizations and different incidence angles.

2. It's recommended to discuss how to separate the signal of gas seepage, and foam generated by wave breaking or ship etc.

3. Appendix A is not mentioned in the text.

Comments on the Quality of English Language

It's better to modify some long sentences to simple and short sentences.

Author Response

The authors are grateful to the Reviewer for his attentive attitude to the manuscript and valuable comments!

1. The Reviewer is certainly right when he speaks about the promise of dual-polarization sensing, which satellite SAR make possible. Based on the difference in polarization, areas covered with foam, wave breaking, oil spills, etc. can be identified. In the experiments described in the paper, dual-polarization radar data was not available. There are also no data at other angles of incidence of radio waves. This is due to the characteristics of the equipment used, which the authors will keep in mind in future studies. However, thanks to the theoretical study of one of the mechanisms for the manifestation of gas seep on the sea surface, namely in the field of wind waves, it becomes possible for a theoretical analysis of the sounding parameters indicated by the Reviewer. And this will definitely be the subject of research for another article. In this work, the focus is on theoretical analysis and on a set of experimental data confirming the theoretical patterns, obtained at grazing angles on HH polarization. The considered features will also manifest themselves at VV polarization and at other sensing angles.
2. The point made by the Reviewer is very important. Indeed, the radar contrasts of the breaking wave are comparable to the contrasts of the gas release to the surface. However, the gas release to the surface has a characteristic difference from breaking waves, which is used in the proposed method. A gas seep on the sea surface is a stationary object, in contrast to random breaking. Temporal averaging of spatial radar images makes it possible to contrast stationary objects and level out random ones.
3. Appendix A was deleted and moved into the text of the manuscript.

Work has been done to simplify long sentences.