Monitoring and Stability Analysis of the Deformation in the Woda Landslide Area in Tibet, China by the DS-InSAR Method

Round 1

Reviewer 1 Report

1    General Comments

The manuscript proposes a SAR-based monitoring approach developing a small variation of DS-Insar and PS-SAR. The manuscript is well-written in the first section and poorly developed from methodology. Moreover, methodological uncertainties are not well covered like atmospheric uncertainties. The manuscript could be of interest to remote sensing readers. The contribution will support landslide early warning systems if the authors correct critical issues.  I suggest a major review due to the methodological flaws.
2    Specific Comments:

Abstract: The abstract needs to be rewrited focusing into the main results related to the precipitation triggering.
Introduction:
Page 1: “Severe plate movements” Please, emphasize tectonics instead of plate movements.
Page 2: “many scholars”. Please, rewrite
Page 2: “one-dimensional deformation rate and the….”. Please use LOS instead of one-dimensional. Moreover, the word “deformation” is overused in the paragraph.
Page 3: Last paragraph. The authors not addressed in detail the knowledge gap that motivate the research. I recommend to include additional cases in the study zone (Baige?).
Study area and datasets
Page 3: “…such as large-scale slides.”. Please, provide examples. The zone has many interesting cases to insert as background.
Methods
Page 4: “By setting reasonable time and space baseline thresholds….”. Please, include temporal and spatial baseline thresholds in the paragraphs.
Page 5: “The phase optimization is evaluated by calculating the temporal coherence, and setting a reasonable temporal coherence threshold to select high quality DS points.”. Please, explain in detail. 
Page 5: “If the available data is less, Equation (4) is….”. Please, explain in detail if you mean points or temporal adcquisitions. 
Page 7: “…wrapped, to construct triangulation networks. Through iterative calculation and regression analysis, we separate the linear deformation, the elevation residual and the residual phase. And from the residual phase, the nonlinear deformation, the atmospheric delay phase and the noise phase are separated.”. This observation is critical. The non-linear component is not just related to atmospheric contribution. The authors should evaluate the atmospheric contribution. For example, https://doi.org/10.1002/2015GL063872 showed the impact of the atmosphere into coarse areas. In the study case, a very complex topography generates turbulent fluxes that increase atmospheric artefacts. I recommend evaluating atmospheric delay in your proposal using numerical modelling (ERA, GACOS), or remote sensing estimations (MODIS).
Figure 3: I suggest these figures into just one.
Results
The authors should reorganize the section. Some paragraphs correspond to methodology. Examples could be found in equations numbers 11 up to 14. Moreover, additional information shown here is not well justified in the methodology. This point does not contribute to scientific reproducibility.
Discussion
Section 5.1 is not introduced in methodology and results. The data shown has serious flaws concerning the source of data. Please explain in detail the origin of the data. Following, move the paragraphs to sections of methodology (first paragraph) and results (following to Figure 13). Finally, the discussion relative to the precipitation effect is weak.
The section 5.2 shown many resources that not contribute to the manuscript. Moreover, the discussion is lazy and mixed with methodology again. First paragraph of 5.2 not explain well how the authors justify “circle shape”. I kindly recommed to include an geotechnical observation or correlation using previous studies over the zone. Moreover, the authors do not shown structural geological data that correlate their impact. Here, faults need to be evaluated in field. Any regional of mesoscale fault could be not representative if is not supported by field data.
I agree with the trird paragraph of section 5.2. However, the data and results showed in the present manuscript not support fully the aseverations. Moreover, additional quantification is needed here. I suggest to evaluate recent studies published in Landslides journal about the zone.
Section 5.3 up to table 3 is methodology. This information is not shown in previous sections. Moreover, table 3 corresponds to the section of results. Please, table 3 and related text to section 4. Finally, the geological constraints to this landslide is not well-covered. I recommend to the authors include additional information or remove the paragraphs. In addition, the approach shown is not validated with field measurements or alternative remote sensing procedures like optical correlation.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Dear Authors,

   Many thanks for your manuscript submission to MDPI Journal of Remote Sensing. This paper has analyzed the influencing factors and deformation mechanism of the Woda land-slide area, justified the surface deformation in relevance of the geological structure and precipitation (which may continue for some time), then finally specified the applicability of their monitoring results on the landslide disaster prevention in mountainous areas.

   After my comprehensive review, I think this set of work contains some good points, while a few problematic issues should be addressed. I may recommend this paper as "Accept with minor edits", in condition that the following major aspects to be improved in the revision process:

   a) Abstract: the current version is a bit too long. Please be as concise as you can for statement. You may shorten it to within 180-200 words. Also, it missed keynote concluding remarks on the quantitative results (i.e. just 1-2 sentences can be good enough). Also, some redundant statements, i.e., several sentences at the front, can be deleted or apply shorter alternatives.

   b) Introduction: I think the first 4 paragraphs require a major re-write. Please delete redundant narrations and replace the gaps with specific understandings on the shortcomings of prior and current research study. Besides, any study after Year 2011 should be clearly statement. Also, this section was absent from a short summary on major contributions on this set of research work, and the authors may consider supplementing the last paragraph on the organization of the related sections. Thanks very much! 

   c) Figures and Tables: several universal issues must be calibrated. First, almost the resolution of each color figure, must be enhanced. Please use better display to present Figure 3 (for the data points). The blur and vague boundary of some figures, i.e., Figs. 9-11 and Fig. 15, should be fixed. The other trivial issues are the missed linespacing of each Table (where MDPI template may require 12 pt before and 6 pt after). Please apply the edits. PS: we suggest you using "Figure" instead of "Fig". Thanks a lot!

   d) Discussion section: It missed a subsection on the limitations of study along with specifying some opening problems. While neither part is mandatory, it is better to contribute a complete set of your research study.

   e) Conclusion: I suggest the authors adding a third paragraph (on future study) with a summary of research challenges and future study orientations, etc. Please consider strengthening this crucial section. Thanks a lot!

   f) References: a few obvious problems must be fixed. (i) Imbalanced citation style. Please apply the italic form of volume after the title of cited journals (MDPI does not require citing the issue number); (ii) Some of the newly added references should be abbreviated when citing the title of journal name. (iii) The authors need to adjust each with the professional standards on citing other papers, i.e., conference proceedings (by checking the MDPI template), not the italic style; (iv) Latest publications in Years 2018-2021 which are similar / parallel to your study area on distributed scatterer interferometric SAR, are recommended to be supplemented.

   A few minor issues recommended for updates in your revised version: 
   a) Please align the proper size of figures (a few of them are left-aligned). Apply the required enhancement on image resolution. Be sure that the size and position of each image comply with the MDPI template. 
   b) In some sections, the literal quality of English should improved. I may suggest the co-authors inviting a native English speaker to polish the literal aspects of this research article, including grammatical checking and careful proofreading in your updated version. 
   c) Another transparent problem is that, the notations of some letters contains obvious distortions, i.e., Eq. (1), Eq. (7) and Eq. (19), which are unprofessional. Similar issues occur in other notations. Please carefully fix such kind of issues.
   d) If you are not using Latex, I suggest that hyphenating a word can be avoided (which currently appears multiple time at the end of some lines to cross-over two adjacent lines). MS word file of MDPI online template has the options to adjust that. Thanks a lot!

   We sincerely wish you the best of luck and prosperity in Year 2022. All the best with your edits towards paper acceptance!

Best wishes,

Yours sincerely,

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript "Monitoring and stability analysis of the deformation in Woda landslide area in Tibet, China by the DS-InSAR method" is submitted as a full manuscript to the Remote Sensing journal (MDPI). The subject is of interest to the SAR remote sensing community and presents a very interesting research application of DINSAR over forest areas that are particularly very difficult. The authors use several Sentinel-1 datasets and propose a procedure to find stable points in order to characterize deformation in both ascending and descending passes. 

The study area is covered by forest (that could be better described in terms of species and biophysical parameters) and whose incidence of landslides is particularly high. My major concern after reading the paper is that the methodological procedures are not fully described making reproducible research difficult. Although some methodological aspects are mentioned in the results section, it would be interesting to have it in a proper place.

I mean with that describing better the procedures with the datasets, where to obtain them, how and where to use them, and which environment was used for data/signal processing. Besides, the steps were performed in which platform or software? It would be interesting for comparisons. The applications shall also be stated in a broader context in order to promote this study application and not make too local.

Below are some general comments (Line numbers were not available for detailed comments)

Abstract: i) DS and PS points are used together and increased. Ok, but how much was this increase? ii) the abstract could end with application into a broader context;

Introduction: i) make Woda landslide into a broader context, it means you could mention the issue of landslides occurrence in natural and planted forest, mainly after extreme events or step areas; ii) besides "and large demand of human and material resources" there is also the safety issue by staying to close to the dangered areas; iii) explain better the NS limitation by relating the SAR system orbits;

Methods: i) explain better where to obtain SRTM and data processing steps to obtain 30m; ii) mention the spatial resolution of 1 arc-second; iii) 

Figure 3: increase size and font size for better visualization;

Another critical point is the use of several methodological sentences in the results, bringing some misunderstandings;

Figure7: please better explain the difference among curves and related it with literature;

Figure 10: please try to add the common overlapping area as a shapefile data layer;

Figure 12: where precipitation records were taken?

Figure 14: from where does this geological map come? scale?

In general, what type of deformations are you considering? uplift or subsidence or both? When mentioning positive and negative values only you may bring some misunderstandings; 

The use of high spatial resolution (when available) would possibly render some nice analyses. Please also consider adding and discussing further open research questions.

Author Response

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Author Response File: Author Response.docx

Reviewer 4 Report

Manuscript ID: remotesensing-1554235

 

Title: Monitoring and stability analysis of the deformation in Woda landslide

area in Tibet, China by the DS-InSAR method

 

Authors: Youfeng Liu, Honglei Yang, Shizheng Wang, Linlin Xu, and Junhuan Peng

 

 

In this paper authors applied the distributed scatterer interferometric SAR (DS-InSAR) and Persistent Scatterers (PS) techniques to monitor and analyse the deformation in the Woda landslide area (Yanbi Township, Jomda County, Eastern Tibet) using a large set of ascending and descending Sentinel-1A images distributed in the time span 2014-2019. Moreover, authors used the hypothesis test of confidence interval (HTCI) algorithm with the highest computational efficiency to select statistically homogeneous pixel (SHP), and the adaptive spatial nonlocal filtering method is then constructed based on the results of SHP selection to perform phase optimization. Finally, authors analysed and discussed the deformation characteristics and inducing factors of the studied landslide area, which can provide reference and guidance for the early investigation, monitoring and timely warning of densely vegetated landslides.

 

In the present form the manuscript remotesensing-1554235 cannot be accepted for publication on Remote Sensing journal, some improvements are necessary to fit the high-quality standards of this journal. In particular:

 

General Remarks:

 

• A revision of the English language is necessary by a native speaker, see journal service.
• All the deformation data indicated by authors in the text of this manuscript in (mm/yr) or even (mm) physical units must be expressed without the decimal places, in fact I don’t think that the methods described here have the sensibility of tents of cents of mm.
• As indicated in the following some section must be reorganised moving text in other paragraphs/chapters.

 

In particular:

 

• From line 7 to 9 of the section “2. Datasets” authors should add the references of SRTM DEMs used and of Google.

 

• After equation (5) of section “2.2. Optimal phase estimation” authors must define the function W(p,m).

 

• In the Figures 3 (a) and (b) authors must indicate the physical units of Bperp and Delta_T.

 

• In the section “1. Monitoring results and analysis” at the beginning of page 8 is evident a discrepancy of 2.6 cm between LOS velocities detected with ascending and descending SAR data, respectively, for this reason authors should in any case compare these measurements with other Geodetic methods like GNSS and levelling.

 

• The results indicated in the 19 lines after Figure 6 at page 9-10 must be summarised in a Table.

 

• At page 11, before equation (11), authors must add the following reference after reference [43]: Remote Sens. 2019, 11(3), 241; https://doi.org/10.3390/rs11030241.

 

• In the Figure 9 (a) (at page 12) the colour scale (incidence angle) in the left lower part of the figure is cut.

 

• In Figure 10 (a), the same problem of figure 9 (a) is evident, the figure is cut.

 

• In the Figure 11 at page 14 (as well as in the Figures 4 and 10), authors should evidence better the pixels affected by deformation (yellow, orange, red in the figures).

 

• In the Figure 13 (a), at page 15, the label of Y-Axis (mm) is cut.

 

• The section “2. Discussion of deformation mechanism” at page 16 and the corresponding Figure 14 must be moved at the beginning of the manuscript, in the section Methods, subsection “2.1. Study area”.

 

• The contents of the following section “3. Evaluation of surface stability” at pages 17 and 18 are more pertinent to the Methods and Results sections as indicated in the third line of this paragraph. The method and equation must be moved in the section methods and the corresponding results in the section Results.

 

• At page 19, in the first line of the section “ Conclusions” an of must be added after the word state-of the-art and before the word DS-InSAR.

 

• At page 19, in the fourth line of the section “ Conclusions”, authors must add a to: “are used to increase”.

 

Author Response

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Author Response File: Author Response.docx

Reviewer 5 Report

Taking the Woda landslide in the upsteam of Jinsha River as example, the DS-InSAR method was utilized for the surface stability monitoring and time series analysis. Although the application and implication of this study is potential, the novelty of this manuscript can be further enhanced. In addition, the structure of the manuscript in this version can also be optimized to improve the readability.

Comments in detail:

1) abstract: "here we use the distributed scatterer interferometric SAR (DS-InSAR) method is used to **" needs tob rephrased.

2) the full termination of GNSS, UAV, PS-InSAR, DS-InSAR and etc. in the Introduction part needs to be provided in their first occurrence.

3) The statement of "the mean temporal intensity of the refrence pixel and that of ther neighborhood pixel is taken as the value of ***, Equation (4)." may not corrected and needs to be fixed.

4) Descriptions of methods in section 4.2 and 5.3 need to be merged into the Methods section.

5) In order to highlight the innovation of the work, the interpolation approach for the deformation time series can be detailed described. From my point of view, this has not be reported in techincally in literatures.

6) The impact of precipitation in 2016 for the deformation acceleration is not significant compared to 2017 and 2018's, please explain why in more detail.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Dear author

I do not have comments about the new version.

Author Response

Thank you very much for your affirmation and support for our first revision. Thanks again.

Reviewer 3 Report

The revised manuscript entitled "Monitoring and stability analysis of the deformation in Woda landslide area in Tibet, China by the DS-InSAR method" addressed all the issues discussed in the first review very well. I am not a native speaker, but I feel that some proofreading is still necessary. However, the editorial team usually performed a very nice job in this regard when necessary. 

In summary, the applied paper is almost ready to be published as an academic journal due to the critical comments of reviewers and the author's appropriate additional explanation and verification. However, some very minor key issues are still necessary (given below):

i) please avoid having methodological aspects and details in results/discussion rather than in methodology. Please shift SWAT tool info in results into the methodology. Same for the geological map. Check for other similar sentences;

ii) your discussion section must be enhanced and relate your own findings with other InSAR findings from the literature;

iii) please increase the font size of the text inside the figures (and increase the size of some figures too). I have the feeling that they are not readable when printing it in B&W printer;

iv) when possible please keep a common shapefile of the overlapping areas (descending and ascending modes) in each figure. This helps to understand the common area of both flights;

v) please add some more recent papers published in the literature about DINSAR applications (including some of this journal);

Since this is a minor review and most probably I am not receiving further reviews to read, this reviewer looks forward to the author's more profound research achievements in the future. Thank you for the opportunity to evaluate this nice research application.

Author Response

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Reviewer 4 Report

Dear Authors,

The manuscript remotesensing-1554235 has been truly improved with the review process. Now the manuscript fits the high-quality standards of Remote Sensing and in my opinion is ready for publication.

Author Response

Thank you very much for your affirmation and support for our first revision. Thanks again.

Reviewer 5 Report

Thanks a lot for the efforts have been taken by authors to improve the quality of this manuscript. I am satisfied with their revision. However, the english style can be further improved such as edited by a native English speaker. At present, there are several typos, for instance:

1) Considering that there are relatively few time series and two-dimensional studies on the Woda landslide area. In this paper

2)...with 1 arc-second spatial resolution from National Aeronautics
and Space Administration (NASA) is used to remove terrain phase and geocode. 

3) 3.3.2 Stability calculation
we analyze ....

In addition, in the section 3.2, why 0.4 temporal coherence was utilized to select high-quality DS points? Please add an relevant citation or give more description on this issue. 

Author Response

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