Distributed Acoustic Sensing: A Promising Tool for Finger-Band Anomaly Detection

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript conducts a straddle-type monorail finger band anomaly monitoring experiment using DAS and analyzes track vibration signals' time-domain and frequency-domain characteristics under different monorail operating conditions.

I will recommend the publication of this paper once the authors have addressed the following points:

1.      Please analyze in detail the ‘shared noise characteristics’ in ‘The shared noise characteristics of the two pulses traveling along the same fiber enable a substantial improvement in the signal-to-noise ratio through heterodyne demodulation.’ Meanwhile, explain the impact of frequency and temporal offset on shared noise.

2.      In Section 2.1.1, there is an extensive description of pivotal components. It is advisable to incorporate illustrative diagrams to enhance the clarity of the description.

3.      How long is the train? Does the type of train influence the experimental results? Were the results in the manuscript obtained from multiple trials?

4.      The expression ‘Figs. 4(a), (b), and (c) depict…’ in the Section 3 is incorrect.

5.      How does Fig. 7 support the following conclusion that the frequency domain of the signals exhibits two distinct primary frequency bands, specifically in the ranges of 0-20 Hz and 20-50 Hz?

6.      How does Fig. 8 support the following conclusion that the effect of finger band loosening on the signal is more pronounced in the 20-50 Hz frequency band? Isn't the impact of the loose bolt more pronounced in the 0-20 Hz?

7.      Does the impact of varying train speeds on bolt loosening into consideration?

8.      ‘When the finger-band was initially loosened by one turn, the change in stiffness may have had a limited impact on the overall vibration response of the rail beam’. Is the 'limited' mentioned here attributable to insufficient train speed or weight?

Comments on the Quality of English Language

None

Author Response

I am extremely grateful to the reviewer. Your comments and suggestions have significantly improved my paper. Your professional insights and rigorous attitude are truly admirable. Your feedback has served as a guiding light, highlighting the issues in my paper and pointing out the direction for improvement. Under your guidance, my paper has been greatly enhanced in terms of content accuracy, structural rationality, and argument rigor. Each of your comments has prompted me to think deeply and make careful revisions, making the paper more refined. Once again, I thank you for the time and effort you have dedicated to my paper. Your comments are a precious asset to me and have greatly enhanced my research and writing skills. 

Comments 1: Please analyze in detail the ‘shared noise characteristics’ in ‘The shared noise characteristics of the two pulses traveling along the same fiber enable a substantial improvement in the signal-to-noise ratio through heterodyne demodulation.’ Meanwhile, explain the impact of frequency and temporal offset on shared noise.

Response 1: Thank you very much for your question. We truly apologize for providing relatively limited explanations here. Nevertheless, due to the constraint of space, we would like to explain this issue by referring to our previous research paper (Ref. 23). We sincerely hope that this answer can meet your expectations and bring you satisfaction.

Comments 2:  In Section 2.1.1, there is an extensive description of pivotal components. It is advisable to incorporate illustrative diagrams to enhance the clarity of the description. 

Response 2: Thank you very much for your suggestion. We have added Fig. 2 in the article to supplement the description of pivotal components.

Comments 3:  How long is the train? Does the type of train influence the experimental results? Were the results in the manuscript obtained from multiple trials?

Response 3: Thank you for your comment. The monorail train used in the experiment has only one section, and the train length is 5.3 meters. We did not discuss the influence of train types on the experimental results. The work in this paper is only a qualitative exploratory experiment to explore whether there is a response relationship between DAS signals and the loosening of finger bands. In subsequent research, we will discuss train types as you suggested.

Comments 4:  The expression ‘Figs. 4(a), (b), and (c) depict…’ in the Section 3 is incorrect.

Response 4: Thank you for your comment. We were indeed careless. We have already made the correction.

Comments 5:  How does Fig. 7 support the following conclusion that the frequency domain of the signals exhibits two distinct primary frequency bands, specifically in the ranges of 0-20 Hz and 20-50 Hz?

Response 5: Thank you for your comment. Our analysis was incorrect. We have deleted the corresponding content in the original text for correction.

Comments 6: How does Fig. 8 support the following conclusion that the effect of finger band loosening on the signal is more pronounced in the 20-50 Hz frequency band? Isn't the impact of the loose bolt more pronounced in the 0-20 Hz?

Response 6: Thank you for your comments. First of all, regardless of whether the bolt is loose or not, there is a strong PSD response at the 0-20Hz frequency band. In contrast, when the bolt is fastened, there is almost no PSD response in the 20-50Hz frequency band. However, when the bolt is loosened, there is a significant PSD response in the 20-50Hz frequency band. Therefore, we believe that the 20-50Hz frequency band is the frequency band that is more significantly affected by the loosening of the bolt. Moreover, in the Discussion part, we mentioned that the 0-20Hz frequency band is more significantly affected by the environment. So, we mainly focused on the 20-50Hz frequency band. Of course, in subsequent research, we will carry out more work, such as targeted noise reduction and environmental factor calibration.

Comments 7:   Does the impact of varying train speeds on bolt loosening into consideration?

Response 7: We did not discuss the influence of train speed on bolt loosening. We believe that under the condition of loose bolts, a faster train speed will bring greater safety risks. Moreover, the safety risk of the bolt loosening experiment we carried out is relatively high. A greater number of train speeds represents a higher number of experiments and also represents a higher experimental risk. However, in order to explore whether there is a correlation between loosening and DAS signals, we adopted the highest train speed under conditions that can ensure safety, which is 4.5 km/h.

Comments 8:  ‘When the finger-band was initially loosened by one turn, the change in stiffness may have had a limited impact on the overall vibration response of the rail beam’. Is the 'limited' mentioned here attributable to insufficient train speed or weight?

Response 8: Thank you for your question. The “limited” here represents the comparison result with more buckling conditions under loosening. Train speed and weight do aggravate the problem of reduced stiffness. However, since the train speed and weight in our experiment are constant, and our purpose is to determine whether there is a relationship between DAS and bolts loosening. So, we ignored the influence of weight and train speed on stiffness. In subsequent research, we will consider more train speeds and weights as much as possible.

Reviewer 2 Report

Comments and Suggestions for Authors

1. Inquiry on Long-Term Stability Testing and Environmental Factor Calibration:
   This research work focuses on studying rail track bolt loosening anomalies. Have the authors conducted long-term stability tests of the system under different environmental conditions? Furthermore, considering the potential impacts of temperature and vibrations on the performance of fiber optic sensors, it is recommended that the authors discuss methods for calibrating and compensating for these environmental factors.

2. Suggestion for Expanding Experimental Scope:
   In their experiments, the authors have only tested two train speeds and conducted bolt loosening tests on a single rail fastener area. It is suggested that the authors expand the experimental scope to include various train speeds, different track structures, and multiple rail fastener areas, in order to verify the universality and reproducibility of the proposed method.

3. Request for Detailed Data Analysis Process:
   The paper utilizes WSST (Wavelet Scale Spectrum Transform) and PSD (Power Spectral Density) analysis to process vibration signals. The authors are requested to provide a more detailed data analysis flow, encompassing steps such as data preprocessing, feature selection, and methods for interpreting the results.

Author Response

I am deeply grateful to the reviewer. Your comments and suggestions have greatly improved my paper. Your professional insights have helped me identify areas for improvement and have enhanced the overall quality and clarity of my research. I truly appreciate the time and effort you have put into reviewing my work.

Comments 1: Inquiry on Long-Term Stability Testing and Environmental Factor Calibration:
This research work focuses on studying rail track bolt loosening anomalies. Have the authors conducted long-term stability tests of the system under different environmental conditions? Furthermore, considering the potential impacts of temperature and vibrations on the performance of fiber optic sensors, it is recommended that the authors discuss methods for calibrating and compensating for these environmental factors.

Response 1: Thank you for your comments. The duration of our experiment is not long. First of all, our experiment is a physical simulation experiment, not an experiment under actual urban traffic conditions. Secondly, a longer experiment time brings higher safety risks. Moreover, our research purpose is to explore whether there is a relationship between DAS signals and bolt loosening. It is only a qualitative analysis experiment. After completing the work in this paper, that is, after verifying that DAS signals are indeed affected by bolt loosening, we will design experiments with higher safety and conduct a larger number of experiments to analyze more carefully. For this reason, at this stage, the experiment does not consider the influence of temperature because the experiment time is short and the outdoor temperature is basically constant. In addition, the object we monitor is track vibration, so we did not correct the vibration itself. We must admit that there are still many areas for improvement in the research work of this paper. We will increase cost investment and conduct more detailed research in subsequent studies.

Comments 2: Suggestion for Expanding Experimental Scope:
In their experiments, the authors have only tested two train speeds and conducted bolt loosening tests on a single rail fastener area. It is suggested that the authors expand the experimental scope to include various train speeds, different track structures, and multiple rail fastener areas, in order to verify the universality and reproducibility of the proposed method.

Response 2: Thank you for your suggestion. Your suggestion is very inspiring to us. We did not discuss the influence of train speed and a greater number of loosened bolts on bolt loosening. We believe that under the condition of loose bolts, a faster train speed will bring greater safety risks. Moreover, the safety risk of the bolt loosening experiment we carried out is relatively high. A greater number of train speeds represents a higher number of experiments and also represents a higher experimental risk. However, in order to explore whether there is a correlation between loosening and DAS signals, we adopted the highest train speed under conditions that can ensure safety, which is 4.5 km/h. The same is true for bolt loosening. More loosening will bring greater experimental safety risks. As mentioned earlier, after verifying that bolt loosening does affect DAS signals, we will carry out more detailed research work. Of course, we will fully refer to your opinions to carry out future research work.

Comments 3: Request for Detailed Data Analysis Process:
The paper utilizes WSST (Wavelet Scale Spectrum Transform) and PSD (Power Spectral Density) analysis to process vibration signals. The authors are requested to provide a more detailed data analysis flow, encompassing steps such as data preprocessing, feature selection, and methods for interpreting the results.

Response 3: Thank you for your suggestion. We have supplemented the data analysis process, as shown in Fig. 12.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I would suggest this manusciprt be pubulished in current status.

Reviewer 2 Report

Comments and Suggestions for Authors

The author has adequately addressed my questions, and I recommend publication.