Experimental Study on Fiber Optic Strain Characterization of Overlying Rock Layer Movement Forms and States Using DFOS

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have provided an interesting scientific paper that uses the latest technologies to solve an important applied problem. In my opinion, this paper can be published in the journal after some comments are corrected. My recommendations on this matter are given below:
1. The BOFDA technology is not the simplest in terms of circuit implementation and not the cheapest, while it faces a number of serious limitations and problems: Brillouin Stokes and anti-Stokes components are a fairly weak signal, and the approach itself requires connecting the optical fiber to the instrument from both sides at once. In turn, I should note that today there are a number of the latest systems based on Rayleigh scattering that are not inferior to this approach in spatial resolution and the length of the interrogated line, while requiring the connection of only one end of the fiber optic sensor. These approaches are described in modern scientific reviews and individual articles included in these reviews:
https://doi.org/10.1109/ACCESS.2021.3061250 ;
https://doi.org/10.1109/JSEN.2022.3227677 ;
http://dx.doi.org/10.3390/s24165432 .
I would suggest that the authors consider similar approaches for further work or even include some of the works from these reviews in the introductory part of this article.
2. In my opinion, the operating principle of the system is not described quite clearly: for example, in line 133, the authors write that "radiation is modulated by a photodetector." In my opinion, usually modulators are involved in modulating radiation. Could you describe this point in more detail?
3. If I understand correctly, as in the time domain, the FD-analyzer ultimately operates on the Brillouin gain spectrum at each location of the fiber sensor. In conventional systems, frequency shift data is extracted using the LCF algorithm. What approach did the authors use?
4. If I understand correctly, the instrument was calibrated for temperature and strain, as shown in Figures 7 and 8. But these two factors usually act simultaneously on the sensor. In my opinion, the paper does not describe the method for discriminating between temperatures and strains in much detail. Typically, two polarization states or two different fibers are used for this, and then the system of two linear equations with two unknowns is solved. Neural network methods can also be used. Please write how this worked in your study?
5. The curves in Figure 12 are almost indistinguishable from each other. I would suggest the authors think about how to split this data into several graphs or present it differently.
6. The manuscript contains figures that are located at the end of the section. I would suggest raising them higher, immediately after the first mention in the text.

Author Response

Author's Reply to the Review Report (Reviewer 1)

Comment 1: 

The BOFDA technology is not the simplest in terms of circuit implementation and not the cheapest, while it faces a number of serious limitations and problems: Brillouin Stokes and anti-Stokes components are a fairly weak signal, and the approach itself requires connecting the optical fiber to the instrument from both sides at once. In turn, I should note that today there are a number of the latest systems based on Rayleigh scattering that are not inferior to this approach in spatial resolution and the length of the interrogated line, while requiring the connection of only one end of the fiber optic sensor. These approaches are described in modern scientific reviews and individual articles included in these reviews:

https://doi.org/10.1109/ACCESS.2021.3061250 ;

https://doi.org/10.1109/JSEN.2022.3227677 ;

http://dx.doi.org/10.3390/s24165432 .

I would suggest that the authors consider similar approaches for further work or even include some of the works from these reviews in the introductory part of this article.

Response 1:

Thank you for pointing this out. We agree with this comment. Therefore, we have revised the manuscript  according to the reviewer. All the revision can be found in page 4, in paragraph 2, from line 145 to line 151.The revised contents are shown below.

OFDR currently stands out as the technology with the highest spatial resolution in overburden deformation monitoring experiments, achieving a resolution of 0.001 m. It is mainly employed for precision measurements in indoor model experiments. It is worth noting that OFDR, based on Rayleigh scattering technology, has demonstrated broad application potential in various sensing fields, such as temperature, strain, vibration, pressure, shape, magnetic field, refractive index, radiation, gas, and flow velocity sensing, due to its high spatial resolution, high sensitivity, and distributed measurement capabilities[22-24].

 

Comment 2: 

In my opinion, the operating principle of the system is not described quite clearly: for example, in line 133, the authors write that "radiation is modulated by a photodetector." In my opinion, usually modulators are involved in modulating radiation. Could you describe this point in more detail?

Response 2:

Thank you for pointing this out. We agree with this comment. Therefore, we have revised the manuscript  according to the reviewer. All the revision can be found in page 5, in the last paragraph , from line 196 to line 234.The revised contents are shown below.

BOFDA stems from measuring a complicated base-band transmitting function that is associated with the amplitude of pump light and Stokes light traveling subtendedly along the optical fiber’s whole length [32], as shown in Fig. 1 below.

Fig. 1  Schematic Diagram of BOFDA Sensing Technology Principle

As soon as the base-band transfer function is determined, it can be subjected to inverse Fourier transform to obtain the impulse response [32-34]. By analyzing the transmission pump intensity of probe light and pump light under different frequency differences, the distribution of fiber optic temperature and strain can be derived [35].

The detailed process of strain or temperature measurement by BOFDA can be briefly explained through BOFDA's system configuration diagram, shown as Fig.2 below.

Fig.2  BOFDA basic configuration.

As shown in the figure, The cw light of a narrow-linewidth pump laser is coupled into one end of a single-mode fiber.At the other end the cw light of a narrow-linewidth probe laser is coupled in,whose frequency is downshifted compared with that of the pump laser by an amount that equals the characteristic Brillouin frequency of the      fiber. At 1.3  this characteristic frequency has a value of some 13 GHz for standard telecommunication single-mode fibers[36].The light of the probe laser is modulated in amplitude with a variable angular modulation frequency . The measurement principle is that for each value of  the alternating parts of the modulated probe wave intensity and of the modulated pump wave intensity are recorded at the end of the sensor fiber. Here the probe-induced Brillouin loss of the pump wave is used.The output signals of the Photo Detectors(PD) are fed to a network analyzer(NWA),which determines the baseband transfer function  of the test fiber.The output of the NWA is digitized by an analog-to-digital converter(A/D) and fed to a signal processor,which calculates the inverse fast Fourier transform(IFFT).This IFFT is a good approximation of the pulse response  of the test fiber and resembles the temperature and strain distribution along the fiber. Finally, stemming from the relationship between the propagation time and spatial span of light in optical fibers , the specific spatial and frequency shift relationship in optical fibers can be obtained. The following mathematical formulas provide a comprehensive explanation for all these processes[28,37-38].

,

(1)

In the formula,  represents the baseband transfer function,  represents the temporal pulse response of fiber optic sensors,  represents the spatial pulse response of fiber optic sensors , c represents the light velocity; n denotes the fiber optic refractive index ; t is the time it takes for light to be launched and received.

 

Comment 3: 

If I understand correctly, as in the time domain, the FD-analyzer ultimately operates on the Brillouin gain spectrum at each location of the fiber sensor. In conventional systems, frequency shift data is extracted using the LCF algorithm. What approach did the authors use?

Response 3:

Thank you for pointing this out. We agree with this comment. Therefore, we have revised the manuscript  according to the reviewer. All the revision can be found in page 6, in the last paragraph , from line 235 to line 244.The revised contents are shown below.

In summary, The key to the entire BOFDA sensing system is the acquisition of the baseband transfer function. BOFDA can reconstruct the Brillouin gain spectrum (BGS) by performing IFFT with the baseband transfer function of the sensing fiber. Then, Based on the spectrum the Brillouin frequency shift (BFS) can be determined by performing Lorentz curve fitting (LCF)[11,36,38] , in the end the strain or temperature changes at each position along the fiber optic cable at each measurement moment are obtained., as demonstrated in Fig.3 below.

 

Figure 3. Description of BFS diagram caused by strain in BOFDA(Sensing Principle of BOFDA).

Comment 4: 

If I understand correctly, the instrument was calibrated for temperature and strain, as shown in Figures 7 and 8. But these two factors usually act simultaneously on the sensor. In my opinion, the paper does not describe the method for discriminating between temperatures and strains in much detail. Typically, two polarization states or two different fibers are used for this, and then the system of two linear equations with two unknowns is solved. Neural network methods can also be used. Please write how this worked in your study?

Response 4:

Thank you for pointing this out. We agree with this comment. Therefore, we have revised the manuscript  according to the reviewer. According to the review comments of the reviewers, a dedicated chapter has been added to this manuscript to discuss the issue of cross sensitivity between temperature and strain, as well as how we addressed it in our experiments. All the revision can be found in page 14, in paragraph 2 , from line 498 to line 527.The revised contents are shown below.

3.1.4 The Influence of Temperature on Strain Testing in BOFDA Measurement

The temperature coefficient of the optical fiber studied in this experiment is eliminated by placing the relaxed and strain free optical fiber in a computer-controlled water bath to eliminate the influence of strain; The strain coefficient is obtained by conducting fiber optic tensile experiments under conditions of essentially constant temperature. Although the temperature strain cross sensitivity problem was basically eliminated in this fiber optic strain coefficient calibration experiment, it is difficult to eliminate the temperature strain cross sensitivity problem in actual fiber optic testing.

(1) When the temperature changes little and remains basically constant.

According to the calibrated fiber temperature coefficient of 1.07 MHz/and strain coefficient of 0.04954 MHz/in this article, it can be seen that the influence of temperature on Brillouin frequency is much smaller than that of strain. Experimental studies have shown that if the temperature change does not exceed 5, the influence of temperature can be ignored. Since our experiment was conducted indoors, the temperature was strictly controlled and maintained at a relatively constant level. The temperature variation throughout the entire experiment was less than 5 degrees, so its impact on strain measurements was negligible.

(2) When temperature changes exceed 5°C, temperature compensation is necessary for fiber optic strain testing to eliminate errors in strain monitoring caused by temperature fluctuations. Several methods can achieve this:

• Parallel Loose Tube Fiber: Place a loose tube fiber of the same length parallel to the strain fiber. Since the loose tube fiber is minimally affected by strain, it can compensate for temperature effects on the strain data.
• Relaxed Fiber Section: Reserve a relaxed fiber section of the same type between the strain-testing fiber and the DFOS analyzer under identical temperature conditions. This section, unaffected by strain, allows subtracting the strain-testing fiber's Brillouin frequency shift from the relaxed fiber's average shift for temperature compensation.
• FBG Temperature Sensors: Connect Fiber Bragg Grating (FBG) temperature sensors in series with strain-sensing fibers at regular intervals for temperature compensation.
• Machine Learning:Apply machine learning and other AI techniques to analyze monitoring data and eliminate temperature-related influences.

Comment 5: 

The curves in Figure 12 are almost indistinguishable from each other. I would suggest the authors think about how to split this data into several graphs or present it differently.

Response 5:

Thank you for pointing this out. We agree with this comment. Therefore, we have revised the manuscript  according to the reviewer. This study has redrawn Fig. 12 in the form of a vector diagram, removing the parts of the fiber optic front-end and back-end that are not affected by the experiment, and proportionally enlarging them to increase their clarity.All the revision can be found in page 17,  , from line 592 to line 594.The revised figures are shown below.

(a)

(b)

Figure 12. The fiber strain distribution of the total optical fibers during the whole excavation  processes: (a) Horizontal fiber optic strain ; (b) Vertical fiber optic strain .

 

Comment 6: 

The manuscript contains figures that are located at the end of the section. I would suggest raising them higher, immediately after the first mention in the text.

Response 6:

Thank you for pointing this out. We agree with this comment.The author have carefully studied the manuscript and moved the figures and tables to the positions mentioned for the first time in the text

 

Finally, I would like to sincerely thank the experts for their valuable review comments. These comments have not only significantly improved the quality of this manuscript but have also greatly benefited the authors. Thank you once again for your time and effort.

 

 

 

 

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

I commend the authors for addressing an important topic in coal mining safety through innovative DFOS technology. However, the manuscript requires substantial revisions to enhance clarity, rigor, and scientific impact. Below are specific recommendations:

1.The abstract is overly verbose and lacks focus. Key quantitative results (e.g., fracture zone height) are buried in lengthy descriptions.

2.The literature review lacks critical analysis of DFOS applications in mining. Recent advancements (e.g., machine learning in DFOS, post-2020 studies) are omitted. Please add a table summarizing DFOS studies (e.g., spatial resolution, limitations) and explicitly highlight gaps (e.g., limited research on rock block kinematics).

3.The BOFDA sensing principle (Section 2.1) is described with excessive mathematical formulas but insufficient conceptual explanation. Please add a simplified schematic (e.g., signal propagation diagram) and clarify how Brillouin frequency shift correlates with strain in mining contexts.

4. Figures 5, 6, and 10–19 are pixelated, lack axis labels (e.g., Figure 12), or omit scale bars (e.g., Figure 13). Please redraw figures using vector graphics, add units (e.g., strain in με, distance in m), and ensure consistency in color schemes.

5.Section 3.1.3 claims "strain transfer efficiency" but lacks validation against industry standards (e.g., ASTM D3039). Please compare results with existing studies (e.g., Cheng et al., 2022) and discuss potential slippage errors.

6.The UDEC simulation (Section 5) lacks boundary condition details (e.g., mesh size, convergence criteria).

7.Table 3 lists errors between methods but does not quantify uncertainties in BOFDA measurements (e.g., spatial resolution limits).

8.The discussion (Section 6) fails to address limitations (e.g., temperature effects ignored beyond 5°C, simplified strata assumptions). Please dedicate a subsection to limitations and propose mitigation strategies (e.g., hybrid temperature-strain sensors).

Comments on the Quality of English Language

Frequent grammatical errors (e.g., "the basic roof maintained this strong voussoir beam movement structure, causing the stepped shape of FOS curve disappear"). Please engage a professional editor to correct syntax, tense consistency, and article usage.

Author Response

Author's Reply to the Review Report (Reviewer 2)

Comment 1: 

The abstract is overly verbose and lacks focus. Key quantitative results (e.g., fracture zone height) are buried in lengthy descriptions.

Response 1:

Thank you for pointing this out. We agree with this comment. Therefore, we have revised the abstract according to the reviewer. All the revision can be found in page 1, paragraph of the abstract, from line 9 to line 24.The revised abstract is shown below.

“Mastering the movement laws of hard overlying rock layers is the foundation of the development of coal mining technology and plays an important role in improving coal mine safety production. Therefore, an indoor similar simulation experiment was conducted based on an actual coal mining face to test the strain variations of the pre embedded optical fibers in the model using distrtibuted fiber optic sensing. Finally, the fiber optic strain distribution curve was used to characterize the movement form and state of the overlying rock layer and fractured rock blocks. The experimental results show that: (1) The strain distribution of horizontally laid optical fibers is characterized by an upward trapezoidal convex platform, reflecting the evolution law of various horizontal movement forms of overlying rock layers: voussoir beamcantilever beamreverse cantilever beam voussoir beam. The strain curve of vertically laid optical fibers is characterized by two levels of right-handed trapezoidal protrusions above and below, representing the motion state of the upper voussoir beam - lower cantilever beam structure of the overburden. (2) In addition, as excavation progresses, the range and height of failure deformation  of the overlying rock layers develop in a stepped shape. (3) In the end, the final vertical development heights of the cantilever beam structure and the voussoir beam structure in the overburden were 90.27m and 24.99m, respectively. The experimental results are highly consistent with the UDEC numerical simulation and mandatory calculation formulas , thus verifying the feasibility of the experiment. These research results provide theoretical and experimental support for safe coal mining in practical working faces”

 

 

Comment 2:

The literature review lacks critical analysis of DFOS applications in mining. Recent advancements (e.g., machine learning in DFOS, post-2020 studies) are omitted. Please add a table summarizing DFOS studies (e.g., spatial resolution, limitations) and explicitly highlight gaps (e.g., limited research on rock block kinematics).。

Response 2:

Thank you for pointing this out. We agree with this comment. According to the review comments of the experts, the following content has been added.

(1) The manuscript enriches the critical analysis of the application of traditional monitoring methods in mining. The modification is located on page 2, in the fifth paragraph, from line 70 to line 87 of the manuscript. The revision is shown as below.

“In SSE, the most important thingis how to accurately obtain parameters such as displacement, strain, temperature, stress and so on within the overburden.  However, in traditional surface subsidence evaluation (SSE), displacement measurement typically relies on close-range photogrammetry, total stations, and dial gauges. For stress-strain monitoring, electromagnetic sensors such as strain gauges, pressure sensors, pressure boxes, and pressure gauges are commonly utilized. However, these conventional measurement methods and sensors are fraught with inherent limitations and drawbacks: (1) They exhibit relatively low accuracy and sensitivity, resulting in significant measurement errors. (2) They are "point-based" measurement tools, which means their layout and operation are complex and prone to missing critical data. (3) They can only monitor surface displacement deformation and localized stress or strain, failing to provide internal distributed monitoring. (4) Continuous and real-time monitoring of the entire model is challenging to achieve.Most critically, the excessive insertion of various "point-type" sensors at different intervals inevitably alters the original state of the tested structure. This leads to deviations in the test results and directly impacts the assessment of the overall stability of the overlying rock mass under mining conditions.Therefore, there is an urgent need to develop a high-precision, real-time, parallel testing method that can achieve full-field and internal deformation and stress monitoring of the model.”

(2) Recent advances in DFOS, particularly the application progress of machine learning in DFOS since 2020, have been incorporated into the manuscript. The revised contents can be found in page 3, in the first paragraph, from line 93 to line 110 of the manuscript. The revision is shown as below.

“DFOS has become a hot topic for scholars due to its inherent advantages in recent years. DFOS technology has developed rapidly, especially with the improvement of computing power and the advancement of interdisciplinary integration. Machine Learning (ML) have propelled DFOS to achieve remarkable progress in both technology and application. For instance, Christos et al.[11] addressed the cross-sensitivity issue between temperature and humidity in DFOS monitoring using ML. They proposed a signal post-processing method based on a convolutional neural network (CNN), which reduced the temperature measurement time of the Brillouin optical frequency domain analyzer (BOFDA) by more than nine times [12]. Additionally, they applied a Bayesian algorithm in ML to resolve the cross-sensitivity problem between temperature and strain in DFOS monitoring [13]. Liu et al. developed a corrosion quantification method based on a ML model for automatic analysis of strain data measured by DFOS, enabling simultaneous monitoring of cracks and corrosion [14].In summary, with the aid of ML, DFOS has achieved smarter data processing, higher resolution, and more accurate predictive modeling. These advancements have expanded the application of DFOS in structural health monitoring, environmental sensing, and industrial monitoring. As ML technology continues to evolve, the integration of artificial intelligence and DFOS is expected to drive further breakthroughs in sensing technology[15].”

(3)The manuscript now includes a statistical comparison table of DFOS application technology for overburden mining deformation monitoring according the reviewer’s comment. It also features a critical analysis of DFOS application in mining from the technical parameters perspective. The revised contents can be found in page 3, in the third paragraph, from line 127 to line 174 of the manuscript. The revision is shown as below.

“The author classified and compared the literature on experimental research on coal mine overburden mining deformation monitoring with DFOS from 2014 to present. The comparison was carried out from several aspects, including the applied technologies [Brillouin optical time domain reflection (BOTDR), Brillouin optical time domain analysis (BOTDA), Pulse Pre-Pump BOTDA (PPP-BOTDA), Brillouin optical frequency domain analysis (BOFDA), optical frequency domain reflection (OFDR)], spatial resolution, application scenarios, advantages, limitations, and experimental research content, as shown in Table 1.

Table 1 Statistical comparison of main DFOS applied in overburden deformation monitoring

Methods	Spatial resolution	Application scenarios	research contents	Number of Papers	Advantage	limitation
BOTDR	1m	Field test	Height of WCFZ	12	Single end testing, No need for loops Good robustness, Easy locate breakpoints Long distance (80km) Wide range Excellent adaptability	Low spatial resolution and strain testing accuracy; Difficult to achieve precise measurement
			Laws of overburden deformation movement	9
			Strata separation	1
			Stress evolution	1
BOTDA	0.5m	Indoor test	state of overlying strata	2	measurement accuracy and spatial resolution are significantly higher than BOTDR	Need testing circuit, with poor environmental adaptability; Unable to measure breakpoints
		Field test	strata separation	1
BOFDA	0.2m	Field test	Settlement displacement	1	Higher spatial resolution and strain testing accuracy than BOTDA	Double ended testing, unable to measure breakpoints; The on-site layout is quite complex
		Indoor test	Height of WCFZ	2
			Evolution of voids	1
			Settlement displacement	1
PPP- BOTDA	0.05/0.1m	Indoor test	Overburden deformation	16	Higher spatial resolution and strain testing accuracy than BOTDA	Dual end testing, poor environmental adaptability; The on-site wiring is relatively complex; The failure rate of on-site application instruments is relatively high
			Height of WCFZ	3
			Coal mine pressure	3
			Evolution of voids and separation	3
			fault activation	2
			Rock- fiber coupling	1
OFDR	0.001m	Indoor test	Overburden deformation	1	High spatial resolution and strain testing accuracy, high signal-to-noise ratio, low probe light power	Poor environmental adaptability; Complex on-site layout Height failure rate on-site application; Smaller range as high-precision measurement
			Key strata stability	1
			Hinge structure & rotating angle	1
			Void evolution	1

As illustrated in Table 1, BOTDR, despite its relatively low spatial resolution of 1 m, is predominantly utilized for on-site testing in coal mines. This preference stems from its notable advantages, including single-ended testing, extensive measurement range, and ease of fiber deployment. Both BOFDA and BOTDA offer higher spatial resolutions of 0.2 m and 0.5 m, respectively. They employ dual-end monitoring methods and are suitable for both on-site monitoring and indoor SSE. BOFDA is particularly recognized for its high accuracy in strain measurement. PPP-BOTDA, with a spatial resolution of 0.05 meters and a dual-end monitoring mode, is primarily applied in indoor similarity model experiments, accounting for 70% of all research content in this field. It has emerged as a crucial tool in SSE research.

OFDR currently stands out as the technology with the highest spatial resolution in overburden deformation monitoring experiments, achieving a resolution of 0.001 m. It is mainly employed for precision measurements in indoor model experiments. It is worth noting that OFDR, based on Rayleigh scattering technology, has demonstrated broad application potential in various sensing fields, such as temperature, strain, vibration, pressure, shape, magnetic field, refractive index, radiation, gas, and flow velocity sensing, due to its high spatial resolution, high sensitivity, and distributed measurement capabilities[16-18].

Given that this experimental study focuses on the engineering application of DFOS, the primary objective is to measure the deformation of overlying rock strata. The key concern is the accuracy of fiber optic strain measurement. Considering the limitations of time and experimental conditions, BOFDA, which combines high spatial resolution and high strain measurement accuracy, was selected for this experimental research.

As shown in Table 1, research on DFOS monitoring of overburden mining deformation primarily focuses on the development height of water-conducting fracture zones (WCFZ), coal mining pressure, the overall deformation status of the overburden, and the evolution laws of rock separation or pores. In contrast, studies on the movement patterns and states of fractured rock blocks, and even individual Key Strata (KS), are relatively limited.

 However, the deformation and movement of the overburden are essentially an integrated manifestation of the motion of the overlying hard strata, known as the KS. As coal mining progresses, these rock layers fracture and form discrete "rock blocks." Fundamentally, this is a mechanical phenomenon characterized by the movement and interaction of these fractured rock blocks[2]. Furthermore, the movement of these fractured rock blocks is a key cause of underground coal mine accidents. Therefore, it is imperative to conduct thorough and meticulous research on the movement forms and states of both the overlying KS and the fractured rock blocks. Regrettably, in the realm of DFOS research, studies that focus on the movement characteristics of rock blocks are currently few and far between.”

 

Comment 3:

The BOFDA sensing principle (Section 2.1) is described with excessive mathematical formulas but insufficient conceptual explanation. Please add a simplified schematic (e.g., signal propagation diagram) and clarify how Brillouin frequency shift correlates with strain in mining contexts.

Response 3:

Thank you for pointing this out. We agree with this comment. According to the review comments of the experts, the following content has been revised.

• According to the expert review comments, the author of this article first added a brief schematic diagram of the BOFDA sensing principle.The added figure can be found in page 5, in the end of the page, from line 199 to line 201 of the manuscript. The revision is shown as below.

Fig. 2  Schematic Diagram of BOFDA Sensing Technology Principle

(2) According to the expert review comments, the author of this paper elaborates on the correlation between Brillouin frequency shift and strain in mining environments.The revision can be found in page 7, in the second page, from line 250 to line 264 of the manuscript. The revision is shown as below.

“Based on the aforementioned analysis, when an optical fiber is subjected to axial tension or compression and experiences strain, the Brillouin frequency monitored by BOFDA will correspondingly increase or decrease. Therefore, when a sensing optical fiber is embedded in the overlying rock strata of a coal mining face, BOFDA can be utilized to monitor the deformation of the overlying rock during coal extraction. Specifically, the following scenarios can be observed:

• When the rock strata are subjected to shear stress or tensile stress, the buried optical fiber will be stretched, resulting in an increase in the Brillouin frequency value.
• The separation of fractured rock blocks, such as when these blocks detach from the original rock layer and collapse or rotate around it, will also exert tensile effects on the buried optical fibers. Consequently, the Brillouin frequency value will increase.
• Conversely, if the loosely stacked fractured rock blocks are subjected to compaction, the gaps between the rock blocks will gradually decrease. The optical fibers embedded in the original rock layer will be correspondingly compressed axially. The tension previously induced in the fibers by the collapse and rotation of the rock blocks will gradually diminish, inevitably leading to a gradual decrease in the Brillouin frequency value.”

 

Comment 4:

Figures 5, 6, and 10–19 are pixelated, lack axis labels (e.g., Figure 12), or omit scale bars (e.g., Figure 13). Please redraw figures using vector graphics, add units (e.g., strain in με, distance in m), and ensure consistency in color schemes.

Response 4:

Thank you for pointing this out. We agree with this comment. According to the review comments of the experts, the author carefully studied the manuscript and made revisions based on expert opinions.The following revised contents have been made.

• Figures 5, 6, and 10-19 have been redrawn and saved in EMF vector format. Vector image files have been inserted instead of using the previous method of copying and pasting.

The revised Fig.5  can be found in page 9,  from line 311 to line 314 of the manuscript.

The revised Fig.6  can be found in page 10,  from line 357 to line 360 of the manuscript.

The revised Fig.10 can be found in page 15,  from line 539 to line 541 of the manuscript.

The revised Fig.11 can be found in page 16,  from line 578 to line 580 of the manuscript.

The revised Fig.12 can be found in page 17,  from line 592 to line 594 of the manuscript.

The revised Fig.13 can be found in page 18,  from line 646 to line 648 of the manuscript.

The revised Fig.14 can be found in page 19,  from line 654 to line 656 of the manuscript.

The revised Fig.15can be found in page 19,  from line 676 to line 678 of the manuscript

The revised Fig.16 can be found in page 20,  from line 687 to line 689 of the manuscript.

The revised Fig.17 can be found in page 20,  from line 703 to line 705 of the manuscript.

The revised Fig.18 can be found in page 22,  from line 753 to line 755 of the manuscript.

The revised Fig.19 can be found in page 23,  from line 821 to line 824 of the manuscript.

• A graph without coordinate axis labels has been added.The revised Fig.12 can be found in page 17,  from line 592 to line 594 of the manuscript.
• Scale bars have been added to the relevant figures(from Fig.13 to Fig.19)in accordance with the expert review comments. 
• Units have been added to the missing coordinate axis labels.In the early stage of the drawing process, some numerical units of coordinate axis labels were omitted. I deeply apologize for any inconvenience caused to the reviewers. I have added these missing units.

 

Comment 5:

Section 3.1.3 claims "strain transfer efficiency" but lacks validation against industry standards (e.g., ASTM D3039). Please compare results with existing studies (e.g., Cheng et al., 2022) and discuss potential slippage errors.（Cheng, Lin, et al. "Experimental verification research of pipeline deflection deformation monitoring method based on distributed optical fiber measured strain." Measurement 199 (2022): 111483.）

Response 5:

Thank you for pointing this out. We agree with this comment. Therefore, we have revised the manuscript according to the reviewer. All the revision can be found in page 13-14,  from line 468 to line 497.The revised contents are shown below.

“This experimental study refers to the existing experimental research [17] and is based on the study conducted by Cheng et.al. Although the principles of the two studies are essentially the same, there are significant differences in the experimental methods. Cheng et al. embedded optical fibers into sandy soil and conducted fiber pull-out tests to investigate the development of stress and strain as the relative displacement between the soil and the sensing fiber gradually increased. They studied the entire process from full coupling to partial coupling and eventually to the fiber slipping and completely detaching, in order to determine the deformation coordination and strain transfer characteristics between the sensing fiber and the surrounding soil. In contrast, the current experimental study involves fully adhering a 2 mm optical fiber to the outer surface of an SS cable. During the pull-out process of the SS cable, the strain changes of the SS cable are obtained by measuring its elongation with a micrometer. Meanwhile, the strain changes of the 2 mm optical fiber are measured using BOTDA (Brillouin Optical Time Domain Analysis). By comparing the two sets of strain data, we aim to determine the ability of the 2 mm optical fiber to characterize the deformation of external structures and its strain transfer efficiency.

As shown in Figure 9(a), before the SS cable is stretched by 2 mm, the strain transfer efficiency gradually deteriorates, falling below 0.0975. This indicates that in the initial stage of fiber stretching, due to the effect of pre-tension stress, the fiber cannot effectively reflect the strain changes of the external structure. When the SS cable is stretched by more than 2 mm, the strain changes of the two become increasingly consistent. This suggests that after a 3 mm stretch, the influence of pre-tension strain transfer is essentially eliminated, and the strain transfer coefficient stabilizes above 0.99. Additionally, as seen in Figures 9(a) and 9(b), when the SS cable is stretched to 16 mm, the strain transfer coefficient no longer increases. This may be due to the effect of sheath slippage, the measurement error of the micrometer, or the measurement error of the BOTDA. The experiment shows that before a structural body undergoes a strain change of 6000 με, the 2 mm optical fiber has a high strain transfer efficiency and can effectively reflect the strain changes of the external measured structure.Therefore, the strain transfer efficiency of the 2 mm fiber satisfies the testing demands.”

Comment 6:

The UDEC simulation (Section 5) lacks boundary condition details (e.g., mesh size, convergence criteria).

Response 6:

Thank you for pointing this out. We agree with this comment. Therefore, we have revised the manuscript according to the reviewer. All the revision can be found in page 24-25, in the end of page 24, from line 872 to line 884. The revised contents are shown below.

 

“The boundary and loading conditions for the numerical simulation software are determined as follows: ① Equal force horizontal constraints are applied to the left and right boundaries of the UDEC model, so the horizontal displacement of the boundaries is zero; ② The bottom boundary of the UDEC model is fixed, with zero displacement; ③ The top boundary of the UDEC model is set as a free boundary. The material failure of the model conforms to the Mohr-Coulomb strength criterion.This UDEC simulation is calculated according to the UDEC default convergence criterion, i.e. the convergence threshold of the equivalent force is 10^-5.

The grid setting of the UDEC model is shown in the figure 20 below, and the mining coal seam and its overlying rock layer adopt the strategy of infill grid, and the upward grid is gradually thinned.

 

Figure 20.  Schematic diagram of UDEC numerical simulation grid model”

 

Comment 7:

Table 3 lists errors between methods but does not quantify uncertainties in BOFDA measurements (e.g., spatial resolution limits).

Response 7:

Thank you for pointing this out. We agree with this comment. Therefore, we have revised the manuscript according to the reviewer. All the revision can be found in page 28, in the second to last paragraph, from line 1006 to line 1013. The revised contents are shown below.

“Furthermore, by averaging the results obtained from the three methods and subtracting the BOFDA characterization from the mean, the error caused by BOFDA with a spatial resolution of 0.2m in deformation measurement can be approximately quantified. That is, the error in the height of the caving zone is 0.944%, and the error in the height of the fracture zone is 0.096%. This is consistent with the results of the comparison above. It is believed that if higher spatial resolution DFOS testing techniques ,such as OFDR etc., are used in the future , more satisfactory results will be achieved.”

Comment 8:

The discussion (Section 6) fails to address limitations (e.g., temperature effects ignored beyond 5°C, simplified strata assumptions). Please dedicate a subsection to limitations and propose mitigation strategies (e.g., hybrid temperature-strain sensors).。

Response 8:

Thank you for pointing this out. We agree with this comment. Therefore, we have revised the manuscript according to the reviewer. All the revision can be found in page 14, in paragraph 2, from line 498 to line 527. The revised contents are shown below.

3.1.4 The Influence of Temperature on Strain Testing in BOFDA Measurement

The temperature coefficient of the optical fiber studied in this experiment is eliminated by placing the relaxed and strain free optical fiber in a computer-controlled water bath to eliminate the influence of strain; The strain coefficient is obtained by conducting fiber optic tensile experiments under conditions of essentially constant temperature. Although the temperature strain cross sensitivity problem was basically eliminated in this fiber optic strain coefficient calibration experiment, it is difficult to eliminate the temperature strain cross sensitivity problem in actual fiber optic testing. When the temperature changes greatly, temperature compensation needs to be considered.

(1) When the temperature changes little and remains basically constant.

According to the calibrated fiber temperature coefficient of 1.07 MHz/and strain coefficient of 0.04954 MHz/in this article, it can be seen that the influence of temperature on Brillouin frequency is much smaller than that of strain. Experimental studies have shown that if the temperature change does not exceed 5, the influence of temperature can be ignored.

(2) When temperature changes exceed 5°C, temperature compensation is necessary for fiber optic strain testing to eliminate errors in strain monitoring caused by temperature fluctuations. Several methods can achieve this:

• Parallel Loose Tube Fiber: Place a loose tube fiber of the same length parallel to the strain-testing fiber. Since the loose tube fiber is minimally affected by strain, it can compensate for temperature effects on the strain data.
• Relaxed Fiber Section: Reserve a relaxed fiber section of the same type between the strain-testing fiber and the DFOS analyzer under identical temperature conditions. This section, unaffected by strain, allows subtracting the strain-testing fiber's Brillouin frequency shift from the relaxed fiber's average shift for temperature compensation.
• FBG Temperature Sensors: Connect Fiber Bragg Grating (FBG) temperature sensors in series with strain-sensing fibers at regular intervals for temperature compensation.
• Machine Learning:Machine learning and other Artificial Intelligent  techniques can be applied  to analyze monitoring data and eliminate temperature-related influences .

 

Comments on the Quality of English Language

Frequent grammatical errors (e.g., "the basic roof maintained this strong voussoir beam movement structure, causing the stepped shape of FOS curve disappear"). Please engage a professional editor to correct syntax, tense consistency, and article usage.

Response : 

Thank you for pointing this out. We agree with the reviewer's comment and express our gratitude for it. Therefore, we have hired several English teachers from our university to proofread and revise the syntax, tense consistency, and article usage of this manuscript based on the opinions of the reviewers.

 

Finally, I would like to sincerely thank the experts for their valuable review comments. These comments have not only significantly improved the quality of this manuscript but have also greatly benefited the authors. Thank you once again for your time and effort.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors practically studied the strata movement laws of mining beam structures with different shapes by the distributed optical fiber sensing technology. The indoor model was demonstrated to predict the overburden movement forms and limit for the safe coal mining process. Major revision is recommended for the following reasons.

1. Figure 1 is not suggested to insert among the introduction section;
2. The special abbreviations should be explained by giving the full name at their first locations;
3. The resolution for the figures should be improved, such as figure 2;
4. Pay attentions to the numbers of figures. Figure 5 should be figure 4, etc.;
5. Figure 7c, it seems that one point is missed near 8000, please explain the reason or include this data;
6. The working range in this work in only one couple of meters, whether the spatial resolution will be better than the whole working range of fTB2505 (25km@0.2m). Please discuss this issue.

Author Response

Comments 1: Figure 1 is not suggested to insert among the introduction section.

Response 1: Thank you for pointing this out. We agree with this comment. Therefore, After careful consideration, the author has moved the revised diagram to Section 2.4, inserting it before discussing the theory of fiber optic strain characterization of overburden mining deformation. All the revision can be found in page 9, in paragraph 1, from line 302 to line 302.The revised contents are shown below.

Comments 2: The special abbreviations should be explained by giving the full name at their first locations.

Response 2:  Thank you for pointing this out. We agree with this comment. Therefore, The author has carefully proofread this article multiple times, checked the names of special abbreviations carefully, and made revisions. .

Comments 3: The resolution for the figures should be improved, such as figure 2.

Response 3:  Thank you for pointing this out. We agree with this comment. Therefore, The author has redrawn and vectorized most of the images in the article, including Figure 2..

Comments 4: Pay attentions to the numbers of figures. Figure 5 should be figure 4, etc.

Response 4:  Thank you for pointing this out. We agree with this comment. Therefore, we have revised the manuscript  according to the reviewer.

Comments 5: Figure 7c, it seems that one point is missed near 8000, please explain the reason or include this data.

Response 5:  Thank you for pointing this out. We agree with this comment. Therefore, The author found through inspection and verification that the data for fiber stretching 18mm was omitted during the drawing process. The author has already supplemented the data and redrawn it. The figure can be found in page 12,  from line 436 to line 438.

Comments 6: The working range in this work in only one couple of meters, whether the spatial resolution will be better than the whole working range of fTB2505 (25km@0.2m). Please discuss this issue.

Response 6:  Thank you for pointing this out. Due to the fact that the highest spatial resolution of BOFDA used in this study is 0.2m, this study has already adopted its highest spatial resolution in the experiment. Although the device can achieve 0.2m spatial resolution testing at 25km, if the 25km range setting is used for measurement within a few meters, it will take a lot of unnecessary measurement time. Therefore, this study used 0.2m maximum spatial resolution testing within a smaller distance range, and its testing effect is consistent with the 25km distance range, and the speed is faster.

Finally, I would like to sincerely thank the experts for their valuable review comments. These comments have not only significantly improved the quality of this manuscript but have also greatly benefited the authors. Thank you once again for your time and effort.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The author's revisions to the paper met the expectations of the reviewers and thus could be accepted.

Reviewer 3 Report

Comments and Suggestions for Authors

My comments' replies are satisfied.