A method for monitoring the grouting effect in goaf areas based on fiber optic sensing

Abstract

This invention discloses a method for monitoring the grouting effect in goaf areas based on fiber optic sensing. The method includes: lowering a sensing optical cable assembly into a vertical grouting hole; then, using a hole-mouth optical cable winch, slowly lowering the assembly to the bottom of the goaf; the optical cable passing through the grouting sealing flange and connecting to a distributed fiber optic DAS device; during grouting, the distributed fiber optic DAS device collects acoustic wave data in real time; based on the energy accumulation and integration value of the acoustic wave data, significant peak channels of the grout surface are identified through mathematical modeling and optimization constraints to locate the grout surface position; combined with the depth information of the optical cable in the hole, the final position of the grout surface within the goaf is determined; and by continuously collecting and processing fiber optic scattering signals through the distributed fiber optic DAS device, real-time dynamic monitoring of the grout level height in the hole is achieved. This invention can automatically monitor changes in the grout surface during grouting, evaluate the grouting effect in real time, and provide data support for real-time control of the grouting process.

Description

Technical Field

[0001] This invention belongs to the field of coal mining technology and relates to a method for monitoring the grouting effect in goaf based on optical fiber sensing. Background Technology

[0002] Goaf areas are cavities formed during coal mining. Their existence can lead to geological disasters such as surface subsidence, collapse, and spontaneous combustion, seriously threatening life, property, and the ecological environment. Surface drilling and grouting is a common method for goaf remediation. By injecting grout materials (such as cement grout, fly ash grout, etc.) into the goaf, the cavities are filled and the rock strata are reinforced, thereby preventing spontaneous combustion, surface subsidence, and collapse. However, traditional monitoring methods for the grouting process, such as core drilling and surface subsidence monitoring, cannot monitor the dynamic changes of the grout in real time, making it difficult to meet the real-time control requirements of the grouting process. Summary of the Invention

[0003] To address the shortcomings of existing technologies, the present invention aims to provide a method for monitoring the grouting effect in goaf areas based on fiber optic sensing. By laying sensing optical cables in vertical grouting holes, a distributed fiber optic DAS device is used to emit pulsed light signals and receive Rayleigh backscattered signals, automatically monitoring the changes in the grout level during the grouting process, evaluating the grouting effect in real time, and providing data support for real-time control of the grouting process.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0005] A method for monitoring the grouting effect in goaf based on fiber optic sensing includes the following steps:

[0006] Step 1: Lower the sensing optical cable assembly into the vertical grouting hole, and then slowly lower the sensing optical cable assembly to the bottom of the goaf area through the orifice optical cable winch. The orifice optical cable passes through the grouting sealing flange and is connected to the distributed optical fiber DAS device.

[0007] Step 2: During the grouting process, the distributed fiber optic DAS device collects acoustic wave data in real time. Based on the energy accumulation integral value of the acoustic wave data, the significant peak channels of the liquid surface are identified through mathematical modeling and optimization constraints to determine the position of the grout surface.

[0008] Step 3: Based on the spatial position of the slurry liquid level corresponding to the optical cable axis, and combined with the optical cable laying depth information in the hole, the actual height of the slurry liquid level in the hole is calculated; the optical fiber scattering signal is continuously collected and processed by the distributed optical fiber DAS equipment to realize real-time dynamic monitoring of the liquid level height in the hole.

[0009] The present invention also includes the following technical features:

[0010] Specifically, the sensing optical cable assembly includes a sensing optical cable, a reflector fused to the front end of the sensing optical cable, and an inner hollow guide device arranged around the reflector.

[0011] Specifically, a high-reflectivity film is coated on the end of the reflector to enhance the reflection intensity of the optical signal and improve the signal sensitivity of the sensing optical cable.

[0012] Specifically, the hollow guide device includes a rear guide, a reflector protective shell, a two-component epoxy resin adhesive, a rear guide straightening plate, a roller, a front guide straightening plate, a counterweight, and a front guide;

[0013] The reflector protective shell is placed around the reflector to provide it with physical protection;

[0014] The two-component epoxy resin adhesive fills the space between the reflector and the reflector protective shell, ensuring the stability and safety of the reflector in complex environments.

[0015] Both the rear guide and the front guide are spring sheets, which are respectively wrapped around the reflector protective shell. The front end of the front guide is connected to the reflector protective shell, and the rear end is connected to the front end of the rear guide. The rear end of the rear guide is connected to the reflector protective shell.

[0016] The counterweight is located at the bottom of the reflector protective housing. The counterweight can be flexibly adjusted according to the drilling depth and the length of the optical cable to ensure that the optical cable is smoothly lowered to the bottom of the hole under the action of gravity.

[0017] The upper part of the reflector protective shell is a tapered rear guide, and the lower end of the counterweight is a tapered front guide.

[0018] Specifically, the rear end of the rear guide straightening plate can slide back and forth on the reflector protective shell. Rollers are provided at the junction of the front guide straightening plate and the rear guide straightening plate to reduce friction. When passing through different apertures, the rear end of the rear guide straightening plate moves backward, and the front guide straightening plate and the rear guide straightening plate deform radially to ensure that the optical cable passes smoothly coaxially under different apertures.

[0019] Specifically, the distributed fiber optic DAS device includes a main control board, a laser, a driver, an acousto-optic modulator, a fiber optic amplifier, a circulator, a photodetector, and a demodulation host;

[0020] The main control board controls the laser to emit highly coherent continuous light. The driver drives the acousto-optic modulator to modulate the continuous light into high-precision nanosecond-level pulse light, which is then amplified by the fiber amplifier and injected into the sensing optical cable through the circulator. When the light pulse propagates in the sensing optical cable, it generates Rayleigh backscattering. The acoustic signal acts on the sensing optical cable, causing changes in the refractive index and length of the fiber, thereby changing the phase and intensity of the scattered light. After the backscattered light returns through the circulator, it is received by a high-sensitivity photodetector through the fiber amplifier. The demodulation host determines the vibration position by analyzing the round-trip time of the light pulse and extracts the phase change characteristics to collect acoustic data.

[0021] Specifically, step 2 includes:

[0022] Step 2.1, Identify the start-up time of the grouting pump:

[0023] Energy distribution of acoustic wave data at different frequencies acquired by distributed fiber optic DAS equipment:

[0024]

[0025] In equation (1), PSD is the power spectral density, X(f) is the amplitude of the frequency domain data, and N is the number of data points;

[0026] Perform the cumulative integral E of PSD in the frequency domain for the effective frequency domain segment:

[0027]

[0028] In equation (2), f1 and f2 are the start and end frequencies of the effective frequency domain segment;

[0029] The operating status of the grouting pump is determined based on the cumulative integral value E of the power spectral density over the effective frequency domain segment. The cumulative integral value E of PSD during pump shutdown is significantly lower than that during pump startup, and the energy of all channels is significantly increased during pump startup. The start-up time of the grouting pump is automatically identified by setting a threshold.

[0030] Step 2.2, determine the range of abrupt changes in the energy accumulation integral value of the acoustic wave data:

[0031] When the grouting pump is running, the sound wave propagating downward along the wellbore is the incident wave. When it encounters the interface where the acoustic impedance of the liquid surface changes drastically, it is reflected. Most of the incident wave propagates in the opposite direction along the original path to form a reflected wave. The incident wave and the reflected wave are superimposed to form a standing wave field. This standing wave field causes a disturbance in the local stress σ, which in turn produces a sudden energy change Δn.

[0032] Δn=p·σ (4)

[0033] Where: p is the photoelastic coefficient;

[0034] The energy mutation Δn causes a change in the Rayleigh scattering characteristics of the laser emitted by the distributed fiber optic DAS device deployed in the sensing optical cable in this area. This change is then detected by the photodetector of the distributed fiber optic DAS device, which manifests as a significant jump in the energy accumulation integral value E of the acoustic data of the corresponding channel. By capturing this energy mutation feature, the channel range corresponding to the mutation of the energy accumulation integral value E can be obtained, which in turn can determine the well depth range where the liquid surface is located.

[0035] Step 2.3: Accurately identify and determine the position of the slurry surface by identifying the abrupt change in the energy accumulation integral value.

[0036] A multi-level signal processing flow is constructed based on the Findpeaks function. This function achieves robust peak detection through mathematical modeling and optimization constraints: First, based on the range of abrupt changes in the energy accumulation integral value obtained in section 2.2, local extremum scanning is performed on the original DAS signal of the channel with a significant jump in the energy accumulation integral value E to extract candidate peaks. Combined with multi-dimensional threshold screening, Savitzky-Golay / wavelet soft thresholding denoising preprocessing is used to improve the signal-to-noise ratio. Based on dynamic parameter optimization and non-maximum suppression algorithms, and combined with peak base width and energy spectrum entropy analysis, significant peaks on the liquid surface are identified, and finally the position of the liquid surface is accurately located.

[0037] Compared with the prior art, the present invention has the following technical effects:

[0038] This invention significantly improves the efficiency of sensor cable deployment and the sensitivity of signal acquisition by integrating a reflector and an internal guide device into the sensor cable. By emitting pulsed light signals and receiving Rayleigh backscattered signals through a distributed optical fiber DAS device, it can automatically identify the start and stop times of the grouting pump, achieving intelligent judgment of the grouting pump's operating status. By identifying the abrupt change point in the energy integration result of the distributed optical fiber monitoring data at the grouting pump's start time, the location range of the liquid level can be determined. Furthermore, based on the principle of energy surge, a liquid level identification method based on energy surge peaks is established, which can automatically identify the precise location of the grouting liquid level in the goaf, providing data support for real-time control of the grouting process. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of an optical cable reflector and an internal guide device.

[0040] Figure 2 This is a three-dimensional structural diagram of the optical cable reflector and the hollow guide device.

[0041] Figure 3 This is a structural diagram of a distributed fiber optic DAS device.

[0042] Figure 4 The integral values ​​accumulated for PSD are: (a) the full-channel integral value in the pump-off state; and (b) the full-channel integral value in the pump-on state.

[0043] Figure 5 This represents the integral value of the PSD across all channels when the pump is running.

[0044] The meanings of the labels in the diagram are as follows:

[0045] 1. Sensor optical cable, 2. Reflector, 3. Rear guide, 4. Reflector protective housing, 5. Two-component epoxy resin adhesive, 6. Rear guide straightener, 7. Roller, 8. Front guide straightener, 9. Counterweight, 10. Front guide. Detailed Implementation

[0046] The following are specific embodiments of the present invention. It should be noted that the present invention is not limited to the following specific embodiments. All equivalent modifications made based on the technical solutions of this application fall within the protection scope of the present invention.

[0047] Example 1:

[0048] This embodiment provides a method for monitoring the grouting effect in goaf areas based on fiber optic sensing. The method includes the following steps:

[0049] Step 1: Lower the sensing optical cable assembly into the vertical grouting hole. Using the orifice optical cable winch, slowly lower the sensing optical cable assembly to the bottom of the goaf. The orifice optical cable passes through the grouting sealing flange and is connected to the distributed fiber optic DAS device.

[0050] Among them, such as Figure 1 The sensing optical cable assembly includes a sensing optical cable, a reflector fused to the front end of the sensing optical cable, and an internal guide device arranged around the reflector.

[0051] Specifically, a high-reflectivity film (such as a gold film, silver film, or multilayer dielectric film) is coated on the top of the reflector, which can achieve a reflectivity of over 90%. The high reflectivity of the reflector can enhance the reflection intensity of the optical signal and improve the signal sensitivity of the sensing optical cable.

[0052] The internal guide device includes a rear guide, a reflector protective shell, a two-component epoxy resin adhesive, a rear guide straightener, rollers, a front guide straightener, a counterweight, and a front guide. The reflector protective shell surrounds the reflector, providing physical protection. The two-component epoxy resin adhesive fills the space between the reflector and the reflector protective shell, ensuring the stability and safety of the reflector in complex environments. Both the rear and front guide straighteners have four spring plates, evenly distributed around the reflector protective shell. The front guide straightener's front end connects to the reflector protective shell, and its rear end connects to the front end of the rear guide straightener. The rear guide straightener's rear end connects to the reflector protective shell and can slide back and forth on it. Rollers are located at the junction of the front and rear guide straighteners to reduce friction. When passing through a diameter change area, the front and rear guide straighteners deform and shrink, and the rear end of the rear guide straightener moves backward to ensure the smooth coaxial passage of optical cables under different aperture sizes. The reflector protective housing integrates an adjustable counterweight module at the bottom, allowing for flexible adjustment of the counterweight based on drilling depth and fiber optic cable length to ensure the cable is smoothly lowered to the bottom of the hole under gravity. The upper part of the reflector protective housing features a tapered rear guide, while the lower end of the counterweight features a tapered front guide.

[0053] Among them, such as Figure 3 Distributed fiber optic DAS equipment includes a main control board, laser, driver, acousto-optic modulator, fiber amplifier, circulator, photodetector, and demodulation host.

[0054] Specifically, the main control board controls the laser to emit highly coherent continuous light. The driver drives the acousto-optic modulator to modulate the continuous light into high-precision nanosecond-level pulses, which are then amplified by an fiber amplifier and injected into the sensing optical cable through a circulator. As the light pulses propagate in the sensing optical cable, Rayleigh backscattering occurs. The acoustic signal acts on the sensing optical cable, causing changes in the fiber's refractive index and length, thereby altering the phase and intensity of the scattered light. The backscattered light returns through the circulator and is received by a high-sensitivity photodetector via the fiber amplifier. The demodulation host determines the vibration location by analyzing the round-trip time of the light pulses and extracts phase change characteristics to acquire acoustic data.

[0055] Step 2: During the grouting process, the distributed fiber optic DAS device stores the real-time acoustic data into a SEG-Y file every 10 seconds to monitor the grout level in the goaf. The specific monitoring method is as follows:

[0056] Step 2.1: Set the time-domain acoustic wave data collected by the distributed fiber optic DAS device into groups of ten minutes each. Randomly select a fixed number of SEG-Y files for each group. Perform a Fourier transform on the time-domain data to convert it into frequency-domain data. Through the Fourier transform, obtain the amplitude and phase information of each frequency component. Convert the amplitude result into power spectral density (PSD) as shown in Equation (1), which is used to describe the energy distribution of the acoustic wave data at different frequencies.

[0057]

[0058] In equation (1), X(f) is the amplitude of the frequency domain data, and N is the number of data points.

[0059] In the frequency domain data results within the range of 0-2000Hz, the 0-0.01Hz range represents ultra-low frequency background noise from the formation, which is not generated during the grouting process and is therefore invalid background noise. The power spectral density (PSD) value in the 200-2000Hz range is close to 0. Therefore, a bandpass filter is applied to the data in the 0.01-200Hz range, and the PSD is accumulated and integrated in the frequency domain for the effective frequency segment, resulting in E.

[0060]

[0061] In equation (2), f1 and f2 are the start and end frequencies of the effective frequency domain segment.

[0062] Based on the cumulative integral value E of the power spectral density over the effective frequency domain segment, the operating status of the grouting pump can be determined, especially during pump shutdown. Figure 4 a) The cumulative integral value E of PSD is significantly lower than that during the pump start-up period ( Figure 4 (b) Furthermore, the energy across all channels significantly increases during pump start-up. By setting a threshold, the start-up time of the grouting pump can be automatically identified.

[0063] Step 2.2, when determining the moment the grouting pump starts, at the grout surface, due to the order-of-magnitude difference between the density ρ and sound velocity c of the liquid and air media, the acoustic impedance Z can be expressed as:

[0064] Z = ρc

[0065] At the interface between two media, liquid and air, the reflection coefficient R is expressed as:

[0066]

[0067] When Z 液体 With Z 空气 The greater the difference in acoustic impedance, the closer the reflection coefficient R is to ±1, so most of the sound waves are reflected.

[0068] The sound waves propagating downward along the wellbore in the slurry are incident waves. When they encounter an interface where the acoustic impedance of the liquid surface changes drastically, they will be reflected. Most of the incident waves will propagate in the opposite direction along the original path to form reflected waves. The incident waves and reflected waves superimpose to form a standing wave field. This standing wave field induces a disturbance in the local stress σ, which in turn produces a significant energy mutation Δn.

[0069] Δn=p·σ (4)

[0070] Where p is the photoelastic coefficient; the energy mutation Δn causes a change in the Rayleigh scattering characteristics of the light emitted by the laser of the distributed fiber optic DAS device deployed in the sensing optical cable in this area, which is then detected by the photodetector of the distributed fiber optic DAS device. This is manifested as a significant jump in the energy accumulation integral value E of the acoustic data of the corresponding channel. Compared with other channels, the slope of the energy curve of this part of the channel changes significantly, and the energy increase far exceeds the growth rate of adjacent channels, showing a sharp upward trend. By capturing this energy mutation feature, the channel range corresponding to the mutation of the energy accumulation integral value E can be obtained, which can also be used to determine the channel range where the liquid surface is located. Figure 5 As shown, based on the PSD energy accumulation integral value E at the time of pump start-up, the distributed fiber optic DAS device detected the range of abrupt changes in the energy accumulation integral value E within the 131-133 channel range.

[0071] Step 2.3, for the precise identification of the abrupt change location range of the energy accumulation integral value E near the liquid surface, a multi-level signal processing flow is constructed based on the Findpeaks function. This function achieves robust peak detection through mathematical modeling and optimization constraints: First, based on the abrupt change location range of the energy accumulation integral value obtained in 2.2, local extremum scanning is performed on the original DAS signal of the channel with a significant jump in the energy accumulation integral value E to extract candidate peaks. Combined with multi-dimensional threshold screening, Savitzky-Golay / wavelet soft thresholding denoising preprocessing is used to improve the signal-to-noise ratio. Based on dynamic parameter optimization (amplitude threshold, minimum peak spacing, peak prominence threshold) and non-maximum suppression algorithm, and combined with peak base width and energy spectrum entropy analysis, significant peaks at the liquid surface are identified, and finally the liquid surface position is accurately located.

[0072] More specifically, the original signal is first subjected to local extremum scanning to define candidate peaks in the discrete signal sequence that satisfy the strict local maximum condition (x[n]>x[n±k], where k is the neighborhood radius). Then, multi-dimensional thresholding is applied in combination with fluid physics characteristics and noise statistics. For the non-stationarity and high-frequency noise interference unique to DAS signals, preprocessing is required using Savitzky-Golay filters or wavelet soft thresholding to improve the signal-to-noise ratio and suppress spurious peaks caused by turbulent vortex breakup or bubble collapse.

[0073] At the parameter optimization level, the amplitude threshold, minimum peak spacing, and peak prominence threshold are dynamically set based on the signal statistical characteristics, and a non-maximum suppression algorithm is used to eliminate multiple detections caused by dense oscillations. For the broadband energy accumulation phenomenon caused by abrupt changes in acoustic impedance at the liquid surface interface, the algorithm verifies the peak significance by jointly calculating the peak floor width (half-maximum width w ≥ 5Δt, where Δt is the time resolution) and the energy spectrum entropy (H = -∑PSD(f)logPSD(f)), thereby distinguishing between the real liquid surface signal and vibration noise.

[0074] Step 3: Based on the spatial position of the liquid level corresponding to the optical cable axis calculated above, and combined with the depth information of the optical cable in the hole, the actual height of the grout level in the hole can be accurately calculated. By continuously acquiring and processing the fiber optic scattering signal through a distributed DAS device, real-time dynamic monitoring of the liquid level height in the hole can be achieved, and a curve showing the liquid level changing over time can be generated. This monitoring result can provide quantitative data support for parameter control during the grouting process, thereby improving grouting efficiency and safety.

[0075] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0076] It should also be noted that the various specific technical features described in the above embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0077] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A goaf grouting effect monitoring method based on optical fiber sensing, characterized in that, Includes the following steps: Step 1: Lower the sensing optical cable assembly into the vertical grouting hole, and then slowly lower the sensing optical cable assembly to the bottom of the goaf area through the orifice optical cable winch. The orifice optical cable passes through the grouting sealing flange and is connected to the distributed optical fiber DAS device. Step 2: During the grouting process, the distributed fiber optic DAS device collects acoustic wave data in real time. Based on the energy accumulation integral value of the acoustic wave data, the significant peak channels of the liquid surface are identified through mathematical modeling and optimization constraints to determine the position of the grout surface. Step 3: Based on the spatial position of the slurry surface corresponding to the optical cable axis, and combined with the optical cable laying depth information in the hole, calculate the actual height of the slurry surface in the hole. By continuously acquiring and processing fiber optic scattering signals through distributed fiber optic DAS equipment, real-time dynamic monitoring of the liquid level in the hole can be achieved. Step 2 includes: Step 2.1, Identify the start-up time of the grouting pump: Energy distribution of acoustic wave data at different frequencies acquired by distributed fiber optic DAS equipment: （1） In formula (1), PSD is a power spectral density, is an amplitude of the frequency domain data, and N is a data point number. The PSD is accumulated in the frequency domain for the effective frequency segment : （2） In formula (2), , are the start and end frequencies of the effective frequency domain segment; According to the accumulated integral value of power spectral density on the effective frequency domain segment , the running state of the grouting pump is judged, the PSD accumulated integral value during pump closing is significantly lower than that during pump opening, and the total channel energy during pump opening is significantly improved; through threshold setting judgment, the opening time of the grouting pump is automatically identified; Step 2.2, determine the range of abrupt changes in the energy accumulation integral value of the acoustic wave data: When the grouting pump is running, the sound wave propagating downwards along the wellbore is considered the incident wave. Upon encountering an interface where the acoustic impedance changes drastically at the liquid surface, it is reflected. Most of the incident wave propagates in the opposite direction along its original path, forming a reflected wave. The superposition of the incident and reflected waves creates a standing wave field, which induces localized stress. The disturbance, in turn, produces a sudden energy change. n; （4） wherein: is the photoelastic coefficient; Energy mutation n The Rayleigh scattering characteristics of the light emitted by the laser of the distributed optical fiber DAS equipment arranged in the liquid level area sensing optical cable are changed, and then recognized by the photodetector of the distributed optical fiber DAS equipment, which is embodied as the energy cumulative integral value of the corresponding channel sound wave data Significant jump, by capturing this energy mutation feature, the energy cumulative integral value can be obtained The mutation of the corresponding channel range can obtain the well depth position range of the liquid level Step 2.3: Accurately identify and determine the position of the slurry surface by identifying the abrupt change in the energy accumulation integral value. A multi-stage signal processing flow is constructed based on the Findpeaks function. This function achieves robust peak detection through mathematical modeling and optimization constraints: First, based on the abrupt change position range of the energy accumulation integral value obtained in section 2.2, the energy accumulation integral value is... Candidate peaks are extracted by local extremum scanning of the original DAS signal of the significantly elevated channel. Combined with multi-dimensional threshold screening, and Savitzky-Golay / wavelet soft thresholding denoising preprocessing is used to improve the signal-to-noise ratio. Based on dynamic parameter optimization and non-maximum suppression algorithm, and combined with peak base width and energy spectrum entropy analysis, significant peaks on the liquid surface are identified, and the liquid surface position is finally accurately located.

2. The method for monitoring the grouting effect in goaf based on fiber optic sensing as described in claim 1, characterized in that, The sensing optical cable assembly includes a sensing optical cable, a reflector fused to the front end of the sensing optical cable, and an inner hollow guide device arranged around the reflector.

3. The method for monitoring the grouting effect in goaf based on fiber optic sensing as described in claim 2, characterized in that, The reflector end is coated with a high-reflectivity film, which enhances the reflection intensity of the optical signal and improves the signal sensitivity of the sensing optical cable by utilizing the high reflectivity of the reflector.

4. The method for monitoring the grouting effect in goaf based on fiber optic sensing as described in claim 2, characterized in that, The hollow guide device includes a rear guide, a reflector protective shell, a two-component epoxy resin adhesive, a rear guide straightening plate, rollers, a front guide straightening plate, a counterweight, and a front guide. The reflector protective shell is placed around the reflector to provide it with physical protection; The two-component epoxy resin adhesive fills the space between the reflector and the reflector protective shell, ensuring the stability and safety of the reflector in complex environments. Both the rear guide and the front guide are spring sheets, which are respectively wrapped around the reflector protective shell. The front end of the front guide is connected to the reflector protective shell, and the rear end is connected to the front end of the rear guide. The rear end of the rear guide is connected to the reflector protective shell. The counterweight is located at the bottom of the reflector protective housing. The counterweight can be flexibly adjusted according to the drilling depth and the length of the optical cable to ensure that the optical cable is smoothly lowered to the bottom of the hole under the action of gravity. The upper part of the reflector protective shell is a tapered rear guide, and the lower end of the counterweight is a tapered front guide.

5. The method for monitoring the grouting effect in goaf based on fiber optic sensing as described in claim 4, characterized in that, The rear end of the rear guide straightener can slide back and forth on the reflector protective shell. Rollers are provided at the junction of the front guide straightener and the rear guide straightener to reduce friction. When passing through different apertures, the rear end of the rear guide straightener moves backward, and the front guide straightener and the rear guide straightener deform and shrink radially to ensure that the optical cable passes smoothly coaxially under different apertures.

6. The method for monitoring the grouting effect in goaf based on fiber optic sensing as described in claim 1, characterized in that, The distributed fiber optic DAS device includes a main control board, a laser, a driver, an acousto-optic modulator, a fiber optic amplifier, a circulator, a photodetector, and a demodulation host. The main control board controls the laser to emit highly coherent continuous light. The driver drives the acousto-optic modulator to modulate the continuous light into high-precision nanosecond-level pulse light, which is then amplified by the fiber amplifier and injected into the sensing optical cable through the circulator. When an optical pulse propagates in a sensing optical cable, it generates Rayleigh backscattering. The acoustic signal acts on the sensing optical cable, causing changes in the fiber's refractive index and length, thereby altering the phase and intensity of the scattered light. The backscattered light returns through a circulator and is received by a high-sensitivity photodetector via an optical fiber amplifier. The demodulation host determines the vibration location by analyzing the round-trip time of the optical pulse and extracts phase change characteristics to collect acoustic data.

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