An ecological coal mining with filling sleeve identification positioning method based on optical fiber sensing

Abstract

The application discloses an ecological coal mining identification and positioning method based on an optical fiber sensing and a follow-up pipe, and comprises the following steps: S1, a cutting sensing system is laid out; a communication optical cable is attached to a vertical hole section on the ground, and a screw optical cable is attached to a horizontal directional long drilling hole section; and a distributed demodulation unit is arranged on the ground and connected with the distributed optical cable; S2, identification and positioning: in the follow-up process, the specific length of the follow-up pipe exposed in the goaf can be identified through the distributed optical cable, the caving condition of the direct roof is verified, and the accurate position that needs to be cut by the water jet is further determined. The distributed optical fiber measurement principle is adopted, different measurement principles and different frequency band signals are used on one sensing optical cable, and multiple parameter monitoring information can be obtained, so that the identification of the follow-up pipe and the cutter head positioning problem can be solved at the same time through only one measurement.

Description

Technical Field

[0001] This invention belongs to the field of coal mining technology and relates to a method for identifying and locating ecological coal mining casing based on fiber optic sensing. Background Technology

[0002] The coal seam roof casing horizontal long borehole with ongoing filling and grouting method achieves the goal of reducing and controlling the subsidence of the overlying strata in the goaf, solving problems such as surface subsidence and environmental damage caused by coal mining. This method involves inserting a filling casing into a horizontal directional long borehole in the immediate roof. During the mining process, the immediate roof strata collapse as mining progresses, exposing the front end of the casing in the goaf. Slurry is then injected into the goaf through the filling casing to fill it. Once the slurry in this section has solidified, the coal mining machine drives the hydraulic support baffle to continue advancing, typically moving 3 to 5 meters. At this point, a water jet is inserted into the filling casing to cut it using hydraulic cutting, ensuring the slurry outlet is located in the goaf to be filled. Because the water jet is connected to a high-pressure hose at the rear, the hose undergoes bending, twisting, and stretching during repeated insertion and withdrawal. This makes it impossible to precisely control the cutting position of the water jet based on the hose length alone. If the cutting position is too far forward, the slurry will fill the coal face, affecting normal mining operations; if the cutting position is too far backward, the slurry will continue to fill the solidified paste area, causing casing blockage. Therefore, controlling the water jet cutting position becomes crucial, requiring precise identification and positioning of the casing as it is filled. Currently, video monitoring is commonly used to manually monitor exposed casing in the goaf. However, due to the limited conditions in the goaf, this method cannot accurately determine the exposed casing length and cutting position, nor can it accurately position the water jet cutter head, resulting in significant errors in the casing cutting position. Summary of the Invention

[0003] To overcome the above problems, this invention proposes a method for identifying and locating the casing during coal mining based on fiber optic sensing. During the charging process, this method uses a distributed optical cable to identify the specific length of the casing exposed in the goaf, verify the collapse of the immediate roof, and further determine the precise location where the water jet needs to cut. After the water jet is lowered, the vibration of the cutter head is monitored via the distributed optical cable to locate its specific position. By tracking the cutter head's movement in real time, the cutter head is positioned to the area to be cut, thus achieving precise positioning of the casing's slurry outlet.

[0004] The specific solutions adopted in this invention include:

[0005] A method for identifying and locating the casing during coal mining based on fiber optic sensing, comprising:

[0006] S1. Deploy the cutting sensing system; complete the horizontal directional long drilling from the ground vertical hole to the top, and install the charging sleeve along the hole; attach distributed optical cables to the outer wall of the charging sleeve along the length of the charging sleeve, attach communication optical cables to the ground vertical hole section, and attach spiral optical cables to the horizontal directional long drilling section; and set up a distributed demodulation unit on the ground, which is connected to the distributed optical cable.

[0007] S2. Identification and Positioning: During the charging process, the specific length of the charging casing exposed in the goaf can be identified through the distributed optical cable, verifying the collapse of the direct roof and further determining the precise location to be cut by the water jet; after the water jet is lowered, the vibration of the cutter head can be monitored through the distributed optical cable to locate the specific position of the cutter head. By tracking the movement of the cutter head in real time, the cutter head is positioned to the area to be cut for cutting, thus completing the precise positioning of the slurry outlet of the charging casing.

[0008] Optionally, in S1, grouting is also performed to seal the space between the filling sleeve and the borehole wall.

[0009] Optionally, S2 specifically includes:

[0010] The system consists of two parts: in-charge sleeve identification and water jet head positioning.

[0011] The identification of the in-flush casing is based on strain inversion using low-frequency signals (0.5Hz–10Hz) from the spiral optical cable segment: After the immediate roof collapses, the pressure of the surrounding rock of the casing is released, and the strain decreases. Through strain inversion of the spiral optical cable, the exposed length of the in-flush casing tail in the goaf can be determined. If the strain value of the optical cable tail section does not decrease significantly, it indicates that the immediate roof in that section has not collapsed directly. By combining the exposed length of the in-flush casing tail with the advance distance of the coal mining machine, the middle area of ​​the goaf can be determined, and the specific location of the water jet cutting can be identified.

[0012] The waterjet cutter head positioning uses a mid-to-high frequency signal (60Hz-300Hz) from the spiral optical cable segment for vibration inversion: The rear end of the waterjet is connected to a high-pressure hose. The waterjet is lowered into the horizontal section of the charging sleeve using a high-pressure water jet delivery method. During the waterjet's descent, the cutter head will vibrate significantly due to the thrust of the high-pressure water. The cutter head collides with the wall of the charging sleeve, and the vibration signal of the cutter head can be sensed through the spiral optical cable in the horizontal section, thereby locating the specific position of the cutter head in real time. When it is determined that the cutter head has advanced to the cutting position, the delivery is stopped and the sleeve cutting begins.

[0013] Optionally, the sleeve identification based on strain inversion using low-frequency signals in the 0.5Hz–10Hz band of the spiral optical cable segment specifically includes:

[0014] First, the original data is subjected to fidelity filtering, and then the filtered data is downsampled. An anti-aliasing filter is used to extract the low-frequency band.

[0015] Secondly, apply a medium-range filter and a DC removal filter to attenuate impulse noise, and set the start time of the "strain 0" data to the moment when the previous goaf filling was completed and the slurry delivery was stopped.

[0016] Finally, by using trapezoidal numerical integration in the time direction, the strain rate domain data is converted into the cumulative strain domain. In the low-frequency dataset, the data with the highest signal-to-noise ratio is selected for waterfall plot imaging and mapped onto a pre-determined coordinate system. After a period of background information acquisition, the working face advances and the top collapses directly. During this stage, strain change measurements are continuously performed to identify the location of the sleeve to be cut, and water jets are started from the orifice.

[0017] Optionally, the waterjet cutter head positioning selects a mid-to-high frequency signal (60Hz–300Hz) from the spiral optical cable segment for vibration inversion, specifically including:

[0018] First, a mid-to-high frequency signal of 60Hz to 300Hz is obtained by setting a bandpass filter;

[0019] Then, the step size is set according to the distance resolution, and a fast Fourier transform is performed. The frequency point is then set to extract the phase information. Mutual differential is performed to eliminate phase noise. After unwinding, the phase signal is high-pass filtered. In the mid-to-high frequency dataset, the data with the highest signal-to-noise ratio is selected to draw the tool trajectory curve and map it onto a pre-determined coordinate system.

[0020] Optional, specifically including:

[0021] Enter the setting state, pre-determine the length and height coordinates based on the wellbore parameters, the length of the charging casing and the spiral optical cable, and determine the zero coordinate starting position of the sensing optical cable;

[0022] Once the measurement is initiated, the laser first generates a linearly swept continuous light. The light is split into two parts by the fiber coupler: one part is emitted as probe light into the fiber under test, and the other part is used as local reference light.

[0023] The electric field expression for linearly swept light is given by [expression], with units of N / C:

[0024]

[0025] In equation (1), P is the optical power in mW, and ω c Here, f0 is the center angular frequency of the laser, in rad / s; f0 is the sweep start frequency, in Hz; k is the sweep speed, in Oct / min; φ(t) is the phase noise term, and t is time, in seconds.

[0026] The photoelectric field expression for the Rayleigh backscattered light generated by the probe light at a certain point in the optical fiber is:

[0027] E RBS(t)=Eiexpj{ω c (t-τ i )+2πf0(t-τ i )+πk(t-τ i ) 2 +φ(t-τ i )} (2);

[0028] E in equation (2) i τ is the photoelectric field intensity of the Rayleigh backscattered light returning from position i on the optical fiber before it reaches the balanced photodetector, in N / C. i The time delay in which the Rayleigh backscattered light generated at a certain position i on the optical fiber returns to the receiver is expressed in seconds.

[0029] The current signal output after beat frequency conversion and photoelectric conversion of Rayleigh backscattered light and local reference light is:

[0030] i(t) = Asin[-2πkτ] i t+πkτ i 2 -2πf0τ i -ω c τ i +φ(t-τ i )-φ(t)] (3);

[0031] The Rayleigh backscattered light at a certain point in the optical fiber beats the local optical signal, which is a single-frequency sinusoidal signal that is positively correlated with the position of the reflection point.

[0032] By performing a Fourier transform on the electrical signal output by the fiber optic sensing system to obtain its spectrum, the Rayleigh scattering information at each point on the fiber can be obtained, thus revealing the relationship between position Z and frequency f.

[0033]

[0034] In formula (4), v g The speed of light is expressed in m / s.

[0035] Two bandpass filters, 0.5–10 Hz and 60–300 Hz, were applied to the dataset at each location.

[0036] Optionally, the distributed demodulation unit includes a laser, an optical modulation module and an optical amplifier arranged along the light propagation direction of the laser, and a communication optical cable connected to the rear end of the optical amplifier. The communication optical cable and the spiral optical cable are connected through a circulator. A light detection module and a demodulation module are arranged sequentially on the return optical path of the communication optical cable, and the demodulation module is also connected to the optical modulation module.

[0037] The advantages of this invention include:

[0038] 1. The problem of casing monitoring is solved by using optical cables arranged on the outer wall of the casing as it is charged. As the coal face advances, the casing is cut, and the optical cable is also cut at the same time. Each time, only the tail of the optical cable is cut. However, the method of measuring strain and vibration by distributed optical fiber is not afraid of the optical cable being cut, and it can always measure the data within the existing length of the optical cable after the cut. This means that the optical cable as a measuring tool does not need to move its position with the advance of the working face. It can be deployed once and measured multiple times, and measured as it is cut. This is the only passive sensor that can be easily deployed in the narrow space of the casing as it is charged.

[0039] 2. By adopting the distributed optical fiber measurement principle, multiple parameter monitoring information can be obtained on a single sensing optical cable through different measurement principles and signals of different frequency bands. Only one measurement is needed to simultaneously solve the problems of identification of the charging sleeve and positioning of the cutter head. Attached Figure Description

[0040] The accompanying drawings are provided to further illustrate the present disclosure and form part of the specification. They are used together with the following detailed description to explain the present disclosure, but do not constitute a limitation thereof. In the drawings:

[0041] Figure 1 This is a schematic diagram illustrating the principle of the charging sleeve identification and positioning of the present invention.

[0042] Figure 2 This is a schematic diagram illustrating the working principle of the distributed demodulation unit used in this invention. Detailed Implementation

[0043] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0044] This invention proposes a method for identifying and locating the charging sleeve in ecological coal mining based on fiber optic sensing. This distributed fiber optic sensing system uses a single optical cable and different measurement principles and signals of different frequency bands to achieve two monitoring modes: strain and vibration. It can simultaneously identify the charging sleeve and accurately locate the cutting head.

[0045] By acquiring low-frequency and very low-frequency signals through a fiber optic sensing system, the strain of different sections of the infill casing can be monitored. As the coal face advances and the immediate roof collapses, monitoring the strain changes in different sections identifies the location of the infill casing exposed in the goaf, verifying the collapse of the immediate roof. When the immediate roof has not completely collapsed, it is necessary to locate the area and use methods such as air cannons to cause the immediate roof to collapse, completely exposing the infill casing in the goaf, and further determining the precise location to be cut by the water jet, thus completing the identification of the infill casing. After the water jet is lowered, the vibration of the water jet can be monitored by acquiring high-frequency signals through the fiber optic sensing system, completing the precise positioning of the cutter head.

[0046] like Figure 1 The schematic diagram of the in-flight casing identification and positioning principle includes: S1, Deploying the cutting sensing system; A horizontal directional long borehole is completed by drilling a vertical hole down to the immediate roof, and an in-flight casing is installed along the hole; Distributed optical cables are attached to the outer wall of the in-flight casing along its length, communication optical cables are attached to the vertical hole section, and spiral optical cables are attached to the horizontal directional long borehole section; A distributed demodulation unit is also installed on the ground, and the distributed demodulation unit is connected to the distributed optical cable; S2, Identification and positioning: During the in-flight process, the specific length of the in-flight casing exposed in the goaf can be identified through the distributed optical cable, verifying the collapse of the immediate roof, and further determining the precise location to be cut by the water jet; After the water jet is lowered, the vibration of the cutter head is monitored through the distributed optical cable, and the specific position of the cutter head can be located. By tracking the movement of the cutter head in real time, the cutter head is positioned to the area to be cut for cutting, completing the precise positioning of the slurry outlet of the in-flight casing. A horizontally directional long borehole is completed by descending to the immediate roof through a vertical borehole. Distributed sensor optical cables are installed on the lower side of the outer wall of the charging casing. The charging casing with the sensor optical cables is then lowered into the horizontal long borehole, and grouting is performed on the gap between the outer wall of the casing and the horizontal borehole to ensure full coupling between the casing and the borehole wall. During the mining process, the immediate roof is usually composed of sandy mudstone or mudstone, which will collapse directly as the coal mining progresses, exposing the front end of the casing in the goaf. Grout is then injected into the goaf through the charging casing to fill it. After the grout in this section solidifies, the coal mining machine drives the hydraulic support baffle to continue advancing, usually moving a distance of 3 to 5 meters. At this point, the tail end of the charging casing needs to be cut to ensure that the grout outlet is located in the goaf to be filled. The spiral optical cable used in this invention is a common spiral optical cable in the field. For example, by winding the optical fiber, a richer combination of optical fiber axial directions can be obtained, namely spiral wound distributed optical fiber acoustic wave sensing (HWCGDAS), so as to ensure the acquisition of signals in the XYZ directions while realizing continuous information acquisition.

[0047] In terms of hardware, the complete cutting sensing system includes a distributed demodulation unit and a distributed sensing optical cable. The distributed sensing optical cable consists of two parts: the vertical section uses a communication cable for transmitting optical signals, while the horizontal section uses a spiral optical cable for better sensing of strain and vibration signals from the charging sleeve. The charging sleeve has grooves and clips on its outer side for securing the optical cable, and is sealed with grout to prevent damage to the cable after the sleeve is lowered into the hole. Combined with... Figure 2The distributed demodulation unit used in this invention employs a laser, with an optical modulation module and an optical amplifier positioned along the laser's light propagation direction. The optical amplifier's rear end is connected to a communication optical cable, which is connected to a spiral optical cable via a circulator. A photodetector module and a demodulation module are sequentially positioned along the return optical path of the communication optical cable, with the demodulation module also connected to the optical modulation module. Since the self-coherent heterodyne modulation demodulation principle requires the use of a modulation signal, the demodulation module still needs information from the optical modulation module. The specific working principle includes: first, the continuous light signal emitted by the laser is modulated; the modulated pulsed light signal is amplified by the optical amplifier and then transmitted through the communication optical cable and spiral optical cable. The Rayleigh scattering signal returning from the optical cable undergoes photoelectric conversion by the photodetector module before reaching the demodulation module. Data demodulation is performed based on the self-coherent heterodyne modulation demodulation principle, and data interpretation is performed simultaneously based on the sleeve identification and tool positioning methods. The distributed demodulation unit is located in a control room next to the ground hole and connected to a distributed sensing optical cable, receiving and processing data collected by the sensing optical cable inside the hole in real time.

[0048] Data processing and interpretation include two parts: casing identification and cutter head positioning. Casing identification is based on strain inversion using low-frequency signals (0.5Hz–10Hz) from the spiral optical cable segment. When the immediate roof collapses, the pressure of the surrounding rock in the casing is released, and the strain decreases. Through strain inversion of the spiral optical cable, the exposed length of the casing tail in the goaf can be determined. If the strain value of the optical cable tail section does not decrease significantly, it indicates that the immediate roof in that section has not collapsed directly, which will affect the subsequent water jet cutting. It is necessary to use an air cannon to completely collapse the immediate roof in the goaf. By analyzing the exposed length of the casing tail and the advancing distance of the coal mining machine, the middle area of ​​the goaf can be determined, thus identifying the specific location for water jet cutting. For water jet head positioning, a mid-to-high frequency signal (60Hz–300Hz) from a spiral optical cable segment is used for vibration inversion. Typically, a high-pressure hose is connected to the rear end of the water jet, and the water jet is lowered into the horizontal section of the casing using high-pressure water transport. During the water jet's descent, the head vibrates significantly due to the thrust of the high-pressure water, colliding with the casing wall. The vibration signal of the head can be sensed through the horizontal spiral optical cable, allowing for real-time positioning of the head. Once the head has reached the cutting position, the transport is stopped, and casing cutting begins.

[0049] The specific processing principle and explanation process are as follows:

[0050] Once in the setup state, the length and height coordinates are determined in advance based on the wellbore parameters, the length of the charging casing and the sensor fiber optic cable, and the zero coordinate starting position of the sensor fiber optic cable is determined.

[0051] Upon entering the measurement state, the laser first generates linearly swept continuous light with high coherence. This light is split into two parts by a fiber coupler: one part is emitted as probe light into the fiber under test, and the other part serves as a local reference light. The electric field expression for the linearly swept light is given by [expression missing], with units of N / C:

[0052]

[0053] In equation (1), P is the optical power in mW, and ω c Here, f0 is the center angular frequency of the laser, in rad / s; f0 is the sweep start frequency, in Hz; k is the sweep speed, in Oct / min; φ(t) is the phase noise term, and t is time, in seconds.

[0054] The photoelectric field expression for the Rayleigh backscattered light generated by the probe light at a certain point in the optical fiber is:

[0055] E RBS (t)=Eiexpj{ω c (t-τ i )+2πf0(t-τ i )+πk(t-τ i ) 2 +φ(t-τ i )} (2);

[0056] E in equation (2) i τ represents the photoelectric field intensity of the Rayleigh backscattered light returning from position i on the optical fiber before it reaches the BPD (balanced photodetector), expressed in N / C; it is related to the reflectivity and path attenuation coefficient. i Let be the time delay in seconds for the Rayleigh backscattered light generated at a certain position i on the optical fiber to return to the receiver.

[0057] The current signal output after beat frequency conversion and photoelectric conversion of Rayleigh backscattered light and local reference light is:

[0058] i(t) = Asin[-2πkτ] i t+πkτ i 2 -2πf0τ i -ω c τ i +φ(t-τ i )-φ(t)] (3);

[0059] The Rayleigh backscattered light at a certain point in the optical fiber beats the local optical signal, which is a single-frequency sinusoidal signal that is positively correlated with the position of the reflection point.

[0060] By performing a Fourier transform on the electrical signal output by the fiber optic sensing system to obtain its spectrum, the Rayleigh scattering information at each point on the fiber can be obtained, thus revealing the relationship between position Z and frequency f.

[0061]

[0062] In formula (4), v gThe speed of light is expressed in m / s.

[0063] Two bandpass filters, 0.5–10 Hz and 60–300 Hz, were applied to the dataset at each location.

[0064] Strain inversion is performed using a 0.5–10 Hz low-pass filter. First, the original data undergoes fidelity filtering, followed by downsampling. An anti-aliasing filter is then used to extract the low-frequency band. Second, a mean-value filter and a DC removal filter are applied to attenuate impulse noise. Because the data exhibits almost no linear trend, the DC removal filter only removes the average value. Since low-frequency strain is slowly varying, data needs to be sampled before the direct roof collapse as a baseline for comparison. Therefore, the location of the "strain 0" point needs careful consideration before applying these noise attenuation methods. This invention sets the start time of the "strain 0" data to the moment when the previous section of the goaf was filled and slurry delivery ceased. Finally, by using trapezoidal numerical integration in the time direction to convert the strain rate domain data into the cumulative strain domain, noise can be eliminated and the signal-to-noise ratio improved under this condition. In the low-frequency dataset, the data with the highest signal-to-noise ratio are selected for waterfall plot imaging and mapped onto a pre-determined coordinate system. After a period of background information collection, the working face advances and collapses directly. During this stage, strain change measurements are continuously performed to identify the location of the casing that needs to be cut, and water jets are then introduced from the orifice.

[0065] Vibration inversion is performed using filtering in the 60–300 Hz mid-to-high frequency band. First, a bandpass filter is used to obtain the mid-to-high frequency signal. The upper and lower limits of the bandpass filter can be adjusted appropriately based on the specific noise characteristics of each charging bushing, generally according to a positive correlation. Then, a step size is set according to the distance resolution, and a fast Fourier transform is performed. Next, a frequency point is set to extract phase information, and phase noise is eliminated through mutual differential analysis. After unwinding, the phase signal is high-pass filtered to eliminate the influence of system components and low-frequency noise. From the mid-to-high frequency dataset, the data with the highest signal-to-noise ratio is selected to plot the tool trajectory curve and map it onto a pre-defined coordinate system.

[0066] The system simultaneously displays strain changes and cutter head vibration trajectory in the same coordinate system, and calculates and outputs the relative coordinate information of the cutter head movement. When the cutter head reaches the sleeve cutting position, it stops moving forward and begins cutting the sleeve. After cutting is completed, the water jet exits and begins slurry conveying. After conveying is completed, it enters the measurement state again.

[0067] The preferred embodiments of this disclosure have been described in detail above with reference to the accompanying drawings. However, this disclosure is not limited to the specific details of the above embodiments. Within the scope of the technical concept of this disclosure, various simple modifications can be made to the technical solutions of this disclosure, and these simple modifications all fall within the protection scope of this disclosure.

[0068] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, this disclosure will not describe the various possible combinations separately.

[0069] Furthermore, various different embodiments of this disclosure can be combined in any way, as long as they do not violate the spirit of this disclosure, they should also be regarded as the content disclosed in this disclosure.

Claims

1. A method for identifying and locating eco-friendly coal mining casing based on fiber optic sensing, characterized in that, include: S1. Deploy the cutting sensing system; complete the horizontal directional long drilling from the ground vertical hole to the top, and install the charging sleeve along the hole; attach distributed optical cables to the outer wall of the charging sleeve along the length of the charging sleeve, attach communication optical cables to the ground vertical hole section, and attach spiral optical cables to the horizontal directional long drilling section; and set up a distributed demodulation unit on the ground, which is connected to the distributed optical cable. S2. Identification and Positioning: During the charging process, the specific length of the charging casing exposed in the goaf can be identified through the distributed optical cable, verifying the collapse of the direct roof and further determining the precise location to be cut by the water jet; after the water jet is lowered, the vibration of the cutter head can be monitored through the distributed optical cable to locate the specific position of the cutter head. By tracking the movement of the cutter head in real time, the cutter head is positioned to the area to be cut for cutting, thus completing the precise positioning of the slurry outlet of the charging casing. Specifically, it includes: Enter the setting state, pre-determine the length and height coordinates based on the wellbore parameters, the length of the charging casing and the spiral optical cable, and determine the zero coordinate starting position of the sensing optical cable; Once the measurement is initiated, the laser first generates a linearly swept continuous light. The light is split into two parts by the fiber coupler: one part is emitted as probe light into the fiber under test, and the other part is used as local reference light. The electric field expression for linearly swept light is given by [expression], with units of N / C: （1）； In formula (1) The unit for optical power is mW. The center angular frequency of a laser is measured in rad / s. 0 represents the sweep start frequency in Hz, and k represents the sweep speed in Oct / min. ( ) represents the phase noise term, and t is time, measured in seconds; The photoelectric field expression for the Rayleigh backscattered light generated by the probe light at a certain point in the optical fiber is: （2）； In formula (2) Position on the optical fiber The photoelectric field intensity of the Rayleigh backscattered light before it reaches the balanced photodetector, expressed in N / C; For a certain location on the optical fiber The time delay in which the Rayleigh backscattered light generated at the point returns to the receiver, measured in seconds; The current signal output after beat frequency conversion and photoelectric conversion of Rayleigh backscattered light and local reference light is: （3）； The Rayleigh backscattered light at a certain point in the optical fiber beats the local optical signal, which is a single-frequency sinusoidal signal that is positively correlated with the position of the reflection point. By performing a Fourier transform on the electrical signal output by the fiber optic sensing system to obtain its spectrum, the Rayleigh scattering information at each point on the fiber can be obtained, thus revealing the relationship between position Z and frequency f. （4）； In formula (4), v g The speed of light is expressed in m / s. Two bandpass filters, 0.5–10 Hz and 60–300 Hz, were applied to the dataset at each location.

2. The identification and positioning method for ecological coal mining casing based on fiber optic sensing according to claim 1, characterized in that, In S1, grouting is also performed to seal the space between the casing and the borehole wall.

3. The identification and positioning method for ecological coal mining casing based on fiber optic sensing according to claim 1 or 2, characterized in that, The S2 specifically includes: The system consists of two parts: in-charge sleeve identification and water jet head positioning. The identification of the in-flush casing is based on strain inversion using low-frequency signals (0.5Hz~10Hz) from the spiral optical cable segment: When the direct roof collapses, the pressure of the surrounding rock of the casing is released, and the strain decreases. Through strain inversion of the spiral optical cable, the exposed length of the in-flush casing tail in the goaf can be determined. If the strain value of the optical cable tail section does not decrease significantly, it indicates that the direct roof in that section has not collapsed directly. By combining the exposed length of the in-flush casing tail with the advance distance of the coal mining machine, the middle area of ​​the goaf can be determined, and the specific location of the water jet cutting can be identified. The waterjet cutter head positioning uses a mid-to-high frequency signal (60Hz-300Hz) from the spiral optical cable segment for vibration inversion: The rear end of the waterjet is connected to a high-pressure hose. The waterjet is lowered into the horizontal section of the charging sleeve using a high-pressure water jet delivery method. During the waterjet's descent, the cutter head will vibrate significantly due to the thrust of the high-pressure water. The cutter head collides with the wall of the charging sleeve, and the vibration signal of the cutter head can be sensed through the spiral optical cable in the horizontal section, thereby locating the specific position of the cutter head in real time. When it is determined that the cutter head has advanced to the cutting position, the delivery is stopped and the sleeve cutting begins.

4. The identification and positioning method for ecological coal mining casing based on fiber optic sensing according to claim 3, characterized in that, The aforementioned sleeve identification, based on strain inversion of low-frequency signals (0.5Hz–10Hz) in the spiral optical cable segment signal band, specifically includes: First, the original data is subjected to fidelity filtering, and then the filtered data is downsampled. An anti-aliasing filter is used to extract the low-frequency band. Secondly, apply a medium-range filter and a DC removal filter to attenuate impulse noise, and set the start time of the "strain 0" data to the moment when the previous goaf filling was completed and the slurry delivery was stopped. Finally, by using trapezoidal numerical integration in the time direction, the strain rate domain data is converted into the cumulative strain domain. In the low-frequency dataset, the data with the highest signal-to-noise ratio is selected for waterfall plot imaging and mapped onto a pre-determined coordinate system. After a period of background information acquisition, the working face advances and the top collapses directly. During this stage, strain change measurements are continuously performed to identify the location of the sleeve to be cut, and water jets are started from the orifice.

5. The identification and positioning method for ecological coal mining casing based on fiber optic sensing according to claim 3, characterized in that, The waterjet cutter head positioning selects a mid-to-high frequency signal (60Hz–300Hz) from the spiral optical cable section for vibration inversion, specifically including: First, a mid-to-high frequency signal of 60Hz to 300Hz is obtained by setting a bandpass filter; Then, the step size is set according to the distance resolution, and a fast Fourier transform is performed. The frequency point is then set to extract the phase information. Mutual differential is performed to eliminate phase noise. After unwinding, the phase signal is high-pass filtered. In the mid-to-high frequency dataset, the data with the highest signal-to-noise ratio is selected to draw the tool trajectory curve and map it onto a pre-determined coordinate system.

6. The identification and positioning method for ecological coal mining casing based on fiber optic sensing according to claim 1 or 2, characterized in that, The distributed demodulation unit is equipped with a laser, and an optical modulation module and an optical amplifier are arranged along the light propagation direction of the laser. The rear end of the optical amplifier is connected to a communication optical cable, and the communication optical cable is connected to the spiral optical cable through a circulator. An optical detection module and a demodulation module are arranged sequentially on the return optical path of the communication optical cable, and the demodulation module is also connected to the optical modulation module.

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