Distributed optical fiber sensing system and method for coal mine settlement monitoring

Abstract

The invention relates to the technical field of optical fiber monitoring, and particularly discloses a distributed optical fiber sensing system and method for coal mine settlement monitoring, the distributed optical fiber sensing system comprises a measuring part and a data acquisition and processing part, the measuring part comprises an optical cable embedded in a coal mine soil body, an FBG-based osmometer and an FBG-based extensometer, and the data acquisition and processing part comprises a data acquisition and processing part. The data acquisition and processing part comprises an OFDR demodulator, an FBG demodulator and a computer, and the optical cable comprises a first optical fiber, a sheath steel pipe and a PUC outer sheath which are sequentially arranged from inside to outside. According to the distributed optical fiber sensing system and method for coal mine settlement monitoring, the optical fiber is used for measuring strain, then the strain is converted into three physical quantities including optical cable deformation, pore pressure and extensometer measurement displacement to represent the coal mine settlement condition, verification is carried out through coal mine field experiments, and the coal mine settlement monitoring accuracy is improved. The method can be used for monitoring the coal mine settlement condition.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber monitoring technology, and in particular to a distributed optical fiber sensing system and method for coal mine subsidence monitoring. Background Art

[0002] With the development and progress of society and the rapid changes in science and technology, the mechanization degree of coal mines has been greatly improved, the mining depth has been continuously increased, and the mining speed has been increasingly accelerated, resulting in the continuous expansion of the goaf area, which in turn caused geological disasters such as landslides, ground collapse, and ground fissures. Therefore, it is very important to conduct full coverage, long-term, and high-precision settlement monitoring in concentrated coal mining areas. It is the primary prerequisite for carrying out geological disaster prevention and control in mining areas.

[0003] There are many traditional methods for monitoring surface subsidence, such as precision leveling, synthetic aperture radar interferometry, global positioning system measurement and remote sensing measurement. Precision leveling uses high-precision optical instruments and high-quality measuring rulers for measurement. Its advantages are high accuracy, good stability and wide application range. However, precision leveling requires professional operation, and data processing is complex and requires professional software for processing; synthetic aperture radar interferometry uses radar signals to scan and perform related processing to obtain interference patterns. Its advantages are high accuracy, fast measurement speed, and real-time monitoring of ground movement and deformation. However, it is difficult to apply to some remote areas due to the coverage and availability of satellite signals; the global positioning system uses signal receivers to receive satellite signals and calculate their time differences. Its advantages are fast measurement speed, high accuracy, and real-time monitoring of ground movement and deformation, which is suitable for large-scale regional subsidence monitoring. However, this method requires high-precision receivers and data processing technology, and is costly; remote sensing measurement is a method of obtaining surface information using satellites or other aircraft. When using remote sensing technology for subsidence monitoring, it is necessary to select appropriate satellites and sensors, and process and analyze the acquired data. Its advantages are large measurement range, fast information acquisition speed, and real-time monitoring of ground movement and deformation, which is suitable for large-scale regional settlement monitoring. However, this method requires professional personnel to operate and process data, and has low accuracy. Therefore, it is urgent to develop an efficient, economical and reliable settlement monitoring method.

[0004] Distributed fiber optic sensing technology has the characteristics of continuous distribution of sensing points, resistance to electromagnetic interference, high strain measurement accuracy, good toughness, strong resistance to deformation and wear, etc., which are suitable for engineering applications. It is very suitable for geological and engineering applications. In recent years, distributed fiber optic sensing technology has begun to be widely used in my country's coal industry. DFOS (distributed fiber optic sensing) enables people to better understand the deformation history, deformation degree and rate of each layer of soil, which is crucial for assessing ground subsidence. Summary of the invention

[0005] The purpose of the present invention is to provide a distributed optical fiber sensing system and method for coal mine subsidence monitoring, which adopts distributed optical fiber sensing technology, utilizes optical frequency domain reflection and fiber Bragg grating to measure the strain change along the depth direction, calculates the deformation, displacement and pore pressure of the sensing optical cable, and monitors the coal mine subsidence.

[0006] To achieve the above-mentioned purpose, the present invention provides a distributed optical fiber sensing system for coal mine subsidence monitoring, including a measuring part and a data acquisition and processing part. The measuring part includes an optical cable buried in the coal mine soil, an FBG-based piezometer and an FBG-based extensometer. The data acquisition and processing part includes an OFDR demodulator, an FBG demodulator and a computer. The optical cable includes a first optical fiber, a sheath steel pipe and a PUC outer sheath which are arranged in sequence from the inside to the outside.

[0007] Preferably, the FBG-based osmometer includes a square shell, a film printed using FDM 3D printing technology, and an optical fiber containing a FBG fiber Bragg grating. The square shell is arranged under the film, a groove is opened on the upper surface of the film, and the optical fiber containing the FBG fiber Bragg grating is installed in the groove. The optical fiber is distributed in the middle and around the film.

[0008] Preferably, the FBG-based extensometer includes an optical fiber containing an FBG fiber Bragg grating strain sensor and a PVC tube. The optical fiber containing the FBG fiber Bragg grating strain sensor is adhered to the outer wall of the PVC tube using epoxy resin. The optical fiber includes five FBG fiber Bragg grating strain sensors, and the FBG fiber Bragg grating strain sensors are connected in series at uniform intervals.

[0009] Preferably, the FBG fiber Bragg grating strain sensor is used to reflect light of the Bragg wavelength and allow light of all other wavelengths to pass through. The central wavelength of the light reflected by the FBG fiber Bragg grating strain sensor depends on the effective refractive index and period of the grating, and the effective refractive index and period are changed by strain or ambient temperature changes.

[0010] Preferably, the PUC outer sheath is made of polyurethane material to ensure the transmission of optical signals in the optical cable.

[0011] The present invention also provides a method for monitoring coal mine subsidence using a distributed optical fiber sensing system for monitoring coal mine subsidence, comprising the following steps: S1. The optical cable is buried in the soil of the coal mine, and the strain is measured by optical frequency domain reflection method to determine the deformation of the optical cable; S2, measuring strain through FBG-based piezometer to determine pore pressure; S3, measuring the strain by using an FBG-based extensometer to determine the measured displacement; S4. Use pore pressure, measured displacement and changes in optical cables to indicate coal mine subsidence.

[0012] Preferably, in S1, the strain is measured by optical frequency domain reflectometry, and the specific process is: When the light source emits continuous light, it is separated into reference light and detection light by the coupler. The detection light generates Rayleigh scattering signal in the optical fiber. The spectral shift value is obtained by correlating the back Rayleigh scattering signal with the reference signal. The spectral shift value is linearly positively correlated with the strain value, and the strain value of the optical fiber under tension or compression is obtained.

[0013] Preferably, S2 is specifically: using an FBG-based piezometer to measure strain, the upper side of the FBG-based piezometer is subjected to pressure due to soil settlement, and the lower side undergoes tensile deformation, the tensile deformation causes the wavelength of the fiber Bragg grating to shift, and a strain relationship between strain and pore pressure is established. The pore pressure is obtained by measuring the change in wavelength, and if the pore pressure decreases, it indicates soil compression.

[0014] Preferably, S3 is specifically: The strain is measured using an FBG-based extensometer. If settlement occurs, the pressure on the FBG fiber Bragg grating changes, and the strain change changes the Bragg wavelength. A mathematical relationship between strain and wavelength, and between wavelength and measured displacement is established, and the strain is converted into measured displacement to represent the settlement.

[0015] Therefore, the present invention adopts the above-mentioned distributed optical fiber sensing system and method for coal mine subsidence monitoring, and the beneficial effects are as follows: The present invention measures strain through optical frequency domain reflection technology and establishes a mathematical equation for converting strain into optical cable deformation; measures strain through fiber Bragg grating and establishes a mathematical relationship between pore pressure, displacement and strain; and conducts field experiments to verify the accuracy of the mathematical equation, providing a scientific basis for monitoring coal mine subsidence using distributed optical fiber sensing technology.

[0016] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of the overall structure of an optical cable of an embodiment of a distributed optical fiber sensing system for coal mine subsidence monitoring of the present invention; Figure 2 It is a cross-sectional view of an optical cable structure of an embodiment of a distributed optical fiber sensing system for coal mine subsidence monitoring of the present invention; Figure 3 It is a schematic structural diagram of a piezometer based on FBG in an embodiment of a distributed optical fiber sensing system for coal mine settlement monitoring of the present invention; Figure 4It is a schematic structural diagram of an FBG-based extensometer in an embodiment of a distributed optical fiber sensing system for coal mine settlement monitoring of the present invention; Figure 5 It is a schematic diagram of the on-site arrangement of an embodiment of a method for monitoring coal mine subsidence using a distributed optical fiber sensing system for monitoring coal mine subsidence according to the present invention; Figure 6 It is a schematic diagram of the installation of a heavy guide of an embodiment of a method for monitoring coal mine subsidence using a distributed optical fiber sensing system for monitoring coal mine subsidence of the present invention.

[0018] Reference numerals 1. Optical cable; 101. First optical fiber; 102. Sheath steel pipe; 103. PUC outer sheath; 2. Osmometer based on FBG; 201. square housing; 202. film; 203. FBG fiber Bragg grating; 204. second optical fiber; 205. groove; 3. FBG-based extensometer; 301. FBG fiber Bragg grating strain sensor; 302. third optical fiber; 303. PVC tube; 4. OFDR demodulator; 5. FBG demodulator; 6. Computer; 7. Heavy guide; 8. Coal mine. DETAILED DESCRIPTION

[0019] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0020] Unless otherwise defined, the technical terms or scientific terms used in the present invention should be understood by people with ordinary skills in the field to which the present invention belongs. The words "first", "second" and similar words used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprise" and similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0021] As shown in the figure, a distributed optical fiber sensing system for coal mine settlement monitoring includes a measuring part and a data acquisition and processing part. The measuring part includes an optical cable 1 buried in the coal mine soil, a piezometer based on FBG 2 and an extensometer based on FBG 3, and the data acquisition and processing part includes an OFDR demodulator 4, an FBG demodulator 5 and a computer 6. Figure 1 ,2 As shown, the optical cable 1 includes a first optical fiber 101, a sheath steel tube 102 and a PUC outer sheath 103 which are arranged in sequence from the inside to the outside.

[0022] The PUC outer sheath 103 is made of polyurethane, which is a polymer with repeating structural units of urethane segments made by the reaction of isocyanate and polyol. Polyurethane has particularly good mechanical properties. The material has extremely strong wear resistance, cut resistance and tear resistance, and can maintain high flexibility even at low temperatures. PUC also has strong resistance to oil, solvents and ultraviolet radiation. In addition, according to the composition of the material, it is halogen-free and flame retardant, which makes the PUC material suitable for many harsh occasions. As the outer sheath of the optical cable, it can protect the optical cable from external factors, increase the service life of the optical cable, and is used to ensure the normal transmission of the optical signal in the optical cable.

[0023] like Figure 3 As shown, the FBG-based osmometer 2 includes a square housing 201, a film 202 made of polylactic acid and printed using FDM 3D printing technology, and a second optical fiber 204 containing a FBG fiber Bragg grating 203. The square housing 201 is arranged below the film 202, a groove 205 is opened on the film 202, the second optical fiber 204 containing the FBG fiber Bragg grating 203 is installed in the groove 205, and the second optical fiber 204 is distributed in the middle and around the film 202.

[0024] Polylactic acid (PLA), also known as polylactide, is a polyester polymer obtained by polymerization of lactic acid as the main raw material. It is a new type of biodegradable material made from starch raw materials from renewable plant resources. PLA is an environmentally friendly material that is easy to print, has a smooth surface, has higher rigidity and strength similar to polycarbonate, does not require a closed cavity, has a low shrinkage rate, does not warp or crack, and can print large-sized models; there is no irritating odor during printing, and it is safe and environmentally friendly.

[0025] FDM technology is a 3D printing technology based on fused deposition modeling. During the printing process, the molten filament is heated to a molten state and then extruded onto the printing platform. The molten filament is then stacked layer by layer at a certain speed and temperature to eventually form a three-dimensional object. FDM technology is low-cost, has a wide range of material options, high flexibility, and strong customizability.

[0026] like Figure 4As shown, the FBG-based extensometer 3 includes a third optical fiber 302 containing a FBG fiber Bragg grating strain sensor 301 and a PVC tube 303. The third optical fiber 302 containing the FBG fiber Bragg grating strain sensor 301 is adhered to the outer wall of the PVC tube 303 using epoxy resin. The third optical fiber 302 includes five FBG fiber Bragg grating strain sensors 301, and the FBG fiber Bragg grating strain sensors 301 are connected in series at uniform intervals.

[0027] The FBG fiber Bragg grating strain sensor 301 is an all-fiber device formed by modulation in the core of an optical fiber, which is used to reflect light of the Bragg wavelength and allow light of all other wavelengths to pass through. The central wavelength of the light reflected by the FBG fiber Bragg grating strain sensor depends on the effective refractive index and period of the grating, and the effective refractive index and period are changed by strain or ambient temperature changes.

[0028] The present invention also provides a method for monitoring coal mine subsidence using a distributed optical fiber sensing system for monitoring coal mine subsidence, comprising the following steps: S1. The optical cable 1 is buried in the soil of the coal mine, and the strain is measured by the optical frequency domain reflection method to determine the deformation of the optical cable 1. The optical frequency domain reflection method measures the strain. The specific process is as follows: When the light source emits continuous light, it is separated into reference light and detection light by the coupler. The detection light generates Rayleigh scattering signal in the optical fiber. The spectral shift value is obtained by correlating the back Rayleigh scattering signal with the reference signal. The spectral shift value is linearly positively correlated with the strain value. Therefore, the strain value of the optical fiber under tension or compression can be obtained. The optical frequency domain reflectometry has extremely high spatial resolution and sensing accuracy.

[0029] S2, the strain is measured by the FBG-based piezometer 2 to determine the pore pressure, specifically: The strain is measured by using the FBG-based piezometer 2. The upper side of the FBG-based piezometer 2 is subjected to pressure due to soil settlement, and the lower side will be stretched and deformed. The stretching deformation will cause the wavelength of the fiber Bragg grating 203 to shift. The strain relationship between the strain and the pore pressure is established. The magnitude of the pore pressure can be obtained by measuring the change in wavelength. If the pore pressure decreases, it means that the soil is compressed, as follows: Strain changes will change the Bragg wavelength. By establishing the relationship between strain and wavelength, and wavelength and pore pressure, strain can be converted into pore pressure: ; Where p is the pore pressure, K is the constant of the ratio of pore pressure to Bragg wavelength, P is the measured Bragg wavelength, is the initial Bragg wavelength of the sensor, then: ; in, is the Bragg wavelength change, is the original Bragg wavelength under unstrained and 0°C conditions, is the effective elastic-optical coefficient, and are the thermal expansion coefficient and the thermo-optical coefficient, and represent strain and temperature changes, respectively.

[0030] S3, by measuring the strain based on the FBG fiber Bragg grating extensometer 3, determine the measured displacement, S3 is specifically: The strain is measured using an FBG-based extensometer 3. If settlement occurs, the pressure on the FBG fiber Bragg grating changes. The strain change will change the Bragg wavelength. By establishing a mathematical relationship between strain and wavelength, and wavelength and measured displacement, the strain can be converted into measured displacement to represent the settlement. The expression is as follows: ; D is the measured displacement, is a constant for the ratio of displacement to the Bragg wavelength, is the measured Bragg wavelength, is the initial Bragg wavelength of the sensor, is the wavelength caused by temperature, is the initial wavelength caused by temperature.

[0031] S4. Use pore pressure, measured displacement and changes in optical cables to indicate coal mine subsidence.

[0032] Embodiment 1 A distributed optical fiber sensing system for coal mine settlement monitoring includes an optical cable 1, a FBG-based piezometer 2, and a FBG-based extensometer 3. The optical cable 1 includes an optical fiber 101, a sheath steel tube 102, and a PUC outer sheath 103, and the diameter of the optical cable 1 is 10 mm.

[0033] The FBG-based osmometer 2 consists of a 40mm×30mm×20mm square housing 201, a film 202 and a second optical fiber 204 containing FBG. The film 202 is made of polylactic acid and is made using 3D printing technology. The film 202 is 3mm thick. A groove 205 with a depth of 1mm is provided at the lower part of the film 202. The second optical fiber 204 containing the FBG fiber Bragg grating 203 is installed in the groove in a tension state, and the second optical fiber 204 is distributed in the middle and around the film.

[0034] The FBG-based extensometer 3 is composed of a PVC tube 303 with a length of 500 mm and a diameter of 10 mm and a third optical fiber 302 containing a FBG fiber Bragg grating sensor 301. There are five FBG fiber Bragg grating strain sensors 301 in the third optical fiber 302, and each FBG fiber Bragg grating strain sensor 301 is connected in series at a uniform interval of 100 mm, and then the third optical fiber 302 is fixed to the outer wall of the PVC tube 303 using epoxy resin.

[0035] Embodiment 2 The method of the system of embodiment 1 is adopted, such as Figure 5 As shown in the figure, when using the distributed optical fiber sensing system to monitor coal mine subsidence on site: First, a monitoring location was selected and a borehole was drilled vertically in the bedrock to a depth of 100 m penetrating the potential sliding surface; Secondly, after reaching the stable bedrock layer, if Figure 6 As shown, the optical cable 1, the FBG-based piezometer 2 and the FBG-based extensometer 3 are placed in the center of the borehole using a heavy-duty guide 7, which can drag the optical cable 1 to the bottom and keep the optical cable 1 taut during the installation process. The heavy-duty guide 7 has a height of 50 cm and a width of 10 cm; Then, the borehole is backfilled to the top with a mixture of gravel, sand and clay. After the backfill is completed, it is left to stand for about a month to ensure that the backfill in the borehole is compacted to a stable state. A fixture (i.e., heavy-duty guide 7) is used to fix the optical cable 1 to prevent the optical cable 1 from shifting during the measurement process and affecting the measurement. After the system is installed, the initial strain of the embedded optical cable can be measured.

[0036] The system measures the longitudinal strain caused by soil settlement, and calculates the corresponding deformation and pore pressure according to the mathematical equation based on the measured strain to represent the settlement of the coal mine. The strain is measured using optical fiber, and then converted into optical cable deformation, pore pressure and extensometer displacement. These three physical quantities can characterize the settlement of the coal mine. In the overall experiment, the optical cable is always kept in tension throughout the installation process to increase the tension strain on the optical cable.

[0037] The distributed optical fiber sensing system involved in the present invention includes an optical cable and an FBG-based piezometer and extensometer. During the settlement of a coal mine, the stress inside the soil changes, and the strain distribution is obtained by optical fiber measurement. The optical frequency domain reflection technology converts the strain into the deformation of the sensing optical cable. The deformation of the optical cable is consistent with the settlement, and the settlement of the coal mine can be monitored. The piezometer is made based on a fiber Bragg grating. The strain change causes the change of the Bragg wavelength, which can be converted into pore pressure. The decrease in pore pressure represents soil compression; the extensometer is made based on a fiber Bragg grating, which can convert the wavelength into a measured displacement, and the displacement is also consistent with the settlement. The data obtained by the system is in good agreement with the settlement of the coal mine, proving the feasibility of the present invention in monitoring the settlement of the coal mine.

[0038] Therefore, the present invention adopts the above-mentioned distributed optical fiber sensing system and method for coal mine subsidence monitoring, measures strain through optical frequency domain reflection technology to determine the deformation of the optical cable; measures strain through a piezometer to determine the pore pressure; measures strain through an extensometer to determine the displacement; the changes in pore pressure, displacement and optical cable can represent coal mine subsidence.

[0039] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A distributed optical fiber sensing system for coal mine subsidence monitoring, characterized in that: It includes a measuring part and a data acquisition and processing part. The measuring part includes an optical cable buried in the coal mine soil, a FBG-based piezometer and a FBG-based extensometer. The data acquisition and processing part includes an OFDR demodulator, an FBG demodulator and a computer. The optical cable includes a first optical fiber, a protective steel pipe and a PUC outer sheath which are arranged in sequence from the inside to the outside.

2. A distributed optical fiber sensing system for coal mine subsidence monitoring according to claim 1, characterized in that: The FBG-based osmometer includes a square shell, a film printed using FDM 3D printing technology, and an optical fiber containing an FBG fiber Bragg grating. The square shell is arranged under the film, a groove is opened on the upper surface of the film, and the optical fiber containing the FBG fiber Bragg grating is installed in the groove. The optical fiber is distributed in the middle and around the film.

3. A distributed optical fiber sensing system for coal mine subsidence monitoring according to claim 2, characterized in that: The FBG-based extensometer includes an optical fiber containing an FBG fiber Bragg grating strain sensor and a PVC tube. The optical fiber containing the FBG fiber Bragg grating strain sensor is adhered to the outer wall of the PVC tube by using epoxy resin. The optical fiber includes five FBG fiber Bragg grating strain sensors. The FBG fiber Bragg grating strain sensors are connected in series at uniform intervals.

4. A distributed optical fiber sensing system for coal mine subsidence monitoring according to claim 3, characterized in that: The FBG fiber Bragg grating strain sensor is used to reflect light of the Bragg wavelength and allow light of all other wavelengths to pass through. The central wavelength of the reflected light of the FBG fiber Bragg grating strain sensor depends on the effective refractive index and period of the grating, and the effective refractive index and period are changed by strain or ambient temperature changes.

5. A distributed optical fiber sensing system for coal mine subsidence monitoring according to claim 4, characterized in that: The PUC outer sheath is made of polyurethane and is used to ensure the transmission of optical signals within the optical cable.

6. A method for monitoring coal mine subsidence using a distributed optical fiber sensing system for monitoring coal mine subsidence according to any one of claims 1 to 5, characterized in that: The following steps are involved: S1. The optical cable is buried in the soil of the coal mine, and the strain is measured by optical frequency domain reflection method to determine the deformation of the optical cable; S2, measuring strain through FBG-based piezometer to determine pore pressure; S3, measuring the strain by using an FBG-based extensometer to determine the measured displacement; S4. Use pore pressure, measured displacement and changes in optical cables to indicate coal mine subsidence.

7. A method for monitoring coal mine subsidence using a distributed optical fiber sensing system for monitoring coal mine subsidence according to claim 6, characterized in that: In S1, the optical frequency domain reflectometry method is used to measure strain. The specific process is as follows: When the light source emits continuous light, it is separated into reference light and detection light by the coupler. The detection light generates Rayleigh scattering signal in the optical fiber. The spectral shift value is obtained by correlating the back Rayleigh scattering signal with the reference signal. The spectral shift value is linearly positively correlated with the strain value, and the strain value of the optical fiber under tension or compression is obtained.

8. The method for monitoring coal mine subsidence using a distributed optical fiber sensing system for monitoring coal mine subsidence according to claim 7, characterized in that: S2 is specifically as follows: strain is measured using an FBG-based piezometer. The upper side of the FBG-based piezometer is subjected to pressure due to soil settlement, and tensile deformation occurs on the lower side. The tensile deformation causes the wavelength of the fiber Bragg grating to shift, and a strain relationship between strain and pore pressure is established. The pore pressure is determined by measuring the change in wavelength. If the pore pressure decreases, it means that the soil is compressed.

9. A method for monitoring coal mine subsidence using a distributed optical fiber sensing system for monitoring coal mine subsidence according to claim 8, characterized in that: S3 is specifically: The strain is measured using an FBG-based extensometer. If settlement occurs, the pressure on the FBG fiber Bragg grating changes, and the strain change changes the Bragg wavelength. A mathematical relationship between strain and wavelength, and between wavelength and measured displacement is established, and the strain is converted into measured displacement to represent the settlement.

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