Continuous mining stress measurement method based on multi-core distributed optical fiber

Abstract

The application relates to the technical field of geotechnical engineering safety monitoring and engineering testing, and provides a continuous mining stress measurement method based on a multi-core distributed optical fiber. The continuous mining stress measurement method comprises the following steps: drilling a cylindrical monitoring drill hole in a to-be-monitored area affected by mining; implanting a flexible cylindrical sensor into the monitoring drill hole, the flexible cylindrical sensor is internally provided with a multi-core distributed optical fiber, the multi-core distributed optical fiber comprises one central core located on a central axis of a sensor base and a plurality of peripheral cores distributed around the central axis, the flexible cylindrical sensor and surrounding rock of the monitoring drill hole form a consolidated coupling body; continuous Brillouin frequency shift data of the central core and the peripheral cores in a mining process are collected; based on a frequency shift variation, in combination with a spatial analytic geometry theory and elastic mechanics parameters of a rock mass, through three-dimensional spatial form reconstruction and a borehole deformation mechanics model, continuous mining stress increment data of the whole monitoring drill hole are solved.

Description

Technical Field

[0001] This application relates to the field of geotechnical engineering safety monitoring and engineering testing technology, specifically to a method for measuring continuous mining stress based on multi-core distributed optical fiber. Background Technology

[0002] In geotechnical engineering projects such as deep mining, tunnel excavation, and dam foundation disturbance, mining activities trigger continuous dynamic evolution of the surrounding rock stress field. Accurate monitoring of mining stress is a core technology for ensuring the stability of engineering structures and the safety of construction and operation. Currently, the industry still relies primarily on traditional point-based monitoring equipment such as hollow inclusion stress gauges and oil bladder stress gauges for mining stress measurement. These monitoring methods have significant technical limitations in practical engineering applications and are difficult to adapt to the full-cycle monitoring needs under complex mining environments.

[0003] Traditional point-based stress monitoring equipment can only collect stress data at discrete points, failing to cover the stress field across the entire structural area of ​​the monitoring region. During the continuous dynamic migration of mining stress with engineering activities, this discrete monitoring mode is prone to missing key nodes of local stress concentration and dynamic stress abrupt changes, failing to fully capture the full-space evolution of the surrounding rock stress field under the influence of mining. Furthermore, the deployment cost of a single point-based monitoring device is high, and the number of devices that can be deployed on-site is extremely limited. It can only extrapolate the overall stress field distribution through data interpolation from a small number of discrete points, unable to form a continuous, high-density stress distribution characterization within the monitoring area, making it difficult to support refined surrounding rock stability analysis and engineering safety assessment. In addition, traditional point-based stress gauges have poor adaptability to the harsh environments of mining projects. The high stress, strong disturbances, and moisture corrosion of deep surrounding rock can easily cause damage and failure of individual monitoring devices. The failure of a single device will directly lead to the loss of data at the corresponding monitoring location, not only disrupting the integrity of the monitoring sequence but also significantly increasing the operation and maintenance costs and data reliability risks of the monitoring system. Summary of the Invention

[0004] This application provides a method for continuous mining stress measurement based on multi-core distributed optical fiber, which solves the problem that mining stress measurement data cannot cover the entire structure in the prior art, and realizes continuous measurement of mining stress.

[0005] A continuous sampling stress measurement method based on multi-core distributed optical fiber according to an embodiment of the first aspect of this application includes the following steps: Drill cylindrical monitoring boreholes in the monitoring area affected by mining activities; A flexible cylindrical sensor is implanted into the monitoring borehole. The sensor substrate of the flexible cylindrical sensor contains a multi-core distributed optical fiber. The multi-core distributed optical fiber includes a central core located on the central axis of the sensor substrate and multiple peripheral cores distributed around the central axis. Both the central core and the peripheral cores extend axially along the sensor substrate and are parallel to each other. A coupling and consolidation layer is provided between the flexible cylindrical sensor and the borehole wall, forming a consolidated coupling body between the flexible cylindrical sensor and the surrounding rock of the monitoring borehole. The end of the multi-core distributed optical fiber is connected to the Brillouin fiber demodulation system. At the initial moment before the disturbance arrives, the initial Brillouin frequency shift data of the central fiber core and the peripheral fiber core are collected as a relative zero reference. During the sampling process, continuous Brillouin frequency shift data of the central fiber core and the peripheral fiber core are collected, and the frequency shift change relative to the relative zero reference is extracted. Based on the frequency shift change, combined with spatial analytical geometry theory and the elastic mechanical parameters of the rock mass, the continuous mining stress increment data of the entire monitoring borehole area is calculated through three-dimensional spatial morphology reconstruction and borehole deformation mechanical model.

[0006] According to one embodiment of this application, drilling a cylindrical monitoring borehole in the monitoring area affected by mining activities includes: According to the preset monitoring plan, monitoring boreholes are drilled in the top, bottom, side or target monitoring body of the surrounding rock in the area to be monitored, along the vertical, horizontal or inclined direction. After the monitoring borehole is completed, high-pressure air or clean water is used to remove rock powder and loose debris from the borehole. The inner diameter of the monitoring borehole is 5mm to 20mm larger than the outer diameter of the sensor, and the depth of the monitoring borehole covers the mining impact range of the area to be monitored.

[0007] According to one embodiment of this application, prior to implanting the flexible cylindrical sensor into the monitoring borehole, the method further includes: A flexible cylindrical substrate adapted to the monitoring borehole was selected as the sensor substrate, and the substrate material has deformation synergy that matches the surrounding rock to be monitored. The multi-core distributed optical fiber is extended along the axial direction of the sensor substrate and arranged parallel to each other. The multi-core distributed optical fiber is then integrated with the sensor substrate through injection molding. The two ends of the sensor substrate are sealed and waterproofed to form a multi-core fiber optic sensing probe with full-range aperture deformation sensing capability.

[0008] According to one embodiment of this application, the deployment structure of the multi-core distributed optical fiber includes: A multi-core distributed optical fiber consists of one central core and at least three peripheral cores; The peripheral fiber cores are distributed at equal angular intervals along the circumference with the central fiber core as the center, and the axes of both the central fiber core and the peripheral fiber cores are parallel to the axis of the sensor substrate.

[0009] According to one embodiment of this application, the multi-core distributed optical fiber adopts a seven-core structure with one central core and six peripheral cores arranged in parallel.

[0010] According to one embodiment of this application, the acquisition of continuous Brillouin frequency shift data of the central fiber core and the peripheral fiber core during the acquisition process includes: Set the spatial sampling interval, acquisition frequency, and measurement range parameters of the Brillouin fiber demodulation system; Throughout the entire sampling process, the Brillouin frequency shift data of all measurement points on the central fiber core and all peripheral fiber cores are simultaneously collected through the demodulation system to obtain a continuous Brillouin frequency shift dataset covering the entire sensor domain.

[0011] According to one embodiment of this application, before calculating the continuous mining stress data of the entire monitoring borehole area based on the frequency shift change, combined with spatial analytical geometry theory and the elastic mechanical parameters of the rock mass, through three-dimensional spatial morphology reconstruction and borehole deformation mechanical model, the method further includes: Using the Brillouin frequency shift data of the central fiber core as a reference, the interference of ambient temperature fluctuations and the overall axial tension of the surrounding rock on the strain data is eliminated; Based on the linear relationship between Brillouin frequency shift and strain, the local strain distribution of each of the peripheral fiber cores along the axial direction of the sensor substrate is calculated; Based on the local strain distribution and spatial arrangement of each peripheral fiber core, an apparent curvature vector is constructed, and the local curvature and bending direction angle of each measuring point are calculated.

[0012] According to one embodiment of this application, after calculating the local curvature and bending direction angle of each measuring point, the method further includes: Calculate the local deflection of the corresponding measuring point based on the bending direction angle of each measuring point; We introduce the Frenet-Serret differential equation system and construct a kinematic space orthogonal frame composed of tangent vector, principal normal vector and binormal vector; Using the borehole anchor point as the spatial integration reference point, continuous integration is performed along the axial direction of the sensor substrate to reconstruct the three-dimensional absolute coordinate trajectory of the borehole at the corresponding sampling time.

[0013] According to one embodiment of this application, based on the frequency shift change, and combining spatial analytical geometry theory with the elastic mechanical parameters of the rock mass, the continuous mining stress increment data of the entire monitoring borehole area is calculated through three-dimensional spatial morphology reconstruction and borehole deformation mechanical model, including: Based on the reconstructed three-dimensional absolute coordinate trajectory, the radial displacement distribution increments on each cross section along the depth of the monitored borehole are extracted; Based on the preset deployment orientation of the multi-core distributed optical fiber, extract at least three dynamic aperture relative deformations with specific included angles on each cross section; By combining the elastic modulus and Poisson's ratio parameters of the surrounding rock to be monitored, the relative deformation of the borehole diameter is substituted into the stress concentration formula at the borehole edge. By solving the multivariate simultaneous equations, the three-dimensional mining stress additional field components and principal stress increment values ​​of each measuring point are obtained.

[0014] According to one embodiment of this application, after calculating the three-dimensional mining stress additional field components and principal stress increment values ​​at each measuring point, the method further includes: The three-dimensional mining stress additional field components and principal stress increment values ​​obtained from each measuring point are correlated one by one with the three-dimensional spatial absolute coordinates of the corresponding measuring points to generate a continuous mining stress data set for monitoring the entire borehole area. Based on the continuous mining stress data set, an axial continuous mining stress profile curve and a three-dimensional spatial mining stress cloud map are generated for the monitoring borehole; the profile curve and cloud map are used for stability analysis and safety assessment of mining operations.

[0015] The above-described one or more technical solutions in the embodiments of this application have at least one of the following technical effects: This application's method for continuous mining stress measurement based on multi-core distributed optical fiber utilizes a flexible cylindrical sensor with integrated multi-core distributed optical fiber. Continuous acquisition of Brillouin frequency shift data for each fiber core is achieved through continuous optical fiber transmission. Continuous mining stress data is then obtained through three-dimensional spatial morphology reconstruction and mechanical transformation calculations. Leveraging the sensitivity of each fiber core's Brillouin frequency shift to bending strain, a sensing system coupling three-dimensional spatial morphology reconstruction and borehole deformation mechanics is constructed, enabling continuous measurement of mining stress across the entire borehole. This eliminates the need for discretely deployed multi-point stress gauges; a four-dimensional spatiotemporal dynamic sensing system can be established solely through multi-core distributed optical fiber. It directly demodulates and monitors continuous mining stress data across the entire borehole area, addressing the problems of insufficient coverage, poor deformation resistance, and lack of three-dimensional spatial deformation sensing capabilities inherent in traditional point-based monitoring. This provides accurate, real-time, and comprehensive stress data support for stability analysis and dynamic disaster early warning in mining engineering. This application breaks through the traditional monitoring mode of combined deployment of rigid point-type hollow inclusion stress gauges, realizing the synchronous perception of continuous mining stress and three-dimensional spatial deformation throughout the borehole. It not only fundamentally solves the core pain points of insufficient coverage, discontinuous data, and susceptibility to damage by large deformation in traditional point-type monitoring, but also realizes a principle upgrade of mining stress measurement from discrete single-point scalar measurement to continuous distributed four-dimensional spatiotemporal measurement throughout the entire area.

[0016] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments or related technologies of this application, the accompanying drawings used in the description of the embodiments or related technologies will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a flowchart illustrating the continuous sampling stress measurement method based on multi-core distributed optical fiber provided in this application.

[0019] Figure 2 This is a schematic diagram of the state of the cylindrical sensor provided in this application implanted in the monitoring borehole (the monitoring borehole is mainly horizontal, but the actual monitoring borehole can also be vertical).

[0020] Figure 3 This is a schematic diagram of the cylindrical sensor with distributed optical fiber provided in this application (solid lines indicate the original positions of the cylindrical sensor and the seven fiber cores, and dashed lines indicate the positions of the cylindrical sensor and the seven fiber cores after being subjected to mining pressure).

[0021] Figure label: 1. Cylindrical sensor; 11. Central fiber core; 12. Peripheral fiber core 2. Grouting materials; 3. Surrounding rock; 31. Monitoring borehole; 4. Brillouin fiber optic demodulation system. Detailed Implementation

[0022] The embodiments of this application will be described in further detail below with reference to the accompanying drawings and examples. The following examples are used to illustrate this application, but should not be used to limit the scope of this application.

[0023] In the description of the embodiments of this application, it should be noted that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application. In addition, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0024] In the description of the embodiments of this application, it should be noted that, unless otherwise explicitly specified and limited, the terms "connected" and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in the embodiments of this application based on the specific circumstances.

[0025] In the embodiments of this application, unless otherwise expressly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "on top of," and "over" the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0026] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the embodiments of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0027] According to an embodiment of the first aspect of this application, a method for continuous sampling stress measurement based on multi-core distributed optical fiber is provided, such as... Figure 1 and Figure 2 As shown, the process includes the following steps: drilling a cylindrical monitoring borehole 31 in the area to be monitored due to mining impact; embedding a flexible cylindrical sensor 1 into the monitoring borehole 31. The flexible cylindrical sensor 1 is a flexible, expandable cylinder with a multi-core distributed optical fiber inside its sensor substrate. The multi-core distributed optical fiber includes a central fiber core 11 located on the central axis of the sensor substrate and multiple peripheral fiber cores 12 distributed around the central axis. Both the central fiber core 11 and the peripheral fiber cores 12 extend along the axial direction of the sensor substrate and are parallel to each other; filling the space between the flexible cylindrical sensor 1 and the borehole wall of the monitoring borehole 31 with high-strength grouting material 2 to form a coupling consolidation layer, thereby achieving full-length permanent consolidation coupling between the flexible cylindrical sensor 1 and the surrounding rock 3 of the monitoring borehole 31; and then... The ends of the multi-core distributed optical fiber are connected to the Brillouin fiber demodulation system 4. At the initial moment before the excavation disturbance reaches the area, the initial Brillouin frequency shift reference data of the central fiber core 11 and the outer fiber core 12 are collected. This state is calibrated as the initial background state of the relative zero stress field for mining stress monitoring (i.e., as the relative zero point reference). Using this reference data as the zero point, the continuous Brillouin frequency shift data of the central fiber core 11 and the outer fiber core 12 during the mining process are collected, and the frequency shift change relative to the relative zero point reference is extracted. Based on this frequency shift change, combined with spatial analytical geometry theory and the elastic mechanical parameters of the rock mass, the continuous mining stress increment data of the entire monitoring borehole 31 is calculated through three-dimensional spatial morphology reconstruction and borehole deformation mechanical model.

[0028] By integrating a multi-core distributed optical fiber within a flexible cylindrical sensor 1, the traditional discrete single-point strain sensing is transformed into a continuous three-dimensional strain sensing system covering the entire domain. Through a single full-length grouting implantation, the fiber, spanning hundreds of meters, can continuously capture minute changes in the pre-support pressure. The monitoring borehole 31 provides the sensor with a tightly fitted installation space within the surrounding rock 3. The coupling consolidation layer formed by the high-strength grouting material 2 eliminates the interface gap between the sensor and the surrounding rock 3, ensuring lossless stress transmission within the surrounding rock 3. This allows the sensor to undergo multi-directional radial deformation and axial three-dimensional spatial bending synchronously with the borehole wall. The Brillouin fiber demodulation system 4 synchronously acquires the Brillouin frequency shift signal of the entire multi-core distributed optical fiber. Utilizing the sensitivity of each fiber core's Brillouin frequency shift to bending strain, based on the continuous three-dimensional shape reconstruction technology of the multi-core fiber and the borehole diameter-following deformation principle, combined with spatial analytical geometry theory, the temporal absolute morphological evolution of the borehole under mining influence in three-dimensional space is continuously reconstructed. Furthermore, a borehole diameter deformation mechanical model is introduced to calculate the mining stress increment, ultimately achieving continuous calculation of mining stress across the entire borehole range.

[0029] The aforementioned continuous mining stress measurement method overcomes the technical limitations of traditional point-based stress monitoring. Through the three-dimensional spatial deployment of multi-core distributed optical fibers, combined with Frenet-Serret spatial analytical geometry theory, it achieves a leap from single-point discrete measurement to continuous measurement across the entire borehole. It eliminates the need for numerous discrete stress gauges; a single sensor can complete three-dimensional stress sensing across the entire borehole, significantly reducing deployment costs and avoiding data loss due to single-point equipment failure. The method accurately captures the dynamic evolution and local concentration characteristics of stress during mining operations. It also visually reconstructs the overall subsidence, delamination, and fault slippage processes of the roof and floor as the working face advances, providing both geometric and mechanical criteria for early warning of coal and rock dynamic disasters. Furthermore, the probe is entirely passive, unaffected by underground electromagnetic interference, gas, and water seepage, greatly improving the long-term stability and survivability of the monitoring system in harsh mining environments.

[0030] According to one embodiment of this application, a cylindrical monitoring borehole 31 is drilled in the area to be monitored by mining, including: drilling the monitoring borehole 31 in the top plate, bottom plate, sidewall or target monitoring body of the surrounding rock 3 in the area to be monitored in a vertical, horizontal or inclined direction according to a preset monitoring plan; after the monitoring borehole 31 is drilled, rock powder and loose debris in the borehole are removed by high pressure air or clean water; wherein, the inner diameter of the monitoring borehole 31 is 5mm to 20mm larger than the outer diameter of the sensor, and the depth of the monitoring borehole 31 covers the mining impact range of the area to be monitored.

[0031] The aforementioned drilling process for monitoring borehole 31 provides an installation space for the flexible cylindrical sensor 1 that conforms to industry standards. Multiple locations and directions allow for flexible placement to meet the monitoring needs of different parts of the surrounding rock 3: vertical boreholes are used for monitoring the top and bottom plates, horizontal boreholes for monitoring the ribs or sidewalls, and inclined boreholes for specific stress field analysis. Hole cleaning eliminates any debris within the borehole that could negatively impact subsequent coupling. The design of the inner diameter and depth parameters ensures successful sensor implantation and provides a uniform filling space for the coupling consolidation layer, guaranteeing the uniformity and accuracy of stress transmission.

[0032] In some embodiments, before drilling the monitoring borehole 31, the inner diameter, depth, orientation, and layout points of the monitoring borehole 31 are determined according to a preset monitoring plan. The borehole opening position is marked, and matching drilling tools are selected to carry out the drilling operation. After the hole cleaning operation is completed, the integrity of the borehole wall and the axial deviation of the monitoring borehole 31 are checked to ensure that there are no problems such as borehole collapse or diameter reduction, and that the axial deviation meets the sensor installation requirements. The actual depth and inner diameter of the monitoring borehole 31 are verified to ensure that the drilling parameters are completely matched with the size of the flexible cylindrical sensor 1 and the monitoring requirements.

[0033] According to one embodiment of this application, before implanting the flexible cylindrical sensor 1 into the monitoring borehole 31, the method further includes: selecting a flexible cylindrical substrate adapted to the monitoring borehole 31 as the sensor substrate, wherein the substrate material has deformation compatibility matching the surrounding rock 3 to be monitored; extending multi-core distributed optical fibers along the axial direction of the sensor substrate and arranging them parallel to each other, and integrating the multi-core distributed optical fibers with the sensor substrate through an injection molding process; and sealing and waterproofing both ends of the sensor substrate to form a multi-core optical fiber sensing probe with full-area aperture deformation sensing capability.

[0034] The sensor prefabrication process is the core foundation for achieving continuous mining stress monitoring across the entire area. A flexible cylindrical substrate adapted to the monitoring borehole 31 provides a stable deployment carrier for the multi-core distributed optical fiber. Simultaneously, through a material design coordinated with rock deformation, it ensures that the sensor can undergo large deformations synchronously with the surrounding rock 3 without being damaged. Injection molding tightly integrates the multi-core distributed optical fiber with the sensor substrate, eliminating relative slippage between the fiber and the substrate and ensuring accurate transmission of strain signals. Sealed and waterproof encapsulation protects the fiber optic connectors from corrosion by the humid underground environment, improving the long-term reliability of the sensor.

[0035] In some embodiments, the sensor substrate is made of polyurethane material, with a substrate diameter 5-20 mm smaller than the inner diameter of the monitoring borehole 31, and a length adapted to the depth of the monitoring borehole 31. During injection molding, the injection pressure and temperature are controlled to ensure that the multi-core distributed optical fiber remains straight and parallel inside the sensor substrate, without bending or twisting. The sensor ends are encapsulated with stainless steel sealing connectors, the inside of which is filled with waterproof sealant, and the outer layer is wrapped with a glass fiber reinforcement layer to improve the sensor's impact and tensile strength. For long-distance monitoring needs, multiple multi-core fiber optic sensing probes can be connected in series via low-loss optical fiber fusion splicing, with the splice encapsulated in a stainless steel sealing protective sleeve to form a long-distance monitoring network.

[0036] According to one embodiment of this application, the deployment structure of the multi-core distributed optical fiber includes: the multi-core distributed optical fiber includes a central fiber core 11 and at least three peripheral fiber cores 12; the peripheral fiber cores 12 are distributed at equal angular intervals along the circumference with the central fiber core 11 as the center, and the axes of the central fiber core 11 and the peripheral fiber cores 12 are parallel to the axis of the sensor substrate.

[0037] Multiple independent fiber cores form the core carrier for constructing the three-dimensional strain sensing system. The central fiber core 11 is located on the central axis of the sensor substrate and is unaffected by bending strain, serving as a reference for temperature and axial tensile force compensation. The peripheral fiber cores 12 are distributed at equal angular intervals along the circumference, enabling them to sense bending strain in different directions and providing multiple sets of independent basic data for three-dimensional spatial morphology reconstruction.

[0038] like Figure 3 As shown, since the central fiber core 11 is located at the central axis of the cylindrical sensor 1, it is assumed that the position of the central fiber core 11 will not shift due to mining stress. The positions of the dashed lines of the cylindrical sensor 1 and the six peripheral fiber cores 12 are for illustrative purposes only and do not indicate that the cylindrical sensor 1 is compressed and shrunk at all angles. In some directions, the cylindrical sensor 1 may also shrink in radial direction, and in other directions, the cylindrical sensor 1 may also increase in radial direction. Similarly, the six peripheral fiber cores 12 are not necessarily offset towards the central fiber core 11, but may also offset towards the direction away from the central fiber core 11. The figure is for illustrative purposes only and does not affect the specific reasoning and calculation process.

[0039] According to one embodiment of this application, such as Figure 3 As shown, the multi-core distributed optical fiber adopts a seven-core structure with one central core 11 and six peripheral cores 12 arranged in parallel. The seven-core structure provides rich geometric feature redundancy information, and by using optimization algorithms such as the least squares method, measurement errors can be significantly reduced and the accuracy of three-dimensional spatial morphology reconstruction can be improved.

[0040] In some implementations, the multi-core distributed optical fiber is a single-mode fiber supporting Brillouin scattering, where the Brillouin frequency shift exhibits a stable linear relationship with strain, and the strain sensitivity is calibrated and fixed, supporting attenuation-free long-distance signal transmission over kilometers. The outer cores 12 are distributed in a regular hexagonal pattern in cross-section, with an included angle of 60° between adjacent outer cores 12, ensuring comprehensive sensing of bending strain in all directions.

[0041] According to one embodiment of this application, continuous Brillouin frequency shift data of the central fiber core 11 and peripheral fiber cores 12 are collected during the sampling process, including: setting the spatial sampling interval, acquisition frequency and measurement range parameters of the Brillouin fiber demodulation system 4; using a Brillouin optical time domain analyzer to set the time step, and dynamically and cyclically collecting the fiber Brillouin frequency shift change under a long period time series throughout the entire sampling process, and synchronously collecting the Brillouin frequency shift data of all measurement points on the central fiber core 11 and all peripheral fiber cores 12 to obtain a continuous Brillouin frequency shift dataset covering the entire sensor domain.

[0042] The Brillouin fiber demodulation system 4 is the core device for converting optical signals from multi-core distributed optical fibers into strain data. The parameter setting process can be matched to the dynamic characteristics of sampling activities, ensuring that the acquisition accuracy and sampling frequency are adapted to monitoring requirements. Synchronous acquisition guarantees precise alignment of measurement points across each fiber core in the time dimension, providing a continuous Brillouin frequency shift dataset covering the entire sensor domain. This provides a time-synchronized and spatially continuous fundamental data source for subsequent three-dimensional spatial morphology reconstruction and sampling stress conversion.

[0043] In some implementations, before setting the acquisition parameters, the first and last ends of the multi-core distributed optical fiber are connected to the corresponding acquisition channels of the Brillouin fiber demodulation system 4 to complete the optical path connection and transmittance performance verification. Before officially starting the acquisition, the initial Brillouin frequency shift data of the multi-core distributed optical fiber before the acquisition activity begins is acquired as the reference value for subsequent acquisition stress calculation. During the data acquisition process, the integrity of the data in each channel is verified in real time, and invalid data with abnormal jumps are removed to ensure the validity of the Brillouin frequency shift dataset.

[0044] According to one embodiment of this application, before calculating the continuous mining stress increment data of the entire monitoring borehole 31 based on the frequency shift change, combined with spatial analytical geometry theory and the elastic mechanical parameters of the rock mass, and through three-dimensional spatial morphology reconstruction and borehole deformation mechanical model, the method further includes: using the Brillouin frequency shift data of the central fiber core 11 as a reference to eliminate the interference of ambient temperature fluctuations and the overall axial tension of the surrounding rock 3 on the strain data; calculating the local strain distribution of each peripheral fiber core 12 along the sensor substrate axis based on the linear relationship between Brillouin frequency shift and strain; establishing a rectangular coordinate system on the local cross section of the optical fiber, and based on the radial distance and azimuth angle of each peripheral fiber core 12 from the center, calculating the sum of the projections of the local strain on the two orthogonal axes of the coordinate system to construct the apparent curvature vector, and calculating the local curvature and bending direction angle of each measuring point.

[0045] This process is a fundamental preprocessing step for reconstructing the three-dimensional spatial morphology. The central fiber core 11 is unaffected by bending strain; its Brillouin frequency shift is solely caused by ambient temperature and overall axial tension, thus serving as a benchmark to eliminate interference from these two factors on the strain data. Based on the linear relationship between Brillouin frequency shift and strain, the Brillouin frequency shift changes of each peripheral fiber core 12 are converted into local strain data. Based on the local strain data of each peripheral fiber core 12 and its spatial position on the cross-section, an apparent curvature vector is constructed, and the local curvature and bending direction angle at any position along the fiber are then calculated.

[0046] In some implementations, the linear relationship between Brillouin frequency shift and strain is as follows: in The local strain of the i-th outer fiber core 12 at axial position s is... Let be the Brillouin frequency shift of the i-th outer fiber core 12 at axial position s. This represents the sensitivity coefficient of the fiber optic Brillouin frequency shift to mechanical strain. The initial reference Brillouin frequency of the optical fiber under initial strain-free conditions and at the current reference temperature.

[0047] According to one embodiment of this application, after calculating the local curvature and bending direction angle of each measuring point, the method further includes: calculating the local deflection of the corresponding measuring point based on the bending direction angle of each measuring point; introducing the Frenet-Serret differential equation system to construct a follower spatial orthogonal frame composed of tangent vector, principal normal vector, and secondary normal vector; using the borehole anchoring point of the monitoring borehole 31 as the spatial integration reference point, continuously integrating along the axial direction of the sensor substrate using a numerical integration algorithm to reconstruct the three-dimensional spatial absolute coordinate trajectory of the monitoring borehole 31 at the corresponding sampling time; and combining the data from each sampling time node to generate a four-dimensional spatial continuous evolution model of the borehole morphology as the mining progresses.

[0048] This process is the core step in reconstructing the three-dimensional spatial morphology. Based on differentiating the bending direction angle of each measuring point with respect to its axial position, the local deflection is obtained. Introducing the Frenet-Serret differential equations, a dynamic spatial orthogonal frame is constructed. Using the local curvature and deflection as input parameters, spatial integration is performed with the borehole anchor point as the reference, thus accurately reconstructing the absolute coordinate trajectory of the monitoring borehole 31 in three-dimensional space. Combining data from various sampling time points, a four-dimensional continuous evolution model of the borehole morphology during the mining process is generated. This model can intuitively recreate the overall subsidence, delamination, and fault slippage processes of the roof and floor as the working face advances.

[0049] According to one embodiment of this application, based on the frequency shift change, combined with spatial analytical geometry theory and the elastic mechanical parameters of the rock mass, the continuous mining stress increment data of the entire monitoring borehole 31 is calculated through three-dimensional spatial morphology reconstruction and borehole deformation mechanical model. This includes: extracting the actual radial displacement distribution increment caused by mining on each cross section along the depth of the monitoring borehole 31 based on the reconstructed three-dimensional spatial absolute coordinate trajectory; extracting at least three dynamic borehole relative deformation variables with specific included angles on each cross section according to the preset layout orientation of the multi-core distributed optical fiber; substituting the borehole relative deformation variables into the borehole edge stress concentration formula by combining the elastic modulus and Poisson's ratio parameters of the surrounding rock 3 to be monitored, and separating the components of the additional stress field on the cross section by solving the multivariate simultaneous equations; and finally obtaining a four-dimensional cloud map showing the continuous distribution of the maximum principal stress increment, the minimum principal stress increment, and the principal stress azimuth angle along the entire length of the deep borehole by traversing the solution along the deep borehole longitudinally and combining it with the time series, thereby realizing the visualization and tracking of the advance influence range of mining stress and the forward shift law of stress peak.

[0050] This process is the core step in calculating mining-induced stress. From the reconstructed three-dimensional absolute coordinate trajectory, the radial displacement distribution increments at each depth cross-section are extracted, resulting in relative aperture deformations in multiple directions. These aperture deformations are substituted into the stress concentration formula at the borehole edge in elasticity, establishing a system of multivariate simultaneous equations. Solving this system yields the three-dimensional mining-induced stress components and principal stress increments at that cross-section, including the maximum and minimum principal stress increments and their azimuths. By traversing the borehole longitudinally and combining the solution with a time series, a four-dimensional cloud map showing the continuous distribution of mining-induced stress along the entire length of the borehole is finally obtained, enabling the visualization and tracking of the leading influence range of mining-induced stress and the forward shift of stress peaks.

[0051] In some implementations, the elastic modulus and Poisson's ratio of the rock mass can be obtained through laboratory confining pressure calibration tests on rock samples from the same batch collected on-site. The calculated three-dimensional mining-induced stress additional field components and principal stress increments, including the magnitude and direction of the three principal stresses, can comprehensively reflect the stress state of the surrounding rock 3.

[0052] According to one embodiment of this application, after calculating the three-dimensional mining stress additional field components and principal stress increment values ​​of each measuring point, the method further includes: associating the calculated three-dimensional mining stress additional field components and principal stress increment values ​​of each measuring point with the three-dimensional spatial absolute coordinates of the corresponding measuring point to generate a continuous mining stress data set for the entire monitoring borehole 31; based on the continuous mining stress data set, generating an axial continuous mining stress profile curve and a three-dimensional spatial mining stress cloud map of the monitoring borehole 31; the profile curve and cloud map are used for stability analysis and safety assessment of mining operations.

[0053] The one-to-one correlation between the additional components of the three-dimensional mining stress field and the principal stress increments with the absolute coordinates in three-dimensional space allows each set of stress data to be precisely mapped to the actual physical location of the monitoring area, forming a complete continuous stress dataset. The axial continuous mining stress profile curve and three-dimensional mining stress cloud map generated based on the dataset can transform abstract numerical data into an intuitive presentation of stress distribution, accurately capturing local stress concentration areas and providing intuitive and accurate results support for the stability analysis and safety assessment of mining projects.

[0054] In some implementations, before associating coordinates with stress values, the correspondence between the spatial coordinates of each measuring point and the actual engineering coordinates of the monitoring borehole 31 is clarified to ensure accurate matching between stress data and actual on-site locations. After generating the continuous mining stress data set, the integrity of the data set is verified, and invalid data sets with duplicate coordinates or abnormal stress values ​​are removed. When generating profile curves and contour maps, the location and values ​​of stress concentration areas are marked, providing intuitive key points for engineering stability analysis.

[0055] For example, the continuous mining stress measurement method based on multi-core distributed optical fiber of this application achieves multi-dimensional, continuous, and long-period four-dimensional spatiotemporal dynamic monitoring of the mining stress field in underground engineering through the deep integration of multi-core optical fiber three-dimensional shape reconstruction technology and borehole diameter follow-deformation principle: Sensor probe implantation without removal and initial background field calibration: A monitoring borehole is drilled in the target coal and rock mass ahead of the excavation face. A multi-core fiber optic sensing probe containing a multi-core distributed optical fiber is pushed into the full length of the borehole, and high-strength grouting material 2 is used to permanently consolidate and couple it with the borehole wall along the entire length. At the initial moment before the excavation disturbance reaches this area, the initial Brillouin frequency shift reference data of the central fiber core 11 and all peripheral fiber cores 12 are collected using a Brillouin optical time domain analyzer. This state is calibrated as the initial background state of the relative zero stress field for mining stress monitoring, and this reference data is used as the zero point for subsequent measurements.

[0056] Dynamic monitoring of full-hole coordinated deformation under excavation disturbance: As the working face advances or excavation continues, the original rock stress balance is disrupted, and the coal and rock mass ahead is subjected to additional mining support pressure, resulting in severe continuous compression and displacement deformation. Due to its tight consolidation with the surrounding rock 3, the multi-core fiber optic sensing probe undergoes radial multi-directional diameter reduction and axial three-dimensional spatial bending synchronously with the borehole wall. Using a Brillouin optical time-domain analyzer to set the time step, dynamic cyclic acquisition of the fiber optic Brillouin frequency shift variation under long-period time series is performed.

[0057] Temporal local geometric features and apparent curvature vector reconstruction: For the Brillouin frequency shift data of each fiber core along the longitudinal arc length acquired at any sampling time in a multi-core distributed optical fiber, the frequency shift change of the central fiber core 11 located at the geometric center of the probe is extracted as a benchmark. The frequency shift changes of each peripheral fiber core 12 are subtracted from this benchmark value to eliminate interference caused by temperature fluctuations within the coal seam and the overall axial migration and stretching of the surrounding rock 3. The local strain distribution of each peripheral fiber core 12 is calculated based on the linear relationship between Brillouin frequency shift and strain. A rectangular coordinate system is established on the local cross-section of the optical fiber. Based on the radial distance and azimuth angle of each peripheral fiber core 12 from the center, the sum of the projections of the local strain onto the two orthogonal axes of this coordinate system is calculated to construct the apparent curvature vector. Based on this vector, the transient bending direction angle and transient local curvature at any position along the fiber at this moment are calculated.

[0058] Reconstruction of four-dimensional spatial morphological evolution based on the Frenet-Serret formula: The transient bending direction angle, which changes continuously along the fiber optic cable, is extracted and its derivative is used to obtain the local transient torsion. The Frenet-Serret differential equations of spatial analytic geometry are introduced to construct a dynamic spatial orthogonal frame on the fiber optic curve, consisting of tangent vectors, principal normal vectors, and secondary normal vectors. Using the transient local curvature and local torsion as input parameters, and setting the deep borehole anchorage point as the spatial integration reference point, a numerical integration algorithm is used to continuously integrate along the fiber arc length direction, accurately reconstructing the absolute coordinate trajectory of the monitored borehole 31 in the underground three-dimensional space at the corresponding sampling time. Combining the data from each sampling time node, a four-dimensional spatial continuous evolution model of the borehole morphology during the mining process is generated.

[0059] Derivation of a continuous profile of time-series mining stress increments based on mechanical principles: Based on the reconstructed temporal three-dimensional spatial morphological evolution model, the actual radial displacement distribution increments caused by mining are extracted from each horizontal cross-section along the depth of the monitoring borehole 31. Combined with the preset deployment orientation of the multi-core distributed optical fibers within the probe, dynamic aperture relative deformation increments in at least three directions with specific included angles are extracted from each cross-section. The elastic modulus and Poisson's ratio parameters of the target coal and rock mass, measured in the laboratory, are obtained. The continuously extracted aperture deformation increments in multiple directions are substituted into the borehole edge stress concentration formula, and a matrix solution is performed using the established multivariate simultaneous equations to separate the components of the additional stress field on that cross-section. By traversing the longitudinal direction of the deep borehole and combining the solution with the time series, a four-dimensional cloud map showing the continuous distribution of the maximum principal stress increment, minimum principal stress increment, and principal stress azimuth angles along the entire length of the deep borehole is finally obtained, enabling the visualization and tracking of the advance influence range of mining stress and the forward shift of stress peaks.

[0060] This method eliminates the need for complex hole removal operations. Through a single full-length grouting injection, it utilizes hundreds of meters of optical fiber to continuously capture minute changes in the pre-support pressure, completely eliminating monitoring blind spots caused by discrete hole stress gauge arrangements. Simultaneously, the four-dimensional morphological evolution model reconstructed based on Frenet-Serret calculus theory can intuitively recreate the overall subsidence, delamination, and fault slippage processes of the roof and floor as the working face advances. This provides both geometric and mechanical criteria for early warning of coal and rock dynamic disasters, significantly improving the comprehensiveness and reliability of mining engineering safety monitoring.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate this application and are not intended to limit this application. Although this application has been described in detail with reference to the embodiments, those skilled in the art should understand that various combinations, modifications, or equivalent substitutions of the technical solutions of this application do not depart from the spirit and scope of the technical solutions of this application and should be covered within the scope of the claims of this application.

Claims

1. A method for continuous sampling stress measurement based on multi-core distributed optical fiber, characterized in that, Includes the following steps: Drill cylindrical monitoring boreholes in the monitoring area affected by mining activities; A flexible cylindrical sensor is implanted into the monitoring borehole. The sensor substrate of the flexible cylindrical sensor contains a multi-core distributed optical fiber. The multi-core distributed optical fiber includes a central core located on the central axis of the sensor substrate and multiple peripheral cores distributed around the central axis. Both the central core and the peripheral cores extend axially along the sensor substrate and are parallel to each other. A coupling and consolidation layer is provided between the flexible cylindrical sensor and the borehole wall, forming a consolidated coupling body between the flexible cylindrical sensor and the surrounding rock of the monitoring borehole. The end of the multi-core distributed optical fiber is connected to the Brillouin fiber demodulation system. At the initial moment before the arrival of the disturbance, the initial Brillouin frequency shift data of the central fiber core and the peripheral fiber core are collected as a relative zero reference. During the sampling process, continuous Brillouin frequency shift data of the central fiber core and the peripheral fiber core are collected, and the frequency shift change relative to the relative zero reference is extracted. Based on the frequency shift change, combined with spatial analytical geometry theory and the elastic mechanical parameters of the rock mass, the continuous mining stress increment data of the entire monitoring borehole area is calculated through three-dimensional spatial morphology reconstruction and borehole deformation mechanical model.

2. The continuous sampling stress measurement method based on multi-core distributed optical fiber according to claim 1, characterized in that, Drilling cylindrical monitoring boreholes in the monitoring area affected by mining activities includes: According to the preset monitoring plan, monitoring boreholes are drilled in the top, bottom, side or target monitoring body of the surrounding rock in the area to be monitored, along the vertical, horizontal or inclined direction. After the monitoring borehole is completed, high-pressure air or clean water is used to remove rock powder and loose debris from the borehole. The inner diameter of the monitoring borehole is 5mm to 20mm larger than the outer diameter of the sensor, and the depth of the monitoring borehole covers the mining impact range of the area to be monitored.

3. The continuous sampling stress measurement method based on multi-core distributed optical fiber according to claim 1, characterized in that, Prior to implanting the flexible cylindrical sensor into the monitoring borehole, the method further includes: A flexible cylindrical substrate adapted to the monitoring borehole was selected as the sensor substrate, and the substrate material has deformation synergy that matches the surrounding rock to be monitored. The multi-core distributed optical fiber is extended along the axial direction of the sensor substrate and arranged parallel to each other. The multi-core distributed optical fiber is then integrated with the sensor substrate through injection molding. The two ends of the sensor substrate are sealed and waterproofed to form a multi-core fiber optic sensing probe with full-range aperture deformation sensing capability.

4. The continuous sampling stress measurement method based on multi-core distributed optical fiber according to claim 3, characterized in that, The deployment structure of the multi-core distributed optical fiber includes: A multi-core distributed optical fiber consists of one central core and at least three peripheral cores; The peripheral fiber cores are distributed at equal angular intervals along the circumference with the central fiber core as the center, and the axes of both the central fiber core and the peripheral fiber cores are parallel to the axis of the sensor substrate.

5. The continuous sampling stress measurement method based on multi-core distributed optical fiber according to claim 4, characterized in that, The multi-core distributed optical fiber adopts a seven-core structure with one central core and six peripheral cores arranged in parallel.

6. The continuous sampling stress measurement method based on multi-core distributed optical fiber according to claim 1, characterized in that, The continuous Brillouin frequency shift data of the central fiber core and the peripheral fiber core during the acquisition process includes: Set the spatial sampling interval, acquisition frequency, and measurement range parameters of the Brillouin fiber demodulation system; Throughout the entire sampling process, the Brillouin frequency shift data of all measuring points on the central fiber core and all peripheral fiber cores are simultaneously collected through the demodulation system to obtain a continuous Brillouin frequency shift dataset covering the entire sensor domain.

7. The continuous sampling stress measurement method based on multi-core distributed optical fiber according to claim 6, characterized in that, Before calculating the continuous mining stress increment data of the entire monitoring borehole area based on the frequency shift change, combined with spatial analytical geometry theory and the elastic mechanical parameters of the rock mass, through three-dimensional spatial morphology reconstruction and borehole deformation mechanical model, the following steps are also included: Using the Brillouin frequency shift data of the central fiber core as a reference, the interference of ambient temperature fluctuations and the overall axial tension of the surrounding rock on the strain data is eliminated; Based on the linear relationship between Brillouin frequency shift and strain, the local strain distribution of each of the peripheral fiber cores along the axial direction of the sensor substrate is calculated; Based on the local strain distribution and spatial arrangement of each peripheral fiber core, an apparent curvature vector is constructed, and the local curvature and bending direction angle of each measuring point are calculated.

8. The continuous sampling stress measurement method based on multi-core distributed optical fiber according to claim 7, characterized in that, After calculating the local curvature and bending direction angle of each measuring point, the method further includes: Calculate the local deflection of the corresponding measuring point based on the bending direction angle of each measuring point; We introduce the Frenet-Serret differential equation system and construct a kinematic space orthogonal frame composed of tangent vector, principal normal vector and binormal vector; Using the borehole anchor point as the spatial integration reference point, continuous integration is performed along the axial direction of the sensor substrate to reconstruct the three-dimensional absolute spatial coordinate trajectory of the borehole at the corresponding sampling time.

9. The continuous sampling stress measurement method based on multi-core distributed optical fiber according to claim 8, characterized in that, Based on the frequency shift change, combined with spatial analytical geometry theory and the elastic mechanical parameters of the rock mass, the continuous mining stress increment data of the entire monitoring borehole area is calculated through three-dimensional spatial morphology reconstruction and borehole deformation mechanical model, including: Based on the reconstructed three-dimensional absolute coordinate trajectory, the radial displacement distribution increments on each cross section along the depth of the monitored borehole are extracted; Based on the preset deployment orientation of the multi-core distributed optical fiber, extract at least three dynamic aperture relative deformations with specific included angles on each cross section; By combining the elastic modulus and Poisson's ratio parameters of the surrounding rock to be monitored, the relative deformation of the borehole diameter is substituted into the stress concentration formula at the borehole edge. By solving the multivariate simultaneous equations, the three-dimensional mining stress additional field components and principal stress increment values ​​of each measuring point are obtained.

10. The continuous sampling stress measurement method based on multi-core distributed optical fiber according to claim 9, characterized in that, After calculating the three-dimensional mining-induced stress additional field components and principal stress increments at each measuring point, the method further includes: The three-dimensional mining stress additional field components and principal stress increment values ​​obtained from each measuring point are correlated one by one with the three-dimensional spatial absolute coordinates of the corresponding measuring points to generate a continuous mining stress data set for monitoring the entire borehole area. Based on the continuous mining stress data set, an axial continuous mining stress profile curve and a three-dimensional spatial mining stress cloud map are generated for the monitoring borehole; the profile curve and cloud map are used for stability analysis and safety assessment of mining operations.

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