A frozen depth monitoring system based on distributed acoustic sensing technology

Abstract

The application relates to the technical field of frozen soil monitoring, and discloses a frozen depth monitoring system based on distributed acoustic sensing technology, which comprises a distributed optical fiber acoustic sensing device, a sensing optical cable arranged in a to-be-measured area, a calibration module and a data processing module, and realizes frozen depth monitoring of a to-be-measured seasonal frozen soil area through'surface wave inversion + borehole calibration'. The sensing optical cable is arranged in a horizontal surface and vertical borehole segmented combination mode, and a 'fishbone type' optical cable distribution network is formed; the horizontal section optical cable collects environmental noise in a large range and carries out surface wave dispersion inversion to obtain a rough frozen layer thickness; a sleeve is arranged on the vertical arrangement section, and the optical cable is spirally wound in the center of the sleeve, so that the sleeve becomes a vertical sensing array, and the vertical section high-resolution vibration signal is used to accurately calibrate a frozen interface. The application realizes full-link frozen depth monitoring from 'large-scale general survey' to 'accurate point calibration' by using the advantages of DAS technology and combining an innovative arrangement mode.

Description

Technical Field

[0001] This invention belongs to the field of geophysical monitoring and permafrost engineering, and specifically relates to a freezing depth monitoring system based on distributed acoustic wave sensing technology. Background Technology

[0002] The freezing depth of seasonally frozen soil is a key parameter for climate change research, infrastructure construction, agricultural production, and geological disaster early warning. Traditional detection methods, such as core drilling and the installation of permafrost detectors, are inefficient, damage the surface, and are difficult to implement for large-scale continuous monitoring.

[0003] Distributed Acoustic Sensing (DAS) is a groundbreaking technology that has emerged in the field of fiber optic sensing in recent years. Essentially, it is a distributed vibration measurement technique based on the phase modulation of coherent Rayleigh scattering light. Its core principle lies in the fact that when sound waves or vibrations act on a sensing optical cable, they change the local refractive index and physical length of the fiber. This perturbation modulates the back-propagating Rayleigh scattering light in the fiber, causing a phase change. By detecting and demodulating these phase changes and utilizing the optical time-domain reflectometry (OTDR) positioning principle, the DAS system can transform an entire ordinary communication optical cable into a continuously distributed, highly sensitive array, thereby achieving continuous, real-time, long-distance monitoring and precise positioning of sound wave and vibration events.

[0004] The unique advantage of the DAS system lies in its ability to achieve truly fully distributed measurement. Unlike traditional point-based or quasi-distributed sensors, DAS does not require pre-fabricated sensing elements (such as gratings) on optical fibers; a single fiber can achieve dense sampling with one sensing point per meter over a range of several kilometers to hundreds of kilometers. This makes it extremely suitable for applications such as security monitoring of large-scale infrastructure (e.g., pipelines, perimeters, railways), seismic wavefield imaging, and urban underground space exploration. In recent years, geophysical methods, such as surface wave dispersion curve inversion based on environmental background noise, have been used to estimate the velocity structure of permafrost. This method has the advantages of being non-destructive and covering a large area. However, its inversion results have multiple solutions and limited depth resolution, making it difficult to accurately capture the abrupt changes in the frozen interface and accurately determine the frozen interface.

[0005] Distributed acoustic sensing (DAS) technology, due to its advantages of high density, long distance, and continuous measurement, has been applied to seismic monitoring. The purpose of this invention is to leverage the advantages of DAS and combine it with innovative deployment methods to achieve end-to-end frozen depth detection, from large-scale surveys to precise point calibration. This invention has broad application prospects in various fields, including research on global warming and permafrost degradation, construction and operation maintenance of railways / highways in high-altitude and cold regions, and early warning of road bulging / collapse / instability disasters caused by frost heave and thaw settlement. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a high-precision, wide-coverage freeze depth monitoring system based on distributed acoustic sensing (DAS) technology. Through specific physical layout of horizontal and vertical segments, as well as the physical entity of a calibration module, it achieves the acquisition of large-scale surface wave signals and high-precision vertical array signals.

[0007] To achieve the above objectives, the present invention discloses a freeze depth monitoring system based on distributed acoustic wave sensing technology, comprising: a distributed acoustic wave sensing device (DAS), a sensing optical cable deployed in the area to be measured, a calibration module, and a data processing module.

[0008] The distributed acoustic wave sensing device (DAS) is connected to the sensing optical cable and is used to collect vibration signals of the area to be measured through the optical cable.

[0009] The sensing optical cable includes a horizontal trunk optical cable laid on the ground and at least one branch optical cable leading from the horizontal trunk optical cable; the end of the branch optical cable is connected to the calibration module, which is installed in a vertical borehole.

[0010] The data processing module is connected to the distributed acoustic wave sensing device (DAS) via a hardware interface, forming a data path for signal reception and processing.

[0011] Furthermore, the horizontal backbone optical cables are buried in shallow trenches dug manually, and their burial depth remains consistent.

[0012] Furthermore, the depth of the vertical borehole is greater than the estimated maximum seasonal freezing depth of the area to be tested, and the space below the calibration module in the vertical borehole is filled with backfill soil.

[0013] Furthermore, the calibration module includes a sheath and a spiral sensing optical cable disposed inside the sheath, the sheath being filled with water.

[0014] Furthermore, the sheath is a cylindrical tube, and the spiral sensing optical cable is suspended at the central axis of the tube.

[0015] Furthermore, the branch optical cable is connected to the calibration module via a quick-pluggable fiber optic connector.

[0016] Furthermore, the horizontal trunk optical cable and multiple branch optical cables together form a "fishbone" optical cable distribution network. The trunk optical cable extends linearly along the area to be tested, and the branch optical cables are connected to each vertical borehole at a preset interval.

[0017] This invention also discloses a method for monitoring freezing depth based on distributed acoustic wave sensing technology, comprising the following steps:

[0018] S1, Deploy sensing optical cables: Deploy sensing optical cables in the area to be measured to form a "fishbone" optical cable distribution network including horizontal surface deployment sections and underground vertical deployment sections. The underground vertical deployment sections are buried through boreholes and integrate calibration modules.

[0019] S2, collect vibration signals: connect the sensor optical cable through the distributed acoustic wave sensor (DAS) to collect environmental noise signals of the horizontal surface deployment section and borehole vibration signals of the underground vertical deployment section.

[0020] S3, Processing horizontal segment signals: Perform surface wave dispersion inversion on the signals collected from the horizontally deployed segment of the ground to generate a preliminary freezing depth model;

[0021] S4, Processing vertical segment signals: Analyze the signals collected from the underground vertical deployment segment, and accurately locate the depth of the frozen interface by identifying the amplitude attenuation or frequency change characteristics of the vibration signal.

[0022] S5, Data Calibration and Output: Using the frozen interface depth obtained in S4 as the calibration point, the preliminary frozen depth model is corrected using geostatistical methods, and the final frozen depth distribution map is output.

[0023] Furthermore, in step S1, the "fishbone" optical cable distribution network is traversed through the test area by a main optical cable, and multiple branch optical cables are branched from the main optical cable to connect to the vertical borehole. The end of the branch optical cable is integrated with a prefabricated calibration module. The calibration module includes a prefabricated spiral sensing optical cable, which is built into a water-filled sleeve to form a vertical sensing array. During deployment, the module is placed into the borehole and connected to the branch optical cable to achieve plug-and-play functionality.

[0024] Furthermore, in step S3, the surface wave dispersion inversion includes background noise cross-correlation calculation and dispersion curve extraction to invert the subsurface shear wave velocity structure and identify the velocity abrupt change layer as a frozen layer.

[0025] Furthermore, in step S4, the analysis of the vertical segment signal includes calculating the amplitude spectrum or frequency components at different depth points on the vertical sensing array, and identifying the frozen interface through abrupt change points.

[0026] Beneficial effects

[0027] This invention discloses a freeze depth monitoring system based on distributed acoustic sensing (DAS) technology. It collects environmental noise signals for surface wave inversion via horizontally deployed sensing optical cables and high-resolution vibration signals containing abrupt changes in the frozen interface via vertical boreholes. Leveraging the advantages of DAS and combining it with an innovative sensing optical cable deployment method, it achieves end-to-end freeze depth monitoring, from large-scale surveys to precise point calibration. The system directly senses the physical properties of natural permafrost, solving the problems of indirect measurement distortion and insufficient spatial resolution in traditional methods. It enables distributed, large-scale freeze depth monitoring, offering extremely high cost-effectiveness and broad application prospects. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of a method for monitoring freezing depth based on distributed acoustic sensing (DAS) technology in an embodiment of the present invention;

[0029] Figure 2 This is a schematic diagram of a "fishbone-shaped" sensor optical cable network deployed in the area to be tested, as an embodiment of the invention.

[0030] Figure 3 This is a longitudinal sectional view of the main optical cable within the area to be tested, as described in an embodiment of the invention.

[0031] Figure 4 This is a longitudinal sectional view of the borehole within the test area, as described in an embodiment of the invention.

[0032] Figure 5 This is a schematic diagram of the borehole sensing module in an embodiment of the invention.

[0033] Among them: 1—main optical cable, 101—backfill soil in artificial trench, 2—branch optical cable, 3—vertical borehole, 301—backfill soil in vertical borehole, 4—calibration module, 401—spiral sensor optical cable, 402—water, 403—sheath. Detailed Implementation

[0034] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0035] Example 1

[0036] This embodiment discloses a freezing depth monitoring system based on distributed acoustic wave sensing technology, including: a distributed acoustic wave sensing device (DAS), a sensing optical cable deployed in the area to be measured, a calibration module, and a data processing module. The DAS is connected to the sensing optical cable and is used to collect vibration signals from the area to be measured via the optical cable. The sensing optical cable adopts a combination of horizontal surface deployment sections and underground vertical deployment sections, forming a "fishbone" optical cable distribution network. The horizontal surface deployment sections are used to collect environmental noise signals over a large area, while the underground vertical deployment sections integrate the calibration module to provide freezing interface calibration signals. The data processing module is communicatively connected to the DAS and is configured to: process the signals collected by the horizontal surface deployment sections, obtain a preliminary freezing depth model through surface wave dispersion inversion; process the signals collected by the underground vertical deployment sections, accurately calibrate the freezing interface by analyzing vibration characteristics; and use the calibration results to correct the preliminary model, outputting a high-precision freezing depth distribution map.

[0037] In this embodiment, as Figure 2 As shown, the "fishbone" optical cable distribution network consists of a main optical cable 1 running through the entire survey area, and multiple branch optical cables 2 branching off from the main optical cable 1. The ends of the branch cables are connected to vertical boreholes 3. The spacing and position of the vertical boreholes 3 can be flexibly adjusted according to the spatial variability of geological conditions, and the failure of a single vertical point does not affect the normal operation of the main cable and other points. A calibration module 4 is installed inside the vertical boreholes 3.

[0038] In the deployment methods of sensing optical cables, such as Figure 3 As shown, the horizontal deployment section is laid linearly on the ground surface, with the main optical cable 1 buried in the backfill soil 101 within the artificial trench, maintaining a consistent burial depth, for large-scale acquisition of surface environmental vibration signals; the vertical deployment section is buried underground by drilling, with a vertical borehole 3 arranged at regular intervals according to the characteristics of the area to be measured, and the borehole depth is greater than the local estimated maximum seasonal freezing depth, for acquisition of vibration signals within small-scale boreholes to achieve freezing interface calibration.

[0039] like Figure 4 As shown, the vertical borehole 3 forms the main body, housing the calibration module 4 and the backfill soil 301 within the vertical borehole. Figure 5As shown, the calibration module is a prefabricated standardized unit, its core component being a factory-prefabricated helical sensing optical cable 401. The helical sensing optical cable 401 is a standard helical structure prefabricated in the factory, making the optical fiber a high-resolution vertical sensing array. The helical sensing optical cable is embedded in the center of a sheath 403, which is filled with water 402, forming a standardized vertical sensing array. Backfill soil 301 in the vertical borehole fills the gap between the calibration module and the borehole wall, ensuring that vibration signals propagate between the surrounding soil and the sensing optical cable in the calibration module. The calibration module features a "plug-and-play" design. During on-site installation, simply insert the module into the drill hole and connect its two ends to the corresponding interfaces of the main optical cable. This avoids problems such as inconsistent quality, low efficiency, and unstable losses caused by manual winding on-site. Factory production allows for precise control of pitch, bending radius, and loss, ensuring the consistency, high resolution, and reliability of each calibration module. It enables rapid deployment and replacement, significantly shortening on-site installation time. Furthermore, the "modular" design allows for quick replacement after damage, facilitating subsequent system maintenance.

[0040] When implementing this system, the first step is to plan the survey area and determine the scope of the area to be measured. Based on geological survey data, a main line route traversing the survey area is planned, and a vertical borehole calibration point is determined every 50m along the main line. A shallow trench approximately 30cm deep is excavated along the planned route, the main optical cable is buried in it, backfilled and compacted, and the route is marked. At each vertical borehole calibration point, a vertical hole 3.5m deep is drilled (ensuring the depth is greater than the maximum freezing depth). After cleaning the borehole, the prefabricated vertical sensing module is slowly placed into the hole, ensuring the module remains vertical. Water is slowly injected into the casing through the water inlet on the top of the module until it is completely filled, ensuring effective acoustic coupling between the sensing fiber and the surrounding medium. The fiber optic connector on the top of the module is fused to the "branch" optical cable (using a lightweight branch cable) leading from the main optical cable, and waterproof protection is ensured. Finally, the borehole gaps are backfilled with fine sand to avoid large voids affecting the signal. Connect the end of the entire "fishbone" optical cable network to the host of the DAS equipment and fused all the connection points.

[0041] Once the equipment is powered on, it continuously acquires vibration signals along the entire sensing optical cable using DAS. The acquired raw signals are then processed by removing the mean, detrending, and bandpass filtering to eliminate instrument noise and low-frequency interference.

[0042] For the surface horizontal section data of passage A: the environmental noise data collected by the main optical cable of the surface horizontal section is extracted separately, the background noise cross-correlation algorithm is used to calculate the dispersion curve and invert the underground shear wave velocity structure. The layer where the velocity suddenly increases is identified as the frozen layer, and its top and bottom depths are the initial freezing depths. The initial freezing depth distribution model of the entire survey area is generated by inversion.

[0043] For precise calibration of the B-channel vertical array: extract vibration signals acquired by individual vertical borehole sensing modules. Analyze the propagation characteristics of vibrations generated by environmental noise (such as surface microseismic activity) on the vertical array. By calculating the amplitude spectrum or frequency components of signals at different depth points, the depth of the abrupt change in amplitude or center frequency is identified as the freezing interface. Repeat this process for each borehole to obtain multiple high-precision freezing depth calibration point data.

[0044] Furthermore, the depth data from multiple precise calibration points obtained through pathway B are used as "ground truth" control points. Geostatistical methods are employed to correct and constrain the preliminary freezing depth model generated by pathway A, effectively overcoming the limitations of surface wave inversion in terms of ambiguity and insufficient depth resolution.

[0045] The final result is a high-precision spatial distribution map of the freezing depth in the survey area, calibrated in the field. This map visually displays the spatial variation trend of freezing depth and can directly output the freezing depth value at any location. By repeating the above data acquisition and processing steps, a dynamic curve of freezing depth changing over time can be generated, enabling real-time and automated monitoring of the entire freeze-thaw process.

[0046] Example 2

[0047] This embodiment discloses a method for monitoring freezing depth based on distributed acoustic wave sensing technology, such as... Figure 1 As shown, the process comprises three stages. The first stage involves system design and deployment, designing a "fishbone" sensor network based on requirements, determining the path of the main optical cable and the distribution of vertical borehole locations. The second stage is data acquisition and signal processing, where the system acquires surface and borehole vibration signals through the deployed network. This includes two paths: surface signal processing and borehole signal processing. Surface signal processing extracts surface environmental noise, processes the raw signals acquired by the horizontal main optical cable, extracts the dispersion characteristics of surface waves from the noise, and inverts the shear wave velocity of the subsurface medium based on the dispersion curve. Significant changes in wave velocity usually correspond to the frozen interface. Finally, a frozen depth distribution model covering the entire survey area, but with relatively low resolution, is formed. Specifically,

[0048] S1, Deploy sensor optical cables: Deploy sensor optical cables in the area to be measured to form a "fishbone" optical cable distribution network that includes horizontal surface deployment sections and underground vertical deployment sections. The underground vertical deployment sections are buried through boreholes and integrate calibration modules.

[0049] S2, Vibration signal acquisition: The distributed acoustic wave sensor (DAS) is connected to the sensing optical cable to acquire environmental noise signals in the horizontally deployed section and borehole vibration signals in the vertically deployed section underground.

[0050] S3-S4, processing horizontal and vertical segment signals: surface wave dispersion inversion is performed on the signals collected from the horizontal surface segment to generate a preliminary freezing depth model; the signals collected from the underground vertical segment are analyzed, and the freezing interface depth is accurately located by identifying the amplitude attenuation or frequency change characteristics of the vibration signal.

[0051] S5, Data Calibration and Output: Using the depth data of multiple precise calibration points obtained in S4 as "ground truth" control points, the preliminary freezing depth model is corrected and constrained using geostatistical methods, effectively overcoming the defects of multiple solutions and insufficient depth resolution in surface wave inversion, and outputting the final freezing depth distribution map.

[0052] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A freeze depth monitoring system based on distributed acoustic sensing technology, characterized in that, include: Distributed acoustic wave sensing device (DAS), sensing optical cable deployed in the area to be tested, calibration module (4) and data processing module; The distributed acoustic wave sensing device (DAS) is connected to the sensing optical cable and is used to collect vibration signals of the area to be measured through the optical cable. The sensing optical cable includes a horizontal trunk optical cable (1) laid on the ground and at least one branch optical cable (2) extending from the horizontal trunk optical cable (1); the end of the branch optical cable (2) is connected to the calibration module (4), and the calibration module (4) is disposed in a vertical borehole (3); The data processing module is connected to the distributed acoustic wave sensing device (DAS) via a hardware interface, forming a data path for signal reception and processing.

2. The system according to claim 1, characterized in that, The horizontal trunk optical cable (1) is buried in a shallow trench dug by hand, and its burial depth remains consistent.

3. The system according to claim 1, characterized in that, The depth of the vertical borehole (3) is greater than the estimated maximum seasonal freezing depth of the area to be tested, and the space below the calibration module (4) in the vertical borehole (3) is filled with backfill soil.

4. The system according to any one of claims 1-3, characterized in that, The calibration module (4) includes a sheath (403) and a spiral sensing optical cable (401) disposed inside the sheath (403), the sheath (403) being filled with water (402).

5. The system according to claim 4, characterized in that, The sheath (403) is a cylindrical tube, and the spiral sensing optical cable (401) is suspended at the central axis of the tube.

6. The system according to claim 1, characterized in that, The branch optical cable (2) is connected to the calibration module (4) via a quick-plug optical fiber connector.

7. The system according to claim 1, characterized in that, The horizontal trunk optical cable (1) and the multiple branch optical cables (2) together form a "fishbone" optical cable distribution network. The trunk optical cable (1) extends linearly along the area to be tested, and the branch optical cables (2) are connected to each vertical borehole at a preset interval.

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