Distributed fiber optic strain and temperature sensors underground in coal mines BOTDR

Sensore di temperatura in fibra ottica, Sistema di monitoraghju intelligente, U fabricatore di fibra ottica distribuitu in Cina

The BOTDR sensing system has the following advantages:

1) It can simultaneously detect temperature and stress;

2) High measurement sensitivity, temperature of 0.2oC, stress of 4 μ ε; three

3) The detection range is far, up to 100 kilometers, and the spatial resolution reaches 5 metri;

4) Low cost.

After excavation of the tunnel, the deformation and failure of the surrounding rock often lead to tunnel failure or collapse. Conventional monitoring techniques, such as extensometers, stress gauges, convergence stations, ecc., can only detect stress or strain data in shallow surrounding rocks, requiring a large amount of manual operation. In più, in the above-mentioned monitoring techniques, the monitoring instruments are installed after the excavation surface, so they cannot detect the strain and deformation that occur before the excavation. In order to overcome these shortcomings, a new type of rock deformation control and monitoring system based on Brillouin optical time-domain reflectometer has been developed. Compared with conventional monitoring systems, this system provides a reliable, accurate, and real-time monitoring method for controlling the deformation of surrounding rocks in wide and elongated tunnels. Installing fiber optic sensors in the drilling holes in front of the excavation face can effectively protect the sensors and study the deformation characteristics of the surrounding rock. The system has been applied in the TBM excavation tunnel of Zhangji Coal Mine. Accurately detecting the deformation behavior of surrounding rock, the monitoring results provide necessary reference basis for the control of surrounding rock deformation.

In the past 20 anni, with the depletion of shallow coal resources, coal mining activities have continuously shifted towards deeper layers. In China, approximately 60% of coal mines are mined at depths of over 800 metri. Deep mining faces challenges from high ground stress and complex geological conditions. These emerging problems have led to significant deformation, damage, and tunnel collapse of the surrounding rock, posing a serious threat to the safety of miners and limiting coal production. Tunnel collapse accidents account for 80% of the total number of coal mine accidents, resulting in 43% of miners dying. Traditional shallow tunnel monitoring techniques, such as extensometers, stress gauges, convergence stations, ecc., due to their low accuracy and excessive manual operation, can no longer meet the monitoring requirements of deep strata.

To solve the problem of deformation monitoring of surrounding rocks in deep coal mines, many emerging measurement technologies have been developed in underground coal mining faces and tunnel excavation. Zhao et al. Using microseismic technology to monitor the damage process of surrounding rock in tunnels. Zhao et al. A displacement monitoring method for overlying strata in coal seams based on fiber optic grating displacement sensors has been proposed. Kajzar et al. applied 3D laser technology to monitor coal pillar deformation and roof in underground tunnels. Yu et al. The deformation of surrounding rocks and convergence of tunnels were studied using a laser rangefinder. Martino and Chandler studied the deformation and damage zone evolution behavior of surrounding rock using borehole camera images [9]. Bl ü ling et al. proposed the long-term process of rock damage using microfocal X-ray tomography. Lubosik et al. proposed a technique for measuring axial force and rock displacement of anchor rods using instrumented anchor rods embedded with strain gauges and tensor sensors. Liu et al. The proposed transient electromagnetic method (TEM) is used to detect the range and deformation of the surrounding rock damage zone. Erich studied the collapse characteristics of coal mine tunnels using seismic reflection method.

Despite some progress in monitoring technology, the above-mentioned monitoring methods still have shortcomings in certain aspects. Microseismic technology and transient electromagnetic and seismic reflection methods can detect the development of fractures in surrounding rocks, but the monitoring accuracy of rock displacement is not high (up to meters). Microfocal X-ray tomography can only measure damage in rock samples and cannot be used for on-site monitoring. Compared with fully distributed fiber optic sensing systems, fiber optic grating systems require too many sensors and have higher costs. In più, most commercially available interrogators can only handle a considerable amount of FBG, set limits on the number of sensing points, and the density along the fiber optic. Drilling camera images can detect damage and fractures within the surrounding rock, but real-time monitoring cannot be achieved, and image analysis relies on manual operation. Due to the limitation of anchor rod length (usually less than 2.5 μ m), instrument anchor rods can only be used to measure the stress and strain in the shallow part of the surrounding rock. 3D laser technology provides a high-precision instrument for tunnel convergence, and the deformation and damage inside the tunnel cannot be measured.

Brillouin optical time-domain reflectometer (BOTDR) is a fully distributed sensing technology used for measuring strain and temperature along all determined regions, where only one fiber is stimulated by a laser pulse, so many discrete sensors can be replaced. BOTDR provides fast and reliable measurements, as well as early detection of deformations that may affect the safety of mining operations, thereby arranging necessary work in advance to mitigate potential risks. In l'ultimi anni, The botdr system has been widely used underground in coal mines. Naruse et al. conducted BOTDR monitoring at the El Teniente mine in Chile. The optical fiber is aligned along the tunnel and set inside the tunnel, so it can measure the convergence of the tunnel. Cheng et al. measured the deformation of overlying strata in coal seams using a botdr based monitoring method. Zhang and Wang established a fiber mesh structure on the surface of the tunnel and conducted botdr strain measurements.

In previous BOTDR applications, optical fibers were installed approximately 5 meters behind the excavation face of the roadway to avoid interference with the installation of supporting structures (anchor rods, cable anchor rods, steel mesh, ecc.). Dunque, only deformation that changes over time can be measured, and deformation that occurs shortly after excavation cannot be studied immediately. Tuttavia, 80% of road damage and collapse accidents occur near excavation surfaces. Dunque, monitoring the entire section of the roadway, including the deeper surrounding rock and excavation face, has always been a key issue in ensuring safe production underground in coal mines.

A monitoring system for the surrounding rock of coal mine underground tunnels based on botdr. The structure of the monitoring system has been modified to enable real-time monitoring of the instantaneous and time-dependent deformation of the surrounding rock. The on-site monitoring of the system in tunnels was proposed, and the monitoring results were analyzed and compared with the measurement results of conventional monitoring techniques.

The basic principle of BOTDR monitoring system

The monitoring system based on botdr has achieved Brillouin scattering, which is a fundamental physical process representing the interaction effect between light and optical media in the propagation medium. When light passes through optical fibers, most of it propagates along the original direction, while a small portion deviates from the original direction, resulting in scattering. There are three types of light scattering in optical fibers: Rayleigh scattering caused by changes in fiber refractive index, Raman scattering caused by optical phonons, and Brillouin scattering caused by acoustic phonons. In Brillouin scattering, the scattered light reaches its peak in its spectrum, and its frequency shifts from the pulse light. This frequency shift is called Brillouin frequency shift.