Optical fiber sensing technology is a high-tech in the field of engineering measurement. Optical fiber sensor takes light as information carrier and optical fiber as information transmission medium. It has the outstanding advantages of anti-electromagnetic interference, corrosion resistance, high sensitivity, fast response, light weight, small volume, variable shape, large transmission bandwidth and reusable distributed measurement. It is widely used in online dynamic detection of high-rise buildings, intelligent buildings, bridges and highways. On 20/0/0, 2006, with the support of the key projects of the Eleventh Five-Year Plan for Science and Technology and the geological survey project of China Geological Survey, a fiber grating monitoring and demodulation system and a distributed optical fiber strain monitoring system with independent intellectual property rights were developed, which were applied to the monitoring of geological disasters in the Three Gorges reservoir area and achieved good results.
Optical fiber sensing technology measures environmental parameters by measuring some parameters (such as intensity, phase, frequency, polarization state, etc.). ) light transmitted in an optical fiber. Distributed optical fiber sensing technology has become one of the most promising technologies in optical fiber sensing technology because of its advantages of reusable, distributed and long-distance transmission, and it is the development trend of optical fiber sensing monitoring technology. Among them, fiber Bragg grating (FBG) and Brillouin optical time domain reflection (BOTDR) are the two most representative distributed optical fiber sensing technologies.
FBG is a quasi-distributed optical fiber sensor. The measurement information obtained by FBG is not continuously distributed in space, but it can simultaneously monitor multiple points in a large range. Bragg grating is a kind of grating whose refractive index changes periodically in optical fiber, and its reflected light wavelength is different in different periods. When an optical fiber with a Bragg grating is stretched or compressed and its temperature changes, its period changes and the wavelength of reflected light also changes. By measuring the wavelength change of reflected light, the strain or temperature value of optical fiber can be known. The measuring principle of fiber Bragg grating is shown in figure 1.
Figure 1 fiber grating measurement principle
BOTDR mainly uses the characteristics that the spectrum and power characteristics of backward Brillouin scattering light generated by light wave propagating in optical fiber are related to the external environment (temperature, strain, etc.). Fig. 2 is a spectral distribution diagram of backscattered light in an optical fiber.
As can be seen from fig. 2, in Brillouin scattering, the frequency of scattered light has Brillouin frequency shift with respect to the pump light. When the material characteristics of optical fiber are affected by temperature or strain, Brillouin frequency shift will change. Therefore, by measuring the frequency shift of backward Brillouin scattering light of pulsed light, distributed temperature and strain measurement can be realized.
Fig. 2 Spectral distribution of backscattered light in optical fiber
A large number of theoretical and experimental studies have proved that when the ambient temperature changes less than or equal to 5℃, the relationship between the axial strain of optical fiber and the frequency drift of Brillouin scattering light can be expressed as follows
VB(ε)= VB(O)+Cε
Where VB(ε) is the shift of Brillouin scattering frequency when the optical fiber is strained; VB(O) is the shift of Brillouin scattering frequency when the fiber is unstrained; C is the strain coefficient of optical fiber, generally 50 MHz/μ ε; ε is the actual axial strain of the optical fiber. The principle of BOTDR strain measurement is shown in Figure 3.
Fig. 3 BOTDR strain measurement principle
Through tackling key problems, the Hydrogeology Environmental Geology Research Center of China Geological Survey has made achievements in optical fiber sensing technology for geological disaster monitoring, including a series of optical fiber grating sensors, optical fiber monitoring instruments and optical fiber monitoring methods. With independent intellectual property rights, it can play a very good role in landslide, slope stability and health monitoring of major national projects.
1) Based on grating sensing technology, grating strain sensors, displacement sensors and grating steel strain gauges have independent intellectual property rights, and their performance reaches the level of similar products. Compared with domestic similar products, the economic cost is reduced by 50%, and the "fiber grating sensor for landslide crack monitoring" (patent number: ZL200820 135234.7) and "fiber grating sensor" are obtained.
Figure 4 Fiber Bragg Grating Monitoring Demodulator
2) The fiber grating sensor array is demodulated by using the optical wavelength division multiplexing technology, and the developed fiber grating monitoring demodulator (Figure 4) can realize the wavelength demodulation range of 40nm, the demodulation accuracy of 5pm, and transmit the monitoring data in real time. The technical performance has reached the level of international similar technical products, and the economic cost is 30% lower than that of similar products at home and abroad, and it has been applied in landslide crack monitoring in the Three Gorges reservoir area.
3) Using microwave electro-optic modulation and optical coherence detection technology, a prototype of distributed optical fiber strain monitoring system (Figure 5) with independent intellectual property rights is developed. The test distance can reach more than 20km, the spatial resolution is 5m, the strain measurement accuracy is 100με, the technical performance is close to the international level of similar products, the economic cost is reduced by 30%, and it has been applied to landslide monitoring in the Three Gorges reservoir area.
Fig. 5 Distributed optical fiber strain monitoring system
4) The application method of combined monitoring of landslide by FBG and BOTDR is explored. The experiment shows that FBG and BOTDR jointly monitor the landslide, laying monitoring optical fiber on the whole landslide, and using BOTDR technology can obtain the general information of the whole landslide; FBG sensor is installed at the key part of landslide deformation-deformation joint. With its high monitoring sensitivity, the strain values of some key parts of landslide can be obtained, which can not only overcome the shortcoming of low spatial resolution of BOTDR monitoring, but also make up for the deficiency that FBG can only measure discrete points, realize landslide monitoring from point to line and then to surface, and obtain more complete strain information of landslide.
Second, the scope of application and application examples
The measurement accuracy of FBG sensor can reach 0.0065438 0%, and the measurement time of the system is short, which can realize real-time monitoring. Based on these advantages, FBG is widely used in structural detection (such as health monitoring of bridges and tunnels). When it is used to monitor geological disasters, it is mainly used to monitor the deformation of structures with large deformation, and when it is used to monitor landslides, it is mainly used to monitor the real-time changes of the trailing edge of landslides or known cracks.
The optical fiber used in BOTDR sensing technology is small, soft and flexible, and can be combined in any form in the matrix structure without affecting the performance of the matrix. As long as the Brillouin scattering power and frequency of the whole test fiber are measured, the strain and temperature distribution of each part of the fiber can be obtained. The biggest advantage of the sensing technology system is that optical fiber is both a sensing element and a transmission medium, which belongs to distributed monitoring, can meet the requirements of long-distance and uninterrupted monitoring, and is convenient for networking with optical fiber transmission systems to realize telemetry and control of the system. When it is used to monitor geological disasters, optical fiber can be implanted into the monitoring body in the form of neural network to monitor the whole process from line to surface.
Application of optical fiber sensing technology in landslide monitoring of Disabled Persons' Federation;
The landslide of the Disabled Persons' Federation is located in the center of the new county town of Wushan County, Chongqing, and belongs to the valley slope topography. Although the scheme of "dredging and load reduction+lattice anchor+toe wall+surface drainage" has been adopted, there is still obvious surface deformation in the lower part of the landslide. In view of this, in August, 2004, it was monitored by BOTDR technology, and in June, 2006, 10, FBG strain sensors were installed at its key deformation parts (Figure 6).
Fig. 6 Installation of distributed optical fiber network and FBG sensors at the lower part of the Disabled Persons' Federation landslide.
BOTDR monitoring shows (Figure 7) that there are four obvious high-strain abnormal segments along the optical fiber, which are symmetrical in pairs (the optical fiber is put back). C 1 abnormal section corresponds to section 92 ~ 93, and C2 abnormal section corresponds to section 142 ~ 143. Macro investigation shows that there are obvious tensile cracks and shear deformation in these two places, and FBG monitoring data (Figure 8) also shows that the deformation is still going on. By May of 20 10, the deformation strain of cracks has reached 642.76με. In this way, through the joint monitoring of FBG and BOTDR, not only the strain distribution of landslide along the profile direction is obtained, but also the monitoring of key deformation parts is strengthened.
Fig. 7 Distribution of BOTDR monitoring strain along optical fiber in the lower part of Disabled Persons' Federation landslide.
Fig. 8 is a schematic diagram of monitoring strain with time by fiber grating.
Third, promote the transformation mode.
The achievements of optical fiber sensing technology for geological disasters can improve market awareness through publicity, conference exchange, personnel training and technical consultation. A series of fiber grating sensors, fiber grating monitoring demodulation instruments and distributed fiber monitoring systems with independent intellectual property rights can be transformed into monitoring technology-related products and sold directly in the market. The technical method of optical fiber monitoring geological disasters can be popularized through demonstration projects.
Technical support unit: Hydrogeological Environmental Geological Survey Center of China Geological Survey.
Contact: Zhang Qing
Mailing address: No.0/305, Qiyi Middle Road, Baoding City, Hebei Province.
Postal code: 07 105 1
Tel: 03 12-59087 18,13503125891.
E-mail: zhqn123 @163.com.