Research on signal propagation model of buried pipe fiber vibration monitoring system
Introduction
Phase-sensitive optical time domain reflectometer (Ф-OTDR) distributed optical fiber perturbation sensing technology can effectively replace the point sensors that require large-scale deployment to achieve regional monitoring and sensitively capture perturbations along the fiber. It can also realize structural health monitoring and safety early warning [1]. Currently, the optical fiber early warning techniques based on Ф-OTDR has applications in various fields, such as the safety monitoring of long-distance natural gas and oil pipelines, the perimeter warning of sensitive facilities, and the health inspection of large structures, including bridges and submarine pipelines, etc. [2], [3], [4], [5], [6], [7], [8]. The buried pipeline Φ-OTDR optical fiber early warning system is a vibration sensing system. The external force of an event causes the surface of the ground to vibrate. These vibrations propagate through the soil in the form of mechanical waves. Due to forced damping vibrations, stress, and strain, the phase of the transmitted light in the optical fiber changes and the backscattered light interferes inside the optical fiber. The photoelectric signal contains information related to external phenomenon. Based on the feature analysis and processing, the external intrusion event is determined. The existing research on optical fiber monitoring system has established conversion models between external physical quantities and optical signals for different sensing mechanisms [9], [10], [11], [12], [13]. However, these models are based on ideal assumptions and lack the procedure for generating external physical quantities. These models are unable to provide a theoretical basis for the analysis and processing of physical quantities. Usually, the signal analysis methods use trial methods, and most of them are limited to the research of optical fiber monitoring system application technology. Based on the signal transmission and conversion process, we adopt elastic half-space theory, forced damping vibration theory, structural mechanics principle and optical fiber interference principle, for modeling soil vibration, optical cable vibration, optical fiber deformation, phase modulation, and photoelectric conversion. Finally, the transfer function of each step is obtained, and their characteristic parameters are derived for obtaining the transfer function of the complete system. Thus, the theoretical model of the distributed optical fiber vibration sensing system is established.
Section snippets
Soil vibration model
When an intrusion event occurs, the disturbance force interacts with the ground surface, simplifying the intrusion event as a point source, and the buried optical fiber is impacted by the vibrations on the ground surface. Ideally, the elastic half-space theory can be used to analyze the vibration characteristics. The elastic half-space theory idealizes the foundation as an isotropic elastic half-space infinite body. The stress waves are generated when the invasion event interacts with the
Discussions
As presented in Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11, Fig. 12, we observe that the simulation signals corresponding to the three types of intrusion events obtained by using the model are similar to the actual signals. In the field experiments, the intrusion events occurred continuously. We compared the vibration signal calculated by the theoretical model with the waveform of the signal collected in the experiment, and compared the amplitude, duration, and main energy range respectively. The
Conclusions
In this work, we analyze the mechanism of vibrations to mathematically model the -OTDR optical fiber early warning system. The propagation of the vibrations in the soil is studied. The optical fiber vibration model is established using the elastic foundation beam model. The phase change of the transmitted light caused by the deformation of the optical fiber is analyzed, and the optical fiber deformation-phase modulation model is established. The photoelectric signal under different types of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Funding: Research Project of Tianjin Education Commission (No. 2018KJ230).
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