Study On Locating Technology Of Distributed Optic-fiber Sensing System Based On Interfering Principle For Long-distance Pipelines Sabotage Acts

Pipeline transportation is one of the main transportations. It has advantages of security, economy, quality-ensuring, without pollution, and it becomes an important way for oil transportation. However, in recent years the behavior, such as stealing oil, construction destroying, badly influences the normal function of oil pipes. The oil-gas pipeline should be monitored in real time to make sure its daily running. However, at present the method of oil-gas pipeline security monitoring can only find the spot of leaking passively after the accident of leakage. It can reduce but can't avoid the loss. Therefore, it is very important to alarm the pipe destruction before leakage.Optical fiber sensing system which combines the fiber interference and fiber distributed sensing technology is one of the most ideal early-alarm technologies of pipe destruction. Based on the interferometer, the distributed fiber interferometer sensing system is regarded to be capable of any potential destruction of the pipe. But its high precision locating technology should be developed further. Twain Mach-Zehnder distributed fiber sensor is presented by Chongqing University in 2005. In the early research, many problems have been solved. But the deeper research on high precision locating technology is not done enough. Because of the great needs of early-alarm and location of pipe destruction, the research on this field has not only great science value but also potential social and economic benefits.Firstly the hypocenter vibration model and earth vibration model are formed after analyzing the generation and transmitting mechanics of the earth vibration signals caused by the destroying behavior such as digging and boring. According to the function style of P wave on the fiber, optical sensing model is proposed. Output signals expression is analyzed when phase changes caused by destroying behavior work on Mach-Zehnder interferometer. And based on some suitable changes of Mach-Zehnder interferometer, Twain Mach-Zehnder distributed fiber sensor is gotten. And the conclusion is gotten that time delay ?τof two interferer signals is the key of the whole positioning technology.Because of the birefringence effect of the single mode optical fiber, polarization degeneration will happen when the linear polarized light transmits in normal single mode fibers. Firstly Jones matrix is used to analyze polarization degeneration effect on sensor performances,and it is found that polarization degeneration not only influences the quality of interference output but also influences positioning accuracy. Based on the Jones matrix analysis, elliptical polarized light interference is used to study interference visibility V,phase error changes with the azimuth angleθand ellipticity angleβof the elliptical polarized light. According to polarization degeneration performances of the system, optical mechanical polarization controllers are used to adjust system polarization, and phase modulation is used to evaluate anti- polarization ability.Cross-correlation method is chosen to realize high-accuracy positioning. Destroying method positioning principle based on cross-correlation method is deduced. The influence is analyzed that when cross-correlation is directly used in the interference signals. A improved positioning method is proposed which is based on generalized cross-correlation method. Many factors are also analyzed which probably make influences on positioning precision. At last it is concluded that the new method can improve the system positioning precision through improving sampling rate, adding the length of data, using wavelet to filter system noise.Based on the analysis above,the entire experimental system is developed: the distributed optical fiber sensing systems enabling to realize positioning of sabotage acts and calibration system to evaluate the measured value. By polarization controlling,increasing the length of the data,wavelet denoising and other means, the positioning accuracy of the system has basically reached the target. Some random deviation appeared during the position, so the law of the average value is used to improve positioning accuracy. Servel measurement results of the same location are taken average.By this means, the random error of a single measurement was eliminated effctively. Measurement error of 4~52m is existed between the average measurement results and the OTDR results. Experimental results show that this system is able to meet the actual needs of the project of positioning sabotage acts of the oil and gas pipeline.