Development of a fiber-optic sensing system for monitoring ground vibrations produced by debris flows

• Main Authors: Chung-RayChu, 朱崇銳
• Other Authors: Ching-Jer Huang
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/46411060699686415610

博士 === 國立成功大學 === 水利及海洋工程學系碩博士班 === 101 === This study presents a fiber-optic sensing system for measuring the ground vibration produced by debris flows. The system comprises a demodulator and four fiber Bragg grating (FBG) accelerometers. The field tests showed that the signal-to-noise-ratio (SNR) of FBG accelerometer was 10 dB higher than that of the geophone. The frequency analysis indicates that in the lower frequency range, 10 – 30 Hz, Fiber Bragg Grating (FBG) accelerometer had approximately the same SNR as geophone; however in the higher frequency range, 30 – 250 Hz, FBG accelerometer had 10 dB higher SNR than geophone. Geophone was commonly used as a device to measure the ground vibration generated by debris flows, but the quality of the detected data is difficult to control because of its low signal-to-noise-ratio (SNR). Following confirmation of the reliability of the proposed sensing system, the system is deployed along the Ai-Yu-Zi Creek in Nautou County, Taiwan, for monitoring debris flows. The optic circuits in parallel and in series construction were individually tested to monitor real debris flows. The series construction is suitable for the deployment of multiple sensors along one creek. The system have detected several debris flows in 2012 and 2013. The monitored data revealed that the frequency range of the acceleration signal of ground vibration was 10 – 150 Hz, which is the same as that of the velocity signal of ground vibration detected by geophone. Ground vibrations result from the hitting of rocks against riverbed and from the rubbing of rocks along the riverbed can be identified from the time series data. The frequency range of the former one is below 50 Hz, while the latter one is 10 – 150 Hz. The estimated speed of debris flow occurred on May 19, 2013 was 4.18 m/s, which is much lower than that of a previous debris flow occurred at the same creek in 2004 with a speed of 13.3 m/s. In addition, the waveforms of ground vibration signals produced by the detected debris flows were different from a triangular shape, which was recognized as a typical signal form for a well-developed debris flow. The ground vibration signals of the previous debris flow in 2004 also showed this characteristic waveform. The reasons for causing the difference in the signal shape may be the following. The rheological behavior of debris flows has been changed because the channel of Ai-Yu-Zi Creek was broaden by a serious debris flow occurred during the period of typhoon Morakot in 2009. The monitored data demonstrate that the detection range of FBG accelerometer for monitoring debris flows is 150 – 230 m, and debris flows can be detected about 40 s earlier before the surge of debris flow reaches the sensor. The quality of the monitored signals depends on the installation location and the material of the creek bed. The performances of geophone and FBG accelerometer were discussed by comparing the monitored data from the same debris flows. The comparison suggests that the FBG accelerometer has better data quality, lower noises than geophone, and the proposed fiber-optic sensing system is highly promising for use in monitoring natural disasters that generate ground vibrations.