本研究欲改善傳統土石流監測中地聲檢知器 (geophone) 量測土石流地表震動之不足，藉由光纖系統之敏感度高、傳輸損耗低、抗外部電磁雜訊強之優勢，克服地聲檢知器監測範圍太小、訊號品質不佳之缺失。本研究結合光纖光柵加速度計、解調儀、及其他光纖元件，發展一套可偵測土石流地表震動之光纖監測系統。現地模擬土石流實驗之量測結果顯示，與目前常用的地聲檢知器系統、麥克風監測系統相較，光纖加速度計所測得地表震動訊號之訊雜比較地聲檢知器高10 dB；頻域部分，10 – 30 Hz頻段光纖加速度計與地聲檢知器量測訊雜比相當，30 – 250 Hz頻段光纖加速度計比地聲檢知器高10 dB；麥克風則容易受環境噪音影響，導致量測訊雜比最差。現地光纖監測系統架設於南投縣信義鄉神木村愛玉子溪，系統光迴路分別採用並聯與串聯方式，沿河岸埋設四組光纖加速度計，並於2012、2013年監測到多場土石流。量測結果顯示，並聯迴路分光後光強度不足，串聯迴路並無此問題。光纖監測系統所測得土石流地表震動加速度訊號頻率範圍為10 – 150 Hz，與文獻結果一致。訊號中可分辯礫石撞擊河床之暫態訊號與礫石磨擦河床之連續訊號，撞擊訊號頻率在50 Hz以下；磨擦訊號頻率在10 – 150 Hz之間。5/19/2013土石流量測結果顯示，該土石流流速約4.18 m/s，與2004年敏督利颱風在愛玉子溪造成之土石流流速13.3 m/s不同，證實2009年莫拉克颱風破壞原有河道後導致土石流流況改變，亦使得土石流至下游後，地表震動訊號也不存在典型三角形波形。由量測結果可估算出光纖加速度計偵測範圍為150 – 230 m，可提前約40 s測得土石流前端訊號，此估算值與埋設位置點及周圍土體材料有關。光纖加速度計與地聲檢知器量測結果比對可知，光纖系統數據品質較佳、雜訊較少，不需經過濾波，原始訊號即可完整量測土石流訊號。

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.

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