以試驗探討土石流都卜勒效應

Experimental Study for Doppler Shift Phenomenon of Debris Flow

蘇得罕(Sudhan Regmi)

指導教授 ： 劉格非

國立臺灣大學/工學院/土木工程學研究所/碩士(2020年)

https://doi.org/10.6342/NTU202003711

摘要

土石流運移過程產生的震聲波是過去監測或偵測土石流的主要方法之一。若能透過地聲接收器的振動訊號來解析土石流發出的震聲波將可使我們更深入的了解土石流的流況，如：流速、流量等相關物理參數。為了探討地聲訊號中是否存在都普勒效應，並了解地聲訊號與土石流流況間的關係，本研究將透過試驗探討固定觀測位置的地聲訊號來分析土石流移動過程產生的震聲源。
本研究在一長10公尺、寬為0.5公尺的水槽中創造一體積量約為0.2 m3的顆粒流來模擬現場的土石流。在渠道的底床上則撲滿0.1公尺深的礫石顆粒，並沿著渠道埋設一系列三軸地聲探測器來記錄顆粒流流動過程產生的震聲波。在訊號分析上，本研究採用離散傅立葉轉換與短時間傅立葉轉換來解析訊號之頻譜，並採用加權非線性最小平方法來回歸主要頻率分布函數與頻率峰值。為了探討流況與地聲訊號間的關聯，本研究則同時透過試驗影片萃取前鋒速度、表面流速、流深、流量等物理參數。
在假設顆粒流主頻介於20-50 Hz範圍的前提下，本文先以此頻率範圍內的頻率峰值與流況參數做關聯性之探討。在頻率峰值與表面流速的比較中，其關係皆不同於傳統的都普勒效應。本研究發現其正規化的頻率與表面流速呈現一非線性關係，且其非線性回歸之相關係數高達0.996。

關鍵字

土石流 ； 震聲波 ； 地聲 ； 都普勒效應 ； 頻率峰值

並列摘要

Seismo-acoustic wave radiated from debris flows motion is one of the main properties used for monitoring and detection of debris flows. Understanding the Seismo-acoustic wave using geophone recordings may give us great insight into the physical process of debris flows such as flow velocity, flow rate. To connect the observation to the debris flows motion, the Doppler effect between moving seismo-acoustic source generated by debris flow and vibration signal recorded from fixed geophones were analyzed. In this study, a granular flow in the volume of 0.2 m3 was generated in a 10 m long and 0.5 m wide hydraulic flume to simulate the debris flow in the field. In this flume, 0.1 m thick granular bed was laid, and a series of three-axis geophones were buried along granular material to record the vibrations produced by granular flows. The discrete Fourier transform and short-term Fourier transform were used to decompose the ground vibrations into a frequency spectrum. Then the weighted non-linear least square regression was adopted to isolate the dominant frequency functions and peak frequency. With video recordings, the physical parameters included front profile, surface velocity, flow depth, and discharge were extracted. Assuming the radiated peak frequency in the moving granular flow is within 20-50 Hz and normally distributed, the isolated peak frequency shift in the fixed geophone location was analyzed with the tracked flow parameters. Compared to the peak frequency shift with surface velocity, we found the relationship is different from the traditional Doppler effect. Results show that the normalized frequency seems to have a non-linear relationship concerning surface velocity with a coefficient of determination (R2) as high as 0.996.

並列關鍵字

Debris flows ； Seismo-acoustic wave ； Geophone ； Doppler effect ； Peak frequency

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