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標題: | 土石流震聲波之理論研究 Theoretical Study of Seismo-acoustic Waves Induced by Debris Flows |
作者: | Shih-Chao Wei 魏士超 |
指導教授: | 劉格非(Ko-Fei Liu) |
關鍵字: | 土石流,震聲源,地聲探測器,波導環境,都卜勒頻移, Debris flow,Seismo-acoustic source,Geophone,waveguide,Doppler shift, |
出版年 : | 2019 |
學位: | 博士 |
摘要: | 以聲波偵測土石流是目前土石流預警系統中重要的方法之一,且在多數研究中已證明聲波的能量與流況間存在正相關的經驗關係,但至今尚未有人透過理論去分析土石流流動過程與土石流聲源兩者間的關係,因此應用於預警上,目前皆停留在偵測土石流的階段,尚無法近一步即時給予土石流規模、影響範圍等後續評估。為了彌補此缺口,本研究嘗試發展土石流與聲波的連結理論,以理論角度探討土石流流況反應在土石流聲源上的機制,並依此嘗試探討土石流聲源向外傳播的特性。
在理論發展的過程,本文統整各國觀測土石流的成果,歸納巨觀土石流運動與微觀土石流聲源特性等尺度特徵。在質量、動量、能量守恆且不代本構關係式的條件下,本文藉由尺度分析發現土石流內部的聲波傳遞將由深度方向的剪力微分項主導,因此在視土石流為一個聲源體的前提下,本文接續導出一組含土石流聲源特性的土石流控制方程式與邊界條件,並另外整理深度積分形式的控制方程式提供後續研究參考。 為了應用導出的控制方程式探討聲源與流況關係,本文先針對定型波土石流的案例做分析,並引入動量積分法搭配土石流之流速剖面導出聲波剪力作用於土石流底床邊界上的近似解。在三種流速剖面的條件下,其聲源剪力近似解皆可被證明為底床沖淤、土石流流深、深度平均流速、土石流波速的一個函數。此近似解除了可以反應出定型波土石流的流況外,也可視為土石流震聲源或震聲波於地層傳播的邊界條件。 透過此震聲源邊界條件,本文則引入傳統的彈性波波傳方程式解析沿渠道的單層波導環境。本文發現在地層中的任意固定位置,其訊號頻率將存在類似都卜勒頻移的現象,且訊號頻率會隨著介質波速的不同而改變,此結論間接證明透過頻率反推土石流前鋒速度的可行性,並說明為何地聲探測器安裝在不同介質中會量測到不同的特徵頻寬。 The acoustic-based detection is one of a popular way to detect debris flows and successfully used in different early warning systems all over the world for decades. From field observation, the high correlation between the energy of ground vibration signal and flow condition has been proved by many studies. However, the flow condition still cannot be quantified using a recording signal due to a lack of theoretical models. Therefore, the present warning system cannot give a real-time subsequent assessment after detecting debris flow such as affected area, influence time, and potential disaster scale. To bridge the gap of theory, this study aimed to develop a connecting theory between debris flow motion and seismo-acoustic wave propagation. Based on mass-, momentum-, and energy- conservation, the governing equations including macro debris flow motion and micro acoustic vibration were derived. With recording data collected from different monitoring stations, the order of magnitude for different variables was estimated for normalization. Hence, the debris-flow governing equations including acoustic properties inside debris flow were deduced. The equations show that the leading order of acoustic propagation inside the debris flow occurred on the depth differentiation terms of acoustic stresses. If we consider debris flow as a seismo-acoustic source, these acoustic stresses can be regarded as the boundary condition for underground seismo-acoustic wave propagation. With governing equations in depth-averaged form, the debris flow of stationary wave type was discussed. To solve the seismo-acoustic stress on the interface of debris flow and bed, a well-known Karman’s momentum integral method was used. With three kinds of velocity profiles, the approximated solutions of seismo-acoustic stress were derived and proved as a function related to bed entrainment, flow depth, depth-averaged velocity, debris-flow phase velocity. To demonstrate the wave propagation from the derived seismo-acoustic stress (seismo-acoustic source induced by debris flow) on the river bed, a single finite layer waveguide along the channel was analyzed. In this case, the conventional elastic wave equation was introduced to govern the seismo-acoustic wave propagation. For any fixed point in this waveguide, the frequency of the recording signal can be solved as a function of debris flow phase velocity and acoustic wave speed. It means the recording signal exists Doppler shift effect and might be used to evaluate debris flow phase velocity. Besides, this result indirectly proves that different superior frequency band would be recorded if the media is different. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/73706 |
DOI: | 10.6342/NTU201903624 |
全文授權: | 有償授權 |
顯示於系所單位: | 土木工程學系 |
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