2024 Fiscal Year Final Research Report

Determination of velocity of debris and mudflows by using optical-fibers and distributed acoustic sensing system

Research Project

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Project/Area Number	23K17773
Research Category	Grant-in-Aid for Challenging Research (Exploratory)
Allocation Type	Multi-year Fund
Review Section	Medium-sized Section 22:Civil engineering and related fields
Research Institution	Kyoto University
Principal Investigator	Nakamichi Haruhisa 京都大学, 防災研究所, 教授 (00420373)
Co-Investigator(Kenkyū-buntansha)	江本賢太郎九州大学, 理学研究院, 准教授 (80707597) 田口貴美子東北大学, 理学研究科, 助教 (80965335)
Project Period (FY)	2023-06-30 – 2025-03-31
Keywords	光ファイバー / 分散型音響センシング / 振動 / 土石流 / 火砕物 / 桜島火山
Outline of Final Research Achievements	Debris flows are typically detected using wire sensors and cameras. However, wire sensors become unusable once the wire is broken by a debris flow, preventing the detection of subsequent flows. Cameras, on the other hand, may miss debris flows that occur at night or during poor weather conditions. In recent years, seismic and volcanic monitoring using fiber optic cables and Distributed Acoustic Sensing (DAS) systems has become more common. In this study, DAS observations were conducted using optical fibers, effectively creating a network of numerous seismometers deployed along the rivers of Sakurajima volcano. By comparing DAS recordings with camera footage, we detected more debris flows than with wire sensors. Furthermore, by analyzing the amplitude distribution and phase of vibrations associated with debris flows, we determined the spatiotemporal changes in the vibration source location and successfully tracked the flow paths of the debris flows.
Free Research Field	火山物理学
Academic Significance and Societal Importance of the Research Achievements	土石流が発生する河川には砂防施設があり、光ファイバーが敷設されている。そのため、火山以外の土石流についてもDASを導入することで検知が可能となる。DASにより土石流に伴う振動を観測するため、天候に左右されずに土石流検知ができる。また、ワイヤーセンサーが補修できない状況においても、土石流検知が可能である。土石流の流動過程をＤＡＳから得られた多数箇所の振動から詳細に明らかした。そのため、DASを多くの河川に導入することで、土石流の流動過程の研究がより進展すると期待される。