| 講演要旨(和文) | | 大規模土石流で被災した砂防ダムの内部亀裂をイメージングすることを目的として高周波地震探査を実施した．副ダムの施工基面の上昇に合わせ，2014年12月から2016年4月にかけて3次にわたって実施した．速度が大きいコンクリート構造物の内部イメージングには，高周波・高速計測が不可欠である．そこでセンサには高周波加速度センサを使用し，収録には2ないし5μs，24bit計測が可能な収録装置を使用した．その結果表層の緩傾斜の亀裂群，被災に伴い分離した打継ぎ面，ダム上流面等をイメージングすることができた． |
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| | 講演要旨(英文) | | High-resolution seismic reflection measurements were conducted repeatedly on the rear wall of a sabo dam, which had been damaged by a huge debris flow occurred on July 9th 2014. The debris flow attack caused distinct dislocation of the body particularly at the lower part of the left bank. To image the internal structure in the concrete dam body by seismic methods, we employed piezoelectric type accelerometers instead of geophones, because high-frequency range measurements were indispensable for obtaining high-resolution spatial data. High-speed and high-resolution recording systems were also used too. Repair work had started in the fall of 2015 enabled us to deploy survey lines at higher levels on the wall and to conduct stepwise measurements. Manual hit by a rock hammer was effective to generate high-frequency signals. Prestack Kirchhoff time migration was adopted to enhance reflected and diffracted events recorded in the obtained data. As a result, clear events indicating internal fractures and frontal face of the dam body were clearly delineated. Distinct attenuation of first breaks were also useful for fracture characterization. The measurements demonstrated the importance of choosing suitable measurement tools and setting appropriate survey parameters. |
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