| 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. |
