High-Resolution Seismic Reflection Survey At A Proposed Dam Site

A high-resolution seismic reflection survey was conducted at a proposed dam site located in an<br>incised valley of Tara Volcano, western Kyushu. The purpose of the survey was to map the bedrock<br>surface and to delineate the shallow reflectors within the overlying volcaniclastic flow deposits. We set<br>three survey lines on river terraces along the stream and covered about 1.1 km long section. Both source<br>and receiver intervals were set to 2 m to have high-resolution data. A mini-vibrator, a drop-hitter, an<br>AWD, and a wooden hammer for hitting the ground surface by hand was used as the P-wave source<br>taking account of the surface condition of the lines.<br>The survey result clearly reconstructed the layered structure in the volcaniclastic deposits, and<br>imaged the unconformity structure between the underlying sandstones. The shallow geologic structure<br>beneath the dam site gently dips southward in concordant with the volcaniclastic flow direction, and<br>neither mappable faulting nor deformed structure was imaged in the section, which means the site has<br>been tectonically stable during the recent geologic period.