Near-Surface Cavity Detection By High-Resolution Seismic Reflection Methods Using Short-Spacing Type Land Streamer

High-resolution seismic reflection surveying by means of short-spacing type Land Streamers was<br>conducted to detect near-surface cavities. The target cavities were inferred to have high potentiality of<br>collapsing because they had been excavated about 60 years ago as air-raid shelter tunnels or drifts of<br>lignite mines. Until now, several surface geophysical methods have been applied to the cavity detection,<br>however, they only provided blurry images for cavities due to the poorness of spatial resolution or<br>insufficiency in obtaining information at the target depths. In contrast, our short-spacing type Land<br>Streamer tools, which were originally developed by the first author, can provide high-resolution images<br>up to 30 m in depth.<br>We applied three types of Land Streamer tools to the cavity detection: a horizontal geophone<br>array at 30 cm spacing, the same array but 20 cm spacing, and an accelerometer array at 20 cm spacing.<br>Each array has 48 channel sensors mounted on a woven belt which is easy to tow by hand. The targets of<br>this study were abandoned drifts of lignite mine at Nagoya, central Japan, and air-raid shelter tunnels<br>excavated in pyroclastic flow deposits at Kanoya, southern Kyushu.<br>As a result, distinct diffraction anomalies were imaged in unmigrated stacked sections at the just<br>points where actual abandoned cavities were checked by sounding or drilling from the surface. Some<br>anomalies occurred at depths shallower than the inferred horizons of shelter tunnels or lignite mines,<br>which indicates roof falling and upward growing of cavities. It is concluded that our Land Streamer tools<br>have high capability of detecting cavities smaller than 2 m in diameter located 5 to 10 m in depth.