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