Naomi Kano

Deep structure of the Nagamachi-Rifu fault deduced from small aperture seismic array observations

A seismic reflection survey using both explosives and vibrators was conducted in June 2001 around the Nagamachi-Rifu fault, northeastern Japan. We carried out observations of four small aperture seismic arrays in the area to reveal detailed structures of the fault. Array analysis was applied to waveform data from 15 explosives to obtain P-wave scatterer distributions in the area. The obtained P-wave scatterer distribution correlates in space with microearthquake activities and heterogeneous structures such as S-wave reflectors, a structure of caldera, and Mohorovicic discontinuity. We could also image that a sub-horizontal layer with a length of about 10 km exists in the deep extension of the Nagamachi-Rifu fault beneath the seismogenic layer.

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.

Time‐Lapse High‐Resolution Seismic Monitoring for a Small‐Scale Carbon Dioxide Gas Injection Test at Shallow Depths

High-resolution seismic reflection monitoring was applied to a small-scale carbon dioxide gas injection field test. In the test, a total of 330-kg CO2 gas was injected over 59 hours into a 62-m deep groundwater observation well drilled in the AIST Tsukuba Site, central Japan. We set 3 seismic lines, on which 40 Hz geophones were planted at 1 m intervals, adjacent to the injection well. P-waves were generated by hitting surface pavement using a wooden hammer.