| We conducted a combined geophysical survey in an abandoned road tunnel for characterizing conditions of pavements and bedrocks. A 2 km long survey line was set along the center line passing through the 1.8 km long tunnel excavated in 1977. Capacitively coupled resistivity measurement using OhmMapper and high-frequency surface wave measurement using Land Streamer were employed for the tunnel survey. Recent increase in collapsing incidents of tunnels in service has led us to utilize geophysical methods for the safety check of existing tunnel structures instead of visual inspection. However, geophysical measurements targeting concrete pavements and the underlying bedrocks have been scarcely implemented due to the difficulty in applying them in active tunnels. We then applied the above two methods in an abandoned road tunnel to verify the optimal acquisition parameters and field performance. As a result, near surface S-wave velocity structures up to 20 m beneath the tunnel floor were clearly imaged. The reconstructed structure was concordant with excavation and maintenance history of the tunnel. In contrast, the capacitively coupled resistivity measurement was very sensitive to the state of concrete pavement slab due to its internal rebar structures. In the repaired zone where rebar was densely arranged and connected each other, measured potentials were very low to reconstruct resistivity structures. Conversely, in unrepaired zones we could obtain potential signals reflecting bedrock resistivity. In conclusion, combined resistivity and surface wave survey is capable for the characterization of physical properties of floor structures and bedrocks of tunnels. |
