Applicability of the time-domain electromagnetic method to investigation of ground classification along tunnel routes

Abstract

There is no standard geological investigation method to acquire geological information along a tunnel route. In this study we applied the time-domain electromagnetic (TDEM) method and acquired geological information along the routes of 25 tunnels to improve the accuracy of geological and geophysical data used for tunnel design and construction. The TDEM method is used mainly for oil and mineral resource explorations and is reliable at depths of 200 m or more. Tunnel-face observations recorded during tunnel construction as well as data of various geophysical survey techniques were analyzed by comparing the correlation coefficients based on a quantitative theory. Quantitative analysis of the comparisons between the TDEM resistivity results and the tunnel-face observations at the same site showed good correlation. The resistivity obtained by the TDEM method was found to have similar survey precision as the seismic velocity obtained by a seismic refraction survey when the overburden was less than 100 m thick. The accuracy of the TDEM results was very high even for an overburden of 200 m. Moreover, TDEM can investigate the low-velocity speed layer below a high-velocity speed layer, where the seismic refraction method does not perform well. The resistivity had strong correlation with the crack intervals in the rocks, and the seismic velocity had strong correlation with the uniaxial compressive strength. Moreover, the resistivity data obtained by the direct-current electrical method in the same way revealed clear differences in the survey accuracy when the overburden exceeded 100 m. Based on this study, we propose the application of the TDEM method to geological surveys along tunnel routes passing deeper than 200 meters.