ABSTRACT: In any engineering geological site investigation, use of geophysical techniques based on the geological model is essential. Geophysical investigation along a tunnel alignment to predict the geological section has usually consisted of seismic refraction and electrical resistivity methods. But, these methods are very hard to apply in high mountain area because of rugged terrain condition and limit of investigation depth. AMT (audio-frequency magnetotelluric) method has been usually applied to explorations for deep seated ground water and underground resources. However, it has been extending its scope to civil engineering areas since it has been developed so as to image underground structures effectively. In this study, AMT survey was carried out along the alignment of the proposed tunnel in Korea to determine the geological boundaries and weak zone such as fracture, fault and coal seams. The frequency range used in AMT survey was 10 Hz-100 kHz in order to obtain the shallow subsurface information. The total length of survey line was 8 km and measurement spacing 100 m. By, interpreting the AMT results and drill logs, four electric media were distinguished, which were identified lithologically as alluvium and weathered sediments (10-1 00 ohm-m), coal bed (1-10 ohm-m), sandstone and shale (100-1000 ohm-m) and limestone (1000-10,000 ohm-m). Also many fracture and fault zones were detected along the tunnel. Finally AMT results were given in the form of a predicted geological section though the tunnel.

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