| In Japan、 seismic prospecting or electric sounding is very popular as geotechnical survey of tunnel construction. Though these methods are very useful for tunnels of ordinary length and overburden、 there are some difficulties to apply them for larger overburden tunnels because they have not sufficient accuracy to detect deeper part of ground. The authors carried out HEM (Helicopter-borne Electromagnetic Method) and CSAMT (Controlled Source Audio-frequency Magnet Telluric method) to study their applicability for detecting geological condition in deep part of a large overburden tunnel being constructed in northeastern Hokkaido、 north Japan. The geology of the tunnel is greenrocks (basalt、 volcanic breccia、 pyroclastic rock、 chert and limestone) in the Tokoro belt. Several low resistivity zones、 mottled or relatively steep gradient part of resistivity was estimated as geotechnically troublesome zones for the tunnel excavating by HEM and by CSAMT、 and by the combination method、 respectively. They were correlated with actual troublesome zones encountered by the tunnel excavation. As a result、 the trobulesome zones estimated by the combination of HEM and CSAMT are found to be most fit to the actual troublesome zones than those by HEM or CSAMT individually. In conclusion、 geotechnically troublesome zones of deeper tunnel were well recognized by the combination method of HEM and CSAMT. |
