The helicopter-borne magnetic survey was carried out to verify its applicability of the geotechnical evaluation along the mountainous long tunnel sites in north eastern Hokkaido, Japan. The geological properties around the tunnel site consists of basalt, chert, pyroclastic rock, hyaloclastite, pillow lava and mixtures of these rocks. During the tunnel construction, the magnetic susceptibility of advanced horizontal boring cores were measured, and precise rock type classification was also carried out to enable comparison with magnetic intensity values obtained from helicopter-borne magnetic survey.

In this study, the obtained data by helicopter-borne magnetic survey was used to estimate the distribution of the geological properties and its condition around the tunnel site, and modeling simulation approach was attempted to estimate of the geological structure and interpret magnetic anomalies. The magnetic intensity distribution was also determined from the helicopter-borne magnetic survey results, and was also translated to create a geological profile using magnetic intensity analytical data. A corrected geological profile along the tunnel sites was created based on the data of the tunnel excavation, and was compared with the details of rock types from advanced horizontal boring cores, the magnetic intensity distribution, and the analysis profile of the observed magnetic curves obtained from helicopter-borne magnetic survey over the tunnel route.

As a results of this survey, The helicopter-borne magnetic survey measurement results showed the some high value peaks of magnetic intensity. As a results of The magnetic susceptibility was highest in the order of massive basalt, hyaloclastite, pillow lava, mixed rock and chert, and these results were in harmony with those for magnetic intensity. The high value peaks of magnetic intensity closely corresponded to the basalt existing area. This can be concluded that magnetic intensity by the helicopter-borne magnetic survey indicates distibution difference of rock type and the survey is useful in clarifying the geological characteristics of tunnel sites. These information will provide auxiliary information to tunnel constructior and geotechnical engineers. This paper describes relationship between geological profiles estimated from the outcomes of tunnel construction records and the magnetic anomalies which was obtained by helicopter-borne magnetometry data.

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