| A helicopter-borne magnetic survey (HMS) was carried out to verify its applicability for geotechnical evaluation of a mountainous tunnel site before the tunnel construction in northeastern Hokkaido、 Japan. The distribution of magnetic intensity (MI) is obtained by HMS results. While the tunnel construction、 magnetic susceptibility (MS) of advanced horizontal boring cores were measured and precise rock type classification was also carried out in order to check the MI by HMS. The geology around the tunnel site is basalt、 chert、 pyroclastic rock、 hyaloclastite、 and those mixed with pillow lava. As a result、 geological profile models along the tunnel site was drawn based on the geological survey results、 and was drawn based on an analysis result of observed magnetic curve by HMS over the tunnel route before its excavation. Both probable models showed an almost similar tendency. MS was highest in massive basalt、 hyaloclastite、 pillow lava、 mixed rock and chert in order of the strength. Consequently、 the geological profile models almost correspond to the rock type distributions. Thus、 the information of geological outline of tunnel site could be grasped by HMS. |
