| Helicopter-borne magnetic survey was carried out to verify its applicability for geological evaluation along a large overburden tunnel before construction in northeastern Hokkaido. The geology around the tunnel is greenstone、 basalt、 pyroclastic sedimentary rock and hyaloclastite、 and those mixed with pillow lava、 chert and limestone、 and many faults are formed due to tectonic movements around there. By the measurement result、 magnetic intensity was high in basalt area、 and low in other rock type like hyaloclastite area、 respectively. The result was combined with geological survey result、 from which inclined geological structure was assumed、 and magnetic intensity model、 and geological section along the tunnel was figured. Many data such as tunnel deformation、 rock type obtained from horizontal pilot boring core observation at all tunnel line、 was obtained by Abashiri Regional Development Office. For the correlation、 we also measured magnetic susceptibility of the boring core. As for the trouble in tunnel construction like water rush or tunnel deformation、 they happened in most part at geological boundary like between chert and basalt. Therefore、 we noticed again the importance of geological judgment beforehand. By the measurement result of boring core with KT-9、 portable measuring equipment、 magnetic susceptibility was highest in massive basalt、 and hyaloclastite、 pillow lava、 mixed rock and chert in order of the strength. These tendencies almost correspond to the helicopter-borne magnetic survey results. By the correlation between result from pilot boring core observation and geological evaluation by helicopter-borne magnetic survey、 field survey and magnetic intensity model、 we found them almost the same except in detail. Therefore、 helicopter-borne magnetic survey is useful to grasp the geological outline and evaluate troublesome part of large overburden tunnel formation. |
