抄録
The Abo Tunnel is planning as a vehicular tunnel with a length of 4.3km from Takayama city to Matsumoto city. The area is occupied by Paleozoic clastic rocks and Quapernary pyroclastic rocks and lava. These rocks distributed in Nakanoyu hot spring, east. portal of this tunnel, were subjected to hydrothermal alteration. So following geological investigations have been conducted for the past 10 years.
1) Survey on distribution characteristics of hydrothermal altered zones.
2) Estimation of underground structure of hydrothermal altered zone by geothermal prospecting.
3) Geophysical logging of vertical and horizontal drilling hole and laboratory tests of obtained core.
The results are summarized as follows.
1) The methods of 1) and 2) are effective at early geological survey stage.
2) Geothermal zones are associated with hydrothermal solution and overheated vapour rich in poisonous gas. Geothermal gradient falls off in proportion to the distance from Azusa river bed.
3) At the same point, the temperature measured in horizontal drilling hole is mostly higher than that of vertical drilling hole because of pressure depression. Long horizontal boring is necessary to estimate a discharge of hydrothermal solution.
4) Along this tunnel, high-temperature zone is traceable for a distance of 1.5km, and there, the temperature reaches from 60°C to 90°C for a distance of 1.0km.
