An Overview of the MIU-4 Borehole Investigations during Phase IV

Kumazaki, Naoki; not registered ; Ikeda, Koki; Amano, Kenji  ; Takeuchi, Shinji; Hama, Katsuhiro ; G.F.D.McCrank*

Surface-based investigations are now being carried out at the MIU site. Drilling of the MIU-4 borehole, one of the planned surface-based investigations in the MIU project, commenced in June 2000. The main aims of the MIU-4 borehole investigations are to address key issues (e.g. hydrochemistry, 3-D rock mechanics, geometry of steeply dipping water-conducting fractures, etc) remaining after the previous borehole investigations and to continue the development of appropriate, comprehensive techniques for assessing the deep subsurface. The borehole was drilled from the centre of the MIU site in a north-easterly direction; the inclination and planned drilling length is 30 from the vertical and 790m respectively. During MIU-4 Phase IV (from October 2000 to July 2001), the borehole was drilled through the hanging wall of the Tsukiyoshi fault in the basement granite (from 134.47 to 673.26m along the borehole) at the MIU site. Planned geological, hydrogeological, hydrochemical and additional investigations were carried out. Quality-controlled information on geology, hydrogeology and hydrochemistry of the hanging wall of the Tsukiyoshi fault in the basement granite was acquired and consequently this advanced the understanding of the geological environment at the MIU site. The techniques and QA/QC procedures adopted for the MIU-4 Phase I to III investigations proved to be effective for the current borehole investigations. In addition, recommendations for the remaining MIU-4 Phases and future borehole investigation programmes are provided.