The University of Tokyo Cosmic Ray Research Institute is planning a new cosmic ray observation facility; called Hyper Kamiokande, at the Kamioka mine in Japan. The detector of cosmic rays for Hyper Kamiokande requires a large cylindrical tank with a capacity of approximately 260,000 m3, and the tank will be set up in an extensive cavern at 650 m underground (the excavation volume is 343,000 m3, and the cavern diameter and height are 76 m and 78 m, respectively). Tens of thousands of ultra-sensitive photosensors will be installed on the wall of the tank. The potential underground area for the construction of the cavern to house the detector spans several hundred meters both horizontally and vertically. It was considered that a 3D seismic survey using existing underground galleries is the most suitable method to investigate and identify the location for the construction of the cavern. However, because case studies of such large-scale 3D seismic surveys are rare, so numerical simulation studies and preliminary surveys were conducted to develop the measurement specifications necessary for the main large-scale 3D seismic surveys.
The main seismic surveys were completed by the end of 2016 and successfully acquired high quality data, seismic tomography and 3D reflection imaging were applied, and the distribution of the 3D seismic velocity and the reflective surface of the rock mass around the planned cavern were clarified. These images corresponded to known geological features, including faults. From the observation results of the existing gallery, the 3D velocity distribution and the reflection imaging result, we extracted a region of good rock quality and set the region suitable for the construction of the cavern.
The cosmic elementary particle research facility "Hyper Kamiokande" is planned to be built within the Kamioka mine in Hida City, Gifu Prefecture, Japan (Fig. 1 a)). Kamiokande and Super Kamiokande were built at 1983 and 1996 respectively and these are located in the northern part of the Kamioka mine. Hyper Kamiokande, however, is planned in the southern part of the same mine (Fig. 1 b)). Comparison of these caverns is shown in Fig. 2. The cavern volume of Hyper Kamiokande is about 60 times larger than that of Kamiokande and about five times that of Super Kamiokande.
