Seismic tomography investigation in 140m Gallery in the Horonobe URL project
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Seismic tomography investigation in 140m Gallery in the Horonobe URL project Yutaka Sugita1, Takahiro Nakamura1, Hiroyuki Sanada1, Takao Aizawa2 and Shunichiro Ito2 Horonobe Underground Research Center, Japan Atomic Energy Agency, Hokushin 432-2, Horonobe, Hokkaido 098-3224, Japan 2 Suncoh Consultants Co., Ltd., 1-8-9, Kameido, Koto-ku, Tokyo 136-5882, Japan 1
ABSTRACT The Japan Atomic Energy Agency (JAEA) established the Horonobe Underground Research Laboratory (URL) Project at Horonobe, in Hokkaido, Japan to enhance reliability of nuclear waste disposal technologies to be developed in deep sedimentary environments. JAEA has undertaken a number of in-situ experiments to determine changes in the properties of the host rock and the extent of the excavation disturbed zone (EDZ) created by the excavation of underground galleries for the disposal of radioactive waste. This paper reports a seismic tomography survey (using a hammer seismic source) of the “140m Gallery” at a depth of 140m below the surface of the Horonobe URL. The observation area was 3m square on the horizontal plane along the sidewall of the 140m Gallery. The measurement was repeated with the progress of excavation of a tunnel. In this experiment, the distribution of seismic velocity in the rock around the new tunnel and its decrease as the tunnel was dug, were observed using a simple small-scale seismic tomography system. The data collected show that this system can be used to capture the EDZ around tunnels. INTRODUCTION In Japan, disposal of high level radioactive waste (HLW) is required to be deeper than 300m underground. When a deep tunnel is excavated for HLW disposal, an excavation disturbed zone (EDZ) is generated around the tunnel. As an EDZ may provide a transfer passage to the radionuclide, it is important to grasp its generation process, extent and variation in the properties, and the change with the time lapse. To understand these phenomena, Japan Atomic Energy Agency (JAEA) conducted a geoscientific research program for EDZ in a deep underground tunnel in the Horonobe URL. Seismic tomography is considered as an effective technique for investigating geological condition around tunnels. The cross-hole tomography survey can investigate and display the geological environment between the boreholes as an image [1, 2], and the visual image facilitates understanding the condition of the bedrock. Large-scale seismic tomography has been used to monitor the sedimentary rocks around the tunnels in the Tono mine [3, 4] and the rocks around the tunnels in the Swedish URL [5]. These seismic tomography investigations took place before and after the tunneling, and were used as a means to record the changes in the vicinity of the tunnel caused by excavation. As such surveys require large and heavy equipment, repeated surveys are not easy. If a seismic tomography investigation were simple enough to be carried out between tunneling operations, the change of the properties of the bedrock around the tunnel could be monitored as the tunneling progre
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