We carried out seismic array observation in mining tunnels of Nakatatsu mine, Fukui, Japan. This observation site is located at about 20 kilometers away from the earthquake fault of Nobi earthquake (1891, M8.0). The major purposes in this observation are: 1) detection of small-scale heterogeneity around the fault; and 2) development of technique to explore structure by using seismic array data. Eighty seismometers which have three components with natural frequency of two hertz were deployed. The observation continued from September 2009 until the middle of February 2010. The sensors were connected to the recording system by cables. The data were converted to digital data by 24bit delta-sigma converter with maximum sampling interval of 1 ms and extracted from memory to storage by self-triggering algorithm in the system. The recording time is 16 – 64 sec. More than 100 earthquakes were recorded in the observation. We were able to obtain seismograms of not only local earthquakes but also distant earthquakes. The good environment for observation with hard rock site and low artificial noise allowed us to obtain high quality data. Some records of the earthquakes have adequate S/N ratios up to hertz. In addition, many scattered phases which could be generated by small-scale heterogeneity in the mid- to lower crust in this area were observed with sufficiently large amplitude
