Research on Active monitoring of crustal state using phased array source

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We have investigated the efficiency of controlled seismic source with phased array for the active monitoring within the earth's crust. The purpose of the research is to investigate (1) the efficiency of the phased array, (2) efficiency of source and receiver array, (3) effect of heterogeneity of the structure and (4) detectability of temporal change with long-term operation.To achieve these purpose, we have re-install the ACROSS (Accurately-Controlled Routine-Operated Signal System) vibrators in the Mikawa observatory of Nagoya University. The vibrator has been being operated in Kagamihara, in Gifu prefecture, and consists of four vibrator units. It was intended to generate a linear motion in arbitrary horizontal direction. As this function has been achieved in different way, we relocate the vibrator from original site to Toyohashi in order to investigate the effect of phased array. In Toyohashi site two vibrator units are separately deployed with 20m separation. The two units are operated in the same time and same frequency range with slight difference in center frequency. The operation is made using PCs that are synchronized to GPS time. The phased array requires several other tips for trouble-less operation, including newly-designed oil lubricator, signal transmission line, anti-vibration insulator, etc. After we have checked operation ability to fix the problem, we installed receiver arrays around the source with a distance of about one kilometer. We operated the source for two months to check the effect of source and receiver arrays. The result of analysis will be soon reported in papers and meetings.