ABSTRACT
On-site experiments of a new source-receiver system for seismic monitoring are presented. The system uses precisely controlled sinusoids to achieve high signal-to-noise ratio without destroying surrounding ground. The temporal variation of travel time is monitored with the system deployed on the surface of where granite is exposed near the fault of 1995 Kobe earthquake. In this site the sources that generate sinusoidal wave with a centrifugal force of 20 ton-f are used. The source waves are received with seismometers at the bottoms of 800 m- and 1800 m-deep hole. During an experiment lasting one-month, the sources were continuously operated with a frequency modulation around 20 Hz and 30 Hz, which produced plural components of sinusoids of constant frequency interval. Both the sources and the receivers are precisely synchronized to GPS clock, resulting in high signal-to-noise ratio for the acquisition of the sinusoids. Data are stacked for every one hour and 24-hour moving average is applied to the data. Each stacked data is converted into time domain to obtain the temporal variation of travel time. Cross-spectral densities are calculated for P- and S-phases in order to obtain relative variation of travel time among stacked data. The variation between the shallower and the deeper two sensors shows that the resolution of travel time is of the order of 10−4 over 1 km distance.
