We are proposing a new geophysical exploration system named ACROSS (Accurately Controlled Routinely Operated Signal System). The purpose of this study is to monitor temporal variations of propagation properties of seismic waves in the crust. The system uses precisely controlled sinusoids, which gives a best way to achieve high signal-to-noise ratio without destroying surrounding ground. The system is deployed near Nojima fault which ruptured during 1995 Kobe earthquake to monitor the temporal variation of the coupling state. In our experimental site, two ACROSS sources that generate 20ton-f centrifugal force are firmly fixed in the ground.
The emitted elastic wave is received with seismometers deployed at 800m and 1700m deep boreholes near the fracture zones of Nojima fault.
To estimate the effect of water diffusion on the seismic velocity and Q, we continuously operated ACROSS system during the period including three runs of water injection; January 22-26, January 31-Februay 5 and March 3-11. The pumping pressure of each run was 3MPa, 3.5Mpa and 4MPa, respectively. We analyzed the temporal variation of travel time at the bottoms of two boreholes, where geophones are deployed. In both records common large variation up to 1 m sec is observed and can be attributed to the temporal velocity variation near the surface. To reduce it we subtract the travel time at the sensor of the shallower hole from that of the deeper hole. As the sensors at two boreholes and ACROSS source are on a straight line, the temporal variation of the relative travel time reflects that of the portion between two sensors. As a result of the operation the variation is reduced by a factor of ten, which corresponds to the velocity variation of 0.04% and 0.02% for P and S wave, respectively. The result shows, however, no meaningful change associated with water injection. This result shows that the injected water flowed in preexisting cracks and channels without changing crack density and water content.
We estimated the resolution of travel time due to the background noise. They are of the order of 10 microseconds, which is 1/10 of the observed fluctuation. This means that there is still room for a reduction of the data variation down to this order. The possible measures are to make a correction of 'source function'. The movement of the source varies depending on condition of the surrounding rock, and should be modeled as a source function. The correction will reduce the variation possibly by a factor of 10.