Simultaneous Near-Field Monitoring During Active Monitoring by seismic ACROSS

ACROSS (Accurately Controlled Routinely Operated Signal System) is a tool for active monitoring of underground structure and state, in which we transmit and receive stationary signal of seismic or electromagnetic wave continuously and monitor the temporal variation of the propagation characteristics. At the present time, the seismic ACROSS transmitters are in continuous operation at the plural sites in Japan and several experiments attempting to monitor underground structures or seismogenic zone by seismic ACROSS have been carried out. Although various temporal changes have been detected by ACROSS observations, most of those changes are considered to occur at the rather shallow part of the ground. We propose a experimental procedure to monitor the neighborhood of a receiver during receiving ACROSS signal. In this procedure, near-field monitoring takes a passive way by use of the ambient noises. The accurate control of the ACROSS signal enables us to deal with the signal and the background noise separately. Because the spectrum of the ACROSS signal is consisted by a group of equally spacing line spectra whose widths are very narrow, we can analyze the remaining parts of the spectra as ambient noise. We developed a simple algorithm for interpolation of the data in which the frequency components of ACROSS signal are absent. The seismic array enables us to distinguish the arrival direction of seismic wave. Array analysis of the ACROSS signal provides us with the information about the propagation path of the arrival phase which involves the temporal change. On the other hand, analysis of the background noise recorded by the seismic array provides the information about the structure near the receiving site. We present the application of the dispersion analysis of the surface-wave phase velocity and seismic interferometry to retrieve the Green"fs function between two seismometers in the array through numerical experiments. In order to apply this procedure to actual observations, some arrangement is necessary, especially in the data stacking system.