To detect CO2 leakage from CO2 geological storage, we describe a seismic monitoring method using a continuous and controlled seismic source system, the Accurately Controlled Routinely Operated Signal System (ACROSS). The method applies surface-wave analysis to monitor the shallow subsurface from the temporal-variation (time-variation) of surface-wave phase velocity. Our numerical simulation study for CO2 leakage through fault zones indicated that the spatial distribution of leaked CO2 can be estimated from small temporal-variation of local phase velocities (~1-3%). To demonstrate the method in a field case, we analyzed continuous seismic records acquired with ACROSS. We clearly extracted a dispersion curve of surface waves in the frequency range excited by the ACROSS (5.015-15.015 Hz). In particular, we obtained reliable estimates of phase velocities in 10-15Hz frequency range, in which the time-variation of phase velocities was better than 1% accuracy. This temporal stability was sufficient to allow us to detect changes in phase velocities associated with CO2 leakage before leaked CO2 reached the surface.
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