Time-lapse monitoring of shallow subsurface in the Aquistore CO₂ storage site from surfacewave analysis using a continuous and controlled seismic source

We performed time-lapse surface-wave analysis to monitor the shallow subsurface at the Aquistore CO₂ storage site, managed by the Petroleum Technology Research Centre, Canada. A continuous and controlled seismic source system called the Accurately Controlled Routinely Operated Signal System (ACROSS) is used to enhance the temporal resolution and source repeatability in the monitoring. We extracted hourly-variation of surface-wave phase velocities from continuous seismic data with 4 hour stacking. As a result, we could monitor phase velocities within 1 % accuracy during 1-9 days in the frequency range of 4.5-6 Hz. We identified 2-5 % seasonal variation of phase velocities. The high phase velocities observed in winter can be explained by the degree of freezing of partially saturated rock. Our time-lapse results contributed to improving the accuracy of monitoring deep reflections from the CO₂ injection reservoir by correcting seasonal variations of nearsurface velocity. The high temporal resolution and accuracy of our monitoring results have the potential to identify sudden changes such as CO₂ leakage from CO2 storage sites.