Shallow characterization and monitoring of the Aquistore CO2 storage site from spatially windowed surface-wave analysis with a permanent seismic source

We characterized and monitored spatial variation of the shallow subsurface at the Aquistore CO2 storage site, managed by the Petroleum Technology Research Centre, Canada. In this study, a continuous and controlled seismic source system called the ACROSS was used to enhance source repeatability and temporal resolution in the monitoring. To extract spatial variations of the surface-wave phase velocity using a fixed single source, we introduced a spatial window in surface-wave analysis. We succeeded in extracting lateral variation of phase velocities consistent with the shallow geological conditions. We also observed seasonal variation of phase velocities. Higher phase velocities observed in winter could be explained by freezing of water in shallow sediments, and their spatial variation might be related to the difference of the degree of freezing. Furthermore, we observed a mode transition between winter and warmer seasons, suggesting the importance of careful mode identification for robust monitoring. In warmer seasons, our monitoring approach showed high temporal stability, indicating the potential to identify the spatial distribution of shallow CO2 leakage.