Quasi-continuous CO₂ monitoring based on combined active-source and microseismic FWI

Xiaohui Cai, Kristopher A. Innanen, Tianze Zhang

Monitoring the subsurface evolution of injected CO₂ requires seismic methods that provide both high spatial resolution and sufficient temporal sampling. Active-source time-lapse full-waveform inversion (FWI) can resolve detailed velocity changes but is typically performed only during infrequent repeat time-lapse surveys, leaving long intervals during which plume migration is unknown. Microseismicity recorded by downhole DAS provides a quasi-continuous passive wavefield, offering potentially denser temporal monitoring. To assess the potential of microseismic data for CO₂ monitoring, we develop an active–passive time-lapse FWI that incorporates passive microseismic wavefields through robust, source-tolerant misfit functions. A source-estimation procedure together with an envelope misfit function, is incorporated into the FWI strategy. Numerical experiments for six CO₂-injection stages demonstrate that the method recovers the primary velocity anomaly even when the assumed passive-source wavelet differs substantially from the true one, and that it significantly improves stability relative to conventional passive FWI. The results show that passive wavefields can effectively complement sparse active surveys and provide a practical route toward cost-effective, time-dense CO₂-storage monitoring.