Crosshole seismic monitoring of CO ₂ plume evolution with an electric sparker source and distributed acoustic sensors

High-resolution crosshole seismic surveying is a powerful technique for monitoring potential CO 2 leakage in the shallow subsurface and tracking the injection process. A set-up employing a high-frequency seismic source in one well and a fibre-optic receiver array in another enables rapid and efficient seismic data acquisition, with repeat surveys allowing for time-lapse analysis. Such data were collected during the controlled shallow CO 2 release experiment at the CO2CRC's Otway International Test Centre in April 2024. Straight-ray tomography images from the initial surveys revealed a velocity anomaly near the injection interval, which migrated upward following the end of injection. To further investigate the CO 2 plume evolution, three additional surveys were conducted 10, 12 and 14 months after injection ceased. In this study, all crosshole datasets were analysed to provide a comprehensive overview of both short-term and long-term monitoring results. A new data processing workflow was developed, incorporating advanced tomography that accounts for seismic wave refraction caused by velocity variations in the medium. The updated results are consistent with previous findings but exhibit reduced uncertainty. The recent monitoring data indicate that the velocity anomaly near the injection zone persists but has decreased in size, while its further evolution continues at a slower rate compared with the initial post-injection period.