The Aquistore project in Saskatchewan, Canada provides carbon dioxide (CO₂) storage for the world's first combined commercial power plant and carbon capture and storage (CCS) project. CO₂ has been injected at a depth of 3.2 km since April 2015 and a permanent near surface geophone array provides passive seismic monitoring. The ability to identify any containment breach is a vital part of risk management and reduction for CO₂ storage sites. We therefore investigate the potential to monitor seismic velocity changes following a hypothetical leak of CO₂ from the reservoir using passive monitoring methods. We estimate the expected shear-wave velocity change with CO₂ saturation, and using data from the geophone array we investigate whether ambient noise interferometry (ANI) and a tomographic inversion for Rayleigh wave group-velocity maps could provide a suitable CO₂ leakage detection tool. To assess the repeatability of the method, we conduct, for the first time, a time-lapse ambient noise tomography survey of a CO₂ storage site to cover time periods preceding and following injection start-up. Sensitivity analysis results indicate that usable surface wave data derived from the current array configuration are sensitive to depths of ∼400 m and shallower. We do not expect to observe any changes due to CO₂ migration at such shallow depths and the estimated seismic velocities pre- and post-injection agree to within 60 m s⁻¹, which is on the order of double the predicted velocity change with CO₂ saturation. Therefore, due to uncertainties in travel-time picks (5–15%) and variations in the obtained velocity structure between consecutive days (up to 20%), we would be unable to resolve the expected seismic velocity change with an influx of CO₂ at 400 m (∼3–4%). Additionally, the noise source variability does not allow stable velocity estimates to be made in the time-frame of currently-available data. Consequently, in the event of a CO₂ leak at the Aquistore site, using the standard ambient noise analysis methods applied herein, Rayleigh wave tomography could be deployed to detect velocity changes due to CO₂ saturation only if (a) a wider aperture surface array was in place to allow longer period surface waves to be used, providing sensitivity at greater depths, (b) arrival times of interferometrically-synthesised surface waves could be picked with increased accuracy, and (c) there is stability of the noise source distribution between repeated surveys. However, a map of three-dimensional near surface velocities, as obtained in this study, could nevertheless be useful for near surface static corrections when using active-source seismic reflection surveys to image and monitor the reservoir. More generally, further similar studies are required to assess the applicability of ANI for leak detection at other CO₂ storage sites.

加拿大萨斯喀彻温省的Aquistore项目为世界上第一个联合商业发电厂和碳捕集与封存（CCS）项目提供二氧化碳（CO ₂）储存。自2015年4月以来，CO _2的注入深度为3.2 km，永久性近地表检波器阵列可进行被动地震监测。识别任何密闭违规的能力是风险管理和减少CO ₂储存地点的重要组成部分。因此，我们研究了使用被动监测方法监测假想的CO ₂从储层泄漏后监测地震速度变化的潜力。我们估计了预期的剪切波速度随CO _2的变化饱和度，并使用地震检波器阵列中的数据，我们调查了环境噪声干涉法（ANI）和瑞利波群速度图的层析成像反演是否可以提供合适的CO ₂泄漏检测工具。为了评估该方法的可重复性，我们首次对CO ₂储存地点进行了延时环境噪声层析成像调查，以涵盖注入启动前后的时间段。灵敏度分析结果表明，从当前阵列构造获得的可用表面波数据对约400 m及更浅的深度敏感。我们预计不会在如此浅的深度观察到由于CO ₂迁移引起的任何变化，并且在注入前和注入后估计的地震速度在60 m s内一致。^-1，是预测速度随CO ₂饱和度变化的两倍。因此，由于行程时间选择的不确定性（5％至15％）以及连续两天之间获得的速度结构变化（最多20％），我们将无法解决随着CO ₂涌入而产生的预期地震速度变化在400 m（〜3-4％）。另外，噪声源的可变性不允许在当前可用数据的时间范围内进行稳定的速度估计。因此，在Aquistore站点发生CO ₂泄漏的情况下，使用此处应用的标准环境噪声分析方法，可以部署瑞利波断层扫描来检测由于CO ₂引起的速度变化仅在以下情况下饱和：（a）设置了更宽的孔径表面阵列以允许使用较长时间的表面波，从而在更大的深度提供了灵敏度，（b）可以以更高的精度选择干涉合成的表面波的到达时间，并且（ c）重复测量之间的噪声源分布稳定。然而，在本研究中获得的三维近地表速度图在使用主动震源地震反射法成像和监测储层时仍可用于近地表静校正。更一般地，需要进一步的相似研究来评估ANI在其他CO ₂储存地点进行泄漏检测的适用性。