Cross-Configuration 4D DAS-VSP Monitoring for Long-Term CO2 Storage

Safe and reliable carbon capture and storage (CCS) requires long-term monitoring to verify secure CO2 containment. Distributed acoustic sensing vertical seismic profiling (DAS-VSP) provides a practical approach for subsurface monitoring due to its permanent installation, dense spatial sampling, and reduced operational costs. However, monitoring programs spanning decades may require hardware upgrades, including interrogator replacements and changes to fibre-optic cable configurations, affecting time-lapse repeatability and interpretability. This field case study evaluates whether the CO2-induced 4D DAS-VSP response remains interpretable across a combined change in interrogator, fibre type and optical path, whose individual contributions cannot be isolated. The assessment uses data from the CO2CRC Otway Project Stage 4, where approximately 10 kilotons of CO2-rich reservoir gas were injected into a saline aquifer at approximately 1500 m depth. Baseline and monitor surveys using different DAS configurations are compared at two monitoring wells, with a same-configuration case as a repeatability benchmark. An established 4D DAS-VSP processing workflow, supplemented by system-specific noise attenuation, is applied to mitigate configuration-dependent differences. The results show that, despite higher non-repeatability, the principal CO2-related time-lapse anomaly remains spatially localised and distinguishable, indicating that the DAS-VSP response can remain interpretable across a combined configuration change under favourable acquisition and signal-quality conditions.