Characterizing structural variation in the shallow subsurface at the Otway Basin, Australia: challenges for environmental monitoring of geological CO ₂ storage

Effective measurement, monitoring and verification (MMV) strategies for geological carbon storage (GCS) and other geological engineering technologies must consider subsurface uncertainties when determining the spatial coverage and resolution of a monitoring programme. MMV of any future leakage to surface or near-surface receptors requires an understanding of the uncertainties in the top 0–100 m, and it is possible that optimal sites for storage may well have non-optimal near-surface geology for characterization of shallow leakage risk. Here we present a multiscale case study of the difficulties in site characterization at such a site. In this paper, we compare field observations from a 45 km section of coastal outcrops and an inland limestone quarry to core and well logs from the Otway International Test Centre (OITC) CO 2 injection project. We find that the type of faulting varies strongly with location, changing from reverse at the coast to strike-slip at the OITC. The observed variability in fault type across this region highlights the need to be cautious when using nearby faults that outcrop at the surface as indicators of local fault architecture, as any variability will impact shallow fluid migration and leakage. This work highlights the need for a better understanding of near-surface processes affecting fault zones, and better methods of geophysical characterization of the shallow subsurface, particularly as there is scale up of GCS projects worldwide.