Evaluation of passive acoustic methods for ambient noise baseline and gas flow rate quantification at a proposed nearshore carbon capture and storage site in Australia

Measurement, monitoring, and verification (MMV) is an integral component of carbon capture and storage (CCS) projects, providing valuable feedback for management practices and assuring integrity. Within an operational MMV equipment, hydrophone-based passive acoustic techniques are used to establish ambient noise baseline and flow rate quantification at short range, specifically to facilitate ‘detect-attribute-quantify’ sequence of an MMV program. But nearshore environments are acoustically complex with overlapping components of biophony, geophony, anthropophony, and deployment noise resulting from multiple instruments used for MMV. These soundscape components can disproportionately dominate ambient noise levels, potentially masking acoustic signatures of bubbles used to quantify seabed gas seeps. Therefore, a robust baseline describing ambient noise variability across the range of frequencies associated with acoustic emissions of gas seeps is required, from which changes can be detected and monitored. In this context, multi-year hydrophone measurements (20 Hz to 24 kHz) from a proposed nearshore CCS site in Australia are analysed to establish a temporally resolved baseline, identifying key drivers causing overall ambient noise variability. These results are compared with acoustic bubble spectrum features and flow rate estimates from a controlled in situ gas release experiment. The experiment was performed using a custom-made bubble generator at 1, 10, and 25 m range from the hydrophone to understand the likelihood of detecting bubbles and quantifying flow rate at the proposed CCS site. Despite the complexities of nearshore environment, the evaluation highlights that low-cost hydrophone-based passive acoustic methods can provide a practical solution to complement quantification component of operational MMV programs.