Passive acoustic monitoring of a natural CO2 seep site – Implications for carbon capture and storage

stimating the range at which an acoustic receiver can detect greenhouse gas (e.g., CO2) leakage from the subseabed is essential for determining whether passive acoustic techniques can be an effective environmentalmonitoring tool above marine carbon storage sites. Here we report results from a shallow water experimentcompleted offshore the island of Panarea, Sicily, at a natural CO2 vent site, where the ability of passive acousticsto detect and quantify gas flux was determined at different distances. Cross-correlation methods determined thetime of arrival for different travel paths which were confirmed by acoustic modelling. We develop an approachto quantify vent bubble size and gas flux. Inversion of the acoustic data was completed using the modelledimpulse response to provide equivalent propagation ranges rather than physical ranges. The results show thatour approach is capable of detecting a CO2 bubble plume with a gas flux rate of 2.3 L/min at ranges of up to 8 m,and determining gas flux and bubble size accurately at ranges of up to 4 m in shallow water, where the bubblesound pressure is 10 dB above that of the ambient noise.