An experiment revealing the ability of a side‐scan sonar to detect CO2 bubbles in shallow seas

With the aim of using side‐scan sonar (SSS) in marine monitoring at offshore CO2 storage sites, an experiment was conducted in a bay with a flat seabed, approximately 32 m deep. CO2 bubbles released at regulated rates between 500 and 5000 mL min−1 were searched for with SSS to elucidate the detection limit, which depends on the towing speed and the altitude of SSS, and the horizontal distance between the release point and SSS. When the towing speed was less than 6 knots, SSS was able to detect CO2 bubbles released at 1000 mL min−1 (approximately 4 tonnes year−1), which is an extremely small rate compared with injection rates in commercial scale storage, O(106) tonnes year−1. The time required for searching an area of 1 km2 with SSS was estimated to be between 1.7 and 3.4 hours, assuming that SSS was towed at an altitude of 27 m. Thus, SSS was demonstrated to be serviceable for marine monitoring. pH measured at 5 m away from the release point dropped by larger than 0.05 four times during the experiment. Only one of them might have been due to the CO2 release, and the others were due to intrusion of water from deeper layers because temperature, salinity and dissolved oxygen simultaneously changed. This indicates that bubble searching with SSS could detect smaller‐scale CO2 leakages than locating anomalous carbonate values, although the latter is still important in case all or most of CO2 dissolve before the emission to the sea. © 2020 Society of Chemical Industry and John Wiley & Sons, Ltd.