Bubble Detection with Side-scan Sonar in Shallow Sea for Future Application to Marine Monitoring at Offshore CO 2 Storage Sites

Abstract

An important challenge in the offshore storage of carbon dioxide (CO2) in deep geological formations is how to monitor storage sites to detect CO2 leakage in the event that it occurs. A promising candidate for the monitoring is to detect CO2 bubbles in the water column using some kinds of sonar. However, the detectability of bubbles is not well known. Here we show the ability of side-scan sonar (SSS) to detect air bubbles in the water column through an in-situ experiment, where two sizes of air bubbles, about 1 cm and 1-2 mm in diameter, released at the seabed around 6 meters deep are observed with SSS. The principal results are the following. When the slant distance between SSS and bubbles is longer than the distance between SSS and the seabed, the detection of the bubbles is difficult because of the echoes from the seabed. Tiny bubbles are much easier to detect with SSS than bubbles with diameter of about 1 cm if the release rates are the same, and the detection limits of the release rate are estimated to be not larger than 20 ml/min for tiny bubbles, and not smaller than 20 ml/min for bubbles with diameter of 1 cm, under the present experimental conditions. Although the results cannot be directly applied to CO2 bubbles because CO2 bubbles are much easier to dissolve in seawater than air bubbles, it is estimated that CO2 bubbles leaking at 4.76 tonnesCO2/year could be detected. This leakage rate is smaller than the release rate at a controlled sub-seabed CO2 release experiment, called QICS, and therefore, SSS will be a useful tool to monitor offshore CO2 storage sites.