Monitoring techniques using Autonomous Underwater Vehicles for potential seepage of CO2 from sub-seafloor storage sites

Although a number of Carbon Capture and Storage sub-seafloor storage sites are now either in operation or planned for CO2, little is known about the effect of potential seepage on marine ecosystems. Here we describe a comprehensive field campaign to the North Sea (RRS James Cook Cruise 77) that used Autosub 6000 to test methods for detection of seepage, including formation fluids, natural gas and CO2, as it passes through the sedimentary overburden and into the water column, and develop monitoring strategies suitable for all offshore carbon capture and storage projects. In this paper we describe the Hugin Fracture, a 2 km long discontinuity imaged on the seabed, and associated fluid flow activity, revealed by geophysical observations including high reflectivity acoustic anomalies within the overburden. Further results in favour of active fluid flow along this fault will be presented, using a combination of multidisciplinary datasets comprising video photography, Eh sensor and sediment samples.