Research paperSeabed bubble flux estimation by calibrated video survey for a large blowout seep in the North Sea

•A new ROV video survey approach was shown to quantify seep bubble emissions.•Best estimated 22/4b seabed emissions was 90 L s−1 in a likely range 50-142 L s−1.•Seepage was constrained largely to a spatially limited ellipse in the crater bottom.

A new approach to estimating seabed bubble emissions was applied to a ROV video survey of the 22/4b blowout crater in the UK North Sea. Video was analyzed to identify and classify bubble plumes and other seabed features. Plume occurrence then was gridded into quadrats and the total number of plumes in the crater's active portion estimated. Almost all of the seepage in the 40 × 45 m crater was localized within a 20 by 18-m ellipse. All five major plumes in this ellipse were measured by direct capture with mean emission of 1.34 L s−1 at 12 bar (range 0.54-3.5 L s−1). The primary source of variability (71%) was temporal, demonstrated by repeat measurements of a single vent. Overall, temporal variability was consistent with highly fluid migration through a thick, near-seabed, coarse-grained sediment bed underlying the 22/4b crater seabed. Occurrence data were converted to flux based on direct capture measurements, survey video, and laboratory experiments where video was collected of bubble plumes of known flow for comparison with field video. The derived best-estimate, total seabed emission flux was 90 L s−1 with an estimated uncertainty of 50%, within a scenario uncertainty range of 50-142 L s−1. This is equivalent to 100 million L dy−1 at Standard Temperature and Pressure or an annualized 25 kton. However, long-term monitoring data showing large temporal variability suggests extrapolation to an annual basis is inappropriate.