Broadband acoustic inversion for gas flux quantification ‐ application to a methane plume at Scanner Pockmark, central North Sea

The release of greenhouse gases from both natural and man-made sites has been identified as a major cause of global climate change. Extensive work has addressed quantifying gas seeps in the terrestrial setting while little has been done to refine accurate methods for determining gas flux emerging through the seabed into the water column. This paper investigates large-scale methane seepage from the Scanner Pockmark in the North Sea with a new methodology that integrates data from both multibeam and single-beam acoustics, with single-beam data covering a bandwidth (3.5 to 200 kHz) far wider than that used in previous studies, to quantify the rate of gas release from the seabed into the water column. The multibeam data imaged a distinct fork-shaped methane plume in the water column, the upper arm of which was consistently visible in the single-beam data, while the lower arm was only intermittently visible. Using a novel acoustic inversion method, we determine the depth-dependent gas bubble size distribution and the gas flux for each plume arm. Our results show that the upper plume arm comprises bubbles with radii ranging from 1 to 15 mm, while the lower arm consists of smaller bubbles with radii ranging from 0.01 to 0.15 mm. We extrapolate from these estimates to calculate the gas flux from the Scanner Pockmark as between 1.6 and 2.7 × 10 6 kg/year (272 to 456 L/min). This range was calculated by considering uncertainties together with Monte Carlo simulation. Our improved methodology allows more accurate quantification of natural and anthropogenic gas plumes in the water column.