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 × 106 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. Plain Language Summary: Understanding the rate of gas release from natural ebullition sites, such as pockmarks, into the water column is a major factor in understanding the input of greenhouse gases, such as methane and carbon dioxide, into the global ocean system. The detection and quantification of gas flux in the marine environment have relied upon acoustics. However, current active acoustic methods are mainly based on single‐frequency quantification, which can never unambiguously quantify the gas flux due to the bubble size distribution and the scattering across a range of frequencies, and lead to an ill‐conditioned inversion problem. This paper proposes a solution to this dilemma using two elements. First, we employ a wider range of frequencies than previously used, so that more of the bubble resonances are encompassed. Second, it assumes a form for the bubble size distribution, further constraining the solution and effectively regularizing the inversion. The broadband methodology enables us to quantify gas flux with frequencies spanning the resonances of all the bubbles in the plume, allowing more accurate quantification of natural and anthropogenic gas plumes in the water column. Key Points: A broadband active acoustic method for determining the bubble size distribution and gas flux of gas plumes in the water column is presentedImaging of the methane plume in the North Sea showed two distinct arms with larger bubbles and smaller bubbles, respectivelyTotal in situ methane flux from the pockmark into the water column is quantified as between 1.6 and 2.7 × 106 kg/year (272 to 456 L/min) [ABSTRACT FROM AUTHOR]