Broadband Acoustic Inversion for Gas Flux Quantification—Application to a Methane Plume at Scanner Pockmark, Central North Sea

Abstract

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.

Seismic Anisotropy Within an Active Fluid Flow Structure: Scanner Pockmark, North Sea

“…Gravity core observations of sediment fluidization features and disseminated iron sulphide, indicates active fluid flow directly beneath the pockmark. Evidence of methane-derived authigenic carbonates and gas ebullition at this site further demonstrates that active methane venting has persisted since 27 (Böttner et al, 2019;Li et al, 2020). As shown by controlled gas release experiments in weakly consolidated sediment (e.g., Roche et al, 2020), mature gas migration from an active, focused source may take place through stable open conduits.…”

“…The Scanner pockmark is a composite feature comprising two overlapping seabed pockmarks (East and West Scanner), with a combined size of ∼900 m × 450 m wide and depth of 22 m, lying in ∼155 m water depth. Direct evidence for methane venting is provided by the water column imaging of Li et al (2020), who calculate a gas flux of 1.6 and 2.7 × 10 6 kg/year (272-456 l/min), as well as the presence of methane-derived authigenic carbonate (MDAC) recovered from within the pockmarks, which formed due to anaerobic oxidation of escaping methane (Judd and Hovland, 2009). The seismostratigraphy and lithostratigraphy hosting the Scanner pockmark complex comprises a ∼600 m-thick Quaternary sediment succession that has been described previously (Stoker et al, 2011;Böttner et al, 2019;Robinson et al, 2020), where it is sub-divided into five units, S1 to S5 (Figures 2, 3).…”

Estimates of Methane Release From Gas Seeps at the Southern Hikurangi Margin, New Zealand

“…The workflow to estimate gas fluxes at the seep locations is illustrated in Figure 1. To estimate the BSD and density, we followed two approaches, one based on the normalized frequency response of the 18 and 38 kHz channels of the EK60 data, and one based on the non-normalized frequency response, similarly to the approach used by Li et al (2020).…”

“…To test the validity of the results, we analyse the dependence of the estimated methane fluxes on different BSD functions: we first parametrise the BSD by assuming log-normal and Weibull probability density functions (PDF), and then compare the inverted results (Figure 11). The choice of these PDFs was made based on published seep studies, which have suggested several distribution functions to describe bubble size data including normal (Römer et al, 2012), log-normal (Veloso et al, 2015;Wang et al, 2016;Li et al, 2020), and Weibull (Dey and Kundu, 2012).…”

“…Although these point-source measurements provide the most accurate observation of bubble parameters, they require long deployment durations and a restricted field of view of less than ~15 m. Moreover, they are also limited to measurements at the seafloor, and cannot provide a way to track the changes in bubble size distribution as they rise through the water column. Broadband hydroacoustic methods provide a more efficient tool to directly estimate bubble parameters by insonifying large areas of the oceans using a range of frequencies (e.g., Veloso et al, 2015;Colbo et al, 2014;Dupré et al, 2015;von Deimling et al, 2011;Li et al, 2020).…”