Sonar gas flux estimation by bubble insonification: application to methane bubble flux from seep areas in the outer Laptev Sea

Abstract

Abstract. Sonar surveys provide an effective mechanism for mapping seabed methane flux emissions, with Arctic submerged permafrost seepage having great potential to significantly affect climate. We created in situ engineered bubble plumes from 40 m depth with fluxes spanning 0.019 to 1.1 L s −1 to derive the in situ calibration curve (Q(σ )). These nonlinear curves related flux (Q) to sonar return (σ ) for a multibeam echosounder (MBES) and a single-beam echosounder (SBES) for a range of depths. The analysis demonstrated significant multiple bubble acoustic scattering -precluding the use of a theoretical approach to derive Q(σ ) from the product of the bubble σ (r) and the bubble size distribution where r is bubble radius. The bubble plume σ occurrence probability distribution function ( (σ )) with respect to Q found (σ ) for weak σ well described by a power law that likely correlated with small-bubble dispersion and was strongly depth dependent. (σ ) for strong σ was largely depth independent, consistent with bubble plume behavior where large bubbles in a plume remain in a focused core.(σ ) was bimodal for all but the weakest plumes. Q(σ ) was applied to sonar observations of natural arctic Laptev Sea seepage after accounting for volumetric change with numerical bubble plume simulations. Simulations addressed different depths and gases between calibration and seep plumes. Total mass fluxes (Q m ) were 5.56, 42.73, and 4.88 mmol s −1 for MBES data with good to reasonable agreement (4-37 %) between the SBES and MBES systems. The seepage flux occurrence probability distribution function ( (Q)) was bimodal, with weak (Q) in each seep area well described by a power law, suggesting primarily minor bubble plumes. The seepage-mapped spatial patterns suggested subsurface geologic control attributing methane fluxes to the current state of subsea permafrost.

A wideband acoustic method for direct assessment of bubble-mediated methane flux

“…The volumetric gas flux and methane mass flux estimated in this research are consistently lower than flux reported in previously published studies (Shakhova et al, 2010;Weber et al, 2014;Leifer et al, 2017). This is not surprising as both our estimates of volumetric gas flux and subsequent estimates of methane flux represent lower bounds of the true regional flux.…”

A Synthesis Review of Emissions and Fates for the Coal Oil Point Marine Hydrocarbon Seep Field and California Marine Seepage

“…Estimates of global seep emissions are based on a very limited number of measurements. Fluxes for individual marine seep vents have been reported for the Gulf of Mexico [33], offshore Norway [34,35], the North Sea [36], offshore Svalbard in the Norwegian Arctic [37], the arctic Laptev Sea [38], and numerous vents in the COP seep field [39].…”

“…Many authors have noted a relationship between seepage and geologic structures [1,2,97,107,108], including faults, diapirs, outcrops, and slumps. Near-seabed geologic structures such as authigenic carbonates [109] and low permeability sediment layers like clay [110] and ancient river valleys [38] also provide spatial control. These structures relate to low permeability migration pathways and/or allow for hydrocarbon accumulation.…”

“…Although studies long have investigated geological controls on the spatial occurrence of seepage [96], few have investigated seep strength with respect to geological controls; e.g., Leifer et al [102] showed fine-scale (<100 m) spatial control of emissions by geological structures (Figure 12), and Leifer et al [38] showed quantitative multibeam sonar emission data that were proposed related to near-surface geological controls on a meter to 10 m scales in both the COP seep field and the Arctic. Bradley et al [53] showed geological controls on trends in the spatial seepage emission distribution-proposed related to vent activation and deactivation based on overpressure and permeability.…”

Supplementary material to &quot;Satellite ice extent, sea surface temperature, and atmospheric methane trends in the Barents and Kara Seas&quot;

“…The Ob and Yenisei Rivers transport significant sediment, underlying the shallowness of the Kara Sea, with extensive proven and proposed petroleum hydrocarbon reservoirs underlying the east and southeast Kara Sea (Rekacewicz, 2005). Given the Kara Sea's shallowness, CH4 seep seabed bubbles can mostly transfer their gas directly to the atmosphere (Leifer et al, 2017;Leifer and Patro, 2002) and indirectly from wind mixing (Wanninkhof and McGillis, 1999), and also from storm sparging (Boitsov et al, 2012;Shakhova et al, 2013), which in the Arctic can extend to 100-200 m depth, i.e., most of the Kara Sea. Figure S2.…”