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Volume 11, issue 3 | Copyright

Special issue: Climate–carbon–cryosphere interactions in the...

The Cryosphere, 11, 1333-1350, 2017
https://doi.org/10.5194/tc-11-1333-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 09 Jun 2017

Research article | 09 Jun 2017

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

Ira Leifer1, Denis Chernykh2,3, Natalia Shakhova3,4, and Igor Semiletov2,3,4 Ira Leifer et al.
  • 1Bubbleology Research International, Solvang, CA 93463, USA
  • 2Russian Academy of Science, Pacific Oceanological Institute, Vladivostok, Russia
  • 3Tomsk Polytechnic University, Tomsk, Russia
  • 4University Alaska Fairbanks, International Arctic Research Center, Fairbanks, AK 99775, USA

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 40m depth with fluxes spanning 0.019 to 1.1Ls−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 (Ψ(σ)) with respect to Q found Ψ(σ) for weak σ well described by a power law that likely correlated with small-bubble dispersion and was strongly depth dependent. Ψ(σ) for strong σ was largely depth independent, consistent with bubble plume behavior where large bubbles in a plume remain in a focused core. Ψ(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 (Qm) were 5.56, 42.73, and 4.88mmols−1 for MBES data with good to reasonable agreement (4–37%) between the SBES and MBES systems. The seepage flux occurrence probability distribution function (Ψ(Q)) was bimodal, with weak Ψ(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.

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Vast Arctic methane deposits may alter global climate and require remote sensing (RS) to map. Sonar has great promise, but quantitative inversion based on theory is challenged by multiple bubble acoustical scattering in plumes. We demonstrate use of a real-world in situ bubble plume calibration using a bubble model to correct for differences in the calibration and seep plumes. Spatial seep sonar maps were then used to improve understanding of subsurface geologic controls.
Vast Arctic methane deposits may alter global climate and require remote sensing (RS) to map....
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