Sonar Gas Flux Estimation by Bubble Insonification: Application to Methane Bubble Fluxes from the East Siberian Arctic Shelf Seabed

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 to derive the in situ calibration curve, Q(σ). Non-linear curves relating volume flux, Q, to sonar return, s, for a multibeam echosounder (MBES) and a single beam echosounder (SBES) for a range of depths demonstrated significant bubble-bubble acoustic interactions – precluding the use of a theoretical calibration function, Q(σ), wherein bubble σ(r) scales with the radius, r, size distribution. Bubble plume sonar occurrence, 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 strongly depth dependent. Ψ(σ) for strong s largely was depth-independent, consistent with bubble plume behavior where large bubbles in a plume remain in a focused core. As a result, Ψ(σ) was bimodal for all but the weakest plumes. Ψ(σ) was applied to sonar observations of natural arctic Laptev Sea, seepage including accounting for volumetric change with a numerical bubble plume. Based on MBES data, values of total mass flux, Qm, the mass flux, were 5.56, 42.73, and 4.88 mmol/s with good to reasonable agreement between the SBES and MBES data (4–37 %) for total Qm. Seepage occurrence, Ψ(Q) was bimodal, with weak Ψ(Q) in each seep area well described by a power law, suggesting primarily minor bubble plumes. Seepage mapped spatial patterns suggested subsurface geologic control attributing methane fluxes to the current state of subsea permafrost.