An acoustic estimate of methane concentration in a water column in regions of methane bubble release

Abstract

A remote acoustic method is presented for estimating the methane flux into water from rising bub bles. With the proposed method and data of hydroacoustic measurements in the Sea of Okhotsk, the dissolved methane concentration profile that forms in the water column in regions of methane bubble release is esti mated. Comparison of the aforementioned methane concentration profile with results of direct measure ments shows good quantitative and qualitative agreement. This testifies to the fair accuracy of the proposed acoustic method and is indicative of the dominant role of bubble transport in the formation of the concentra tion of dissolved methane in the water column in such regions.

Sonar Estimation of Methane Bubble Flux from Thawing Subsea Permafrost: A Case Study from the Laptev Sea Shelf

“…Quantifying a methane bubble flux from sonar data is challenging and poorly amenable to laboratory modeling. Currently the problem has been solved by combined theoretical calculations, laboratory testing, and field experiments [12,13,15,[22][23][24][25][26][27][28]. We consider several methods of remote active sonar estimation of bubble methane fluxes based on data from ship-mounted single-and multibeam echosounders operated in a monostatic mode when the acoustic transmitter also acts as a receiver.…”

“…The size dependence of the rise velocity [37] can be used to estimate the equivalent spherical radius r i and hence the volume ϑ i of each i-th bubble. The observed maximum height of gas plumes (also called flares) provides another tool for estimating bubble sizes [15,22,27]. It corresponds to the sea depth where the plumes are still seen in echograms as continuous zones of high scattering.…”

“…If single bubbles in a gas plume are irresolvable but their size, velocity, and shape distributions are known, the methane flux can be inferred from the total back scattering cross section [11,12,15,22,27,38,39], assuming that it equals the sum of cross sections for single bubbles, in the single scattering approximation [40]. In this approximation, the cross section of backscattering from a bubble cloud σ bs is given by [29]:…”

“…The bubble gas flux F (moles per second) is related with the size and shape distribution functions of n(r) and Φ(r), respectively [15,27], as…”

“…Strictly speaking, the 38 kHz frequency often used for methane flux estimation [21,28] is too high, because kr = 1 already at r = 6 mm. However, the validity of (5) can be extended into the kr > 1 domain to a satisfactory accuracy if the real nonspherical shape of bubbles is taken into account [27]. On the other hand, according to (5), the backscattering cross section of bubbles smaller than the resonance radius, decreases with decreasing bubble size at r 6 , i.e., the contribution of these bubbles to the backscattered signal is vanishing, which is especially essential for deep-sea seeps.…”