Inference of Methane Bubble Rise Speed Using Broadband Acoustic Backscattering Techniques

Andone Lavery, Applied Ocean Physics & Engineering

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2015 OEI Funded Project

Abstract

Methane gas seeps of microbial and thermogenic origin have been observed throughout the oceans, with widespread seeps recently reported along the northern US Atlantic margin [Skarke et al, 2014].  Some of the methane (CH4), a greenhouse gas, seeping from the seafloor may directly reach the atmosphere and has the potential to contribute to climate [Judd, 2004; Kessler, 2014] and some is dissolved in the water-column and has the potential to contribute to ocean acidification.  Many factors contribute to the transport and ultimate fate of methane seeping from the seabed, including the depth of the seabed source of methane, presence of surfactants, ocean currents, dissolution rates, initial size of the bubbles, and bubble rise speed.  There is substantial evidence indicating the importance of a hydrate coating significantly affecting the dissolution rate of the methane bubbles if the bubbles originate below the gas-hydrate stability zone (GHSZ).  This decreased dissolution rate results in extended methane bubble lifetimes and allows the bubbles to rise well above the depths where they would otherwise be expected [Rehder et al, 2002, 2009; McGinnis, 2006].  It is necessary to understand the impact of these myriad factors, many of which are not well constrained or even independent, in order to understand the ultimate fate of methane seeping from the seabed and to estimate the total contribution of methane to the atmosphere.  Advanced sonar technologies have been instrumental in the discovery and imaging of methane gas seeps, including single and split-beam scientific echosounders, and more recently, multibeam sonar mapping systems [Merewether et al, 1985; Greinert et al, 2006, Heeschen et al, 2003; Weber et al, 2014].  For the most part, however, these advanced acoustic technologies are narrowband and/or hull-mounted, and though very well suited to locating the gas seeps and imaging the plumes in the water-column, these systems are not as well suited to quantify parameters such as bubble size and/or rise speed.  Instead, these later measurements are typically performed in situ by high-resolution cameras [Leifer and McDonald, 2003; Weber et al, 2014].
 
Emerging broadband acoustic scattering techniques [Lavery et al, 2010; Stanton et al, 2010] were recently used to image and quantify methane bubbles escaping from the seabed at depths below the methane GHSZ in the Gulf of Mexico.  The broadband acoustic systems were deployed on the ROV Hercules as a tag-along experiment on a recent Gulf Integrated Spill Response (GISR) Consortium research expedition on board the E/V Nautilus from 8-21 April 2015, funded by BP/Gulf of Mexico Research Initiative (GoMRI).  The unique feature of broadband techniques is the ability to map the frequency spectrum of sound scattered from different targets, in this case, methane gas bubbles.  The primary goal of this effort was to exploit the location of the gas bubble resonance to remotely determine the size of methane bubbles.  This is only possible at the relatively close ranges (10s of meters) enabled by the ROV, as hull mounted systems simultaneously ensonify multiple bubbles in the acoustic sampling volume, and at very far ranges relevant to ship board measurements the bubbles only partially fill the acoustic scattering volume.  A secondary benefit of broadband systems is the greatly improved range resolution.  Exploiting this resolution, it was possible during this research expedition to acoustically image and track individual bubbles over large portions of their ascent.  This leads to the unique possibility of determining the rise speed of individual bubbles of different sizes at different elevations above the sea bed, which is important as the fate of methane bubbles depends strongly upon their size.  This proposal requests funding to perform the rise-speed analyses on the unique acoustic data collected during this recent research expedition.