Acoustic estimates of methane gas flux from the seabed in a 6000 km² region in the Northern Gulf of Mexico

Abstract

Seeps of free methane gas escaping the seabed can be found throughout the ocean basins. To understand the role of methane gas seeps in the global carbon cycle-including both gas added to the atmosphere and that which is dissolved and potentially oxidized in the ocean volume-it is important to quantify the amount of methane escaping the seabed. Few large-scale mapping projects of natural methane seeps have been undertaken, however, and even among these, quantitative estimates of flux are rare. Here we use acoustic mapping techniques to survey 357 natural methane seeps in a large region (6000 km 2 ) of the northern Gulf of Mexico and outline a general approach for methane seep mapping using a combination of multibeam and split-beam echo sounders. Using additional measurements collected with a remotely operated vehicle (ROV) together with the acoustic mapping results, we estimate the total gas flux within the 6000 km 2 region to be between 0.0013 and 0.16 Tg/yr, or between 0.003 and 0.3% of the current estimates for global seabed methane seepage rates.

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

“…For bubbles larger than resonance, σ varies within 5 or so dB; for bubbles smaller than resonance, it decreases precipitously -35 dB for a factor of 2 decrease in r e (Weber et al, 2014). Integrating over the bubble emission size distribution, which is the number of bubbles in a r e bin, passing through the measurement plane combined with the bubble vertical velocity (V Z (r e )), which is a function of r e over the measurement volume yields the total plume cross section if bubbles are acoustically noninterfering (no multiple scattering) and the bubble-sonar interaction is isotropic -i.e., σ B is independent of angle despite bubble eccentricity.…”

Detecting and Measuring Gas Seepage

“…Side-scan sonars and multibeam echosounders are typically employed and numerous examples exist for submarine seepage detection (e.g., Papatheodorou et al 1993;Orange et al 2002;Rollet et al 2006;Judd and Hovland 2007;Weber et al 2014 and references therein). These techniques allowed the development of important theoretical models for the transfer of methane from the seabed to the atmosphere and determined that, in general, gas only reaches the sea surface if the seep is shallower than 300-400 m (Schmale et al 2005;McGinnis et al 2006).…”

Submarine vortices derived from natural gas hydrate conversion: a mechanism for ocean mixing

“…Undirected hydrostatic pressures decrease during bubble ascent resulting in gas exsolution and bubble expansion and fragmentation, which in turn results in increased buoyancy, possible expansion of the bubble plume, and changes in the interfacial tension of hydrate-shelled bubbles. Vortex tube generation could explain the consistent relatively slender morphologies associated with plumes seen in multibeam data (Solomon et al, 2009;Colbo et al, 2014;Skarke et al, 2014;Smith et al, 2014;Tudino et al, 2014;Weber et al, 2014;Garcia-Pineda et al, 2015;Wilson et al, 2015) and recent observations indicate that vertical rise of buoyant plumes may be augmented by spiral flow geometries (von Deimling et al, 2015). Because the reflection and scattering from a bubble plume is a bulk effect, rotation has not been observed in seismic reflection data.…”