In-situ Optical Characterization of Methane Seeps and Bubble Plumes

Methane seeps are potentially keys contributors to atmospheric methane and to the global greenhouse gas budget. Improved estimates of methane flux from ocean floor seeps are required to understand the magnitude and characteristics of this potential source. In high-intensity methane seeps the bubble density, speed and size may permit a significant fraction of the gas to reach the atmosphere. However, quantifying methane within the water column in the free gas phase (i.e., in bubbles) remains challenging. Current approaches rely either on indirect acoustic methods or direct collection of bubbles. Acoustic methods have the disadvantage of requiring extensive calibration, and can fail to distinguish the bubble signal from other sources of acoustic noise. Gas-capture techniques are mechanically complex and can potentially alias episodic events. In both cases the fine scale structure such as heterogeneity of the rising bubbling plume is lost. We describe a vision-based system to characterize bubble plumes and the seep features from which they emanate. Image data is processed to estimate each bubble's volume and velocity; then integrated to produce an estimate of volumetric flux rate. This technique can reveal fine scale variability in the spatial and temporal structure within the plume. The system can be configured to be dormant until triggered by chemical sensors indicating high concentrations of methane in order to conserve power and extend deployment times. The imaging package was deployed over a methane hydrate site in the Mississippi Canyon for several days on a fixed mooring together with an array of chemical sensors. Preliminary results from field and flume tests suggest that vision-based sensing is a viable approach for determining gas bubble fluxes.