HR: 1340h
AN: OS43A-0617    [Abstracts]
TI: Advances in Optical Characterization of Methane Seeps and Bubble Plumes
AU: Pizarro, O
EM: o.pizarro@acfr.usyd.edu.au
AF: Woods Hole Oceanographic Institution, 266 Woods Hole Rd, Woods Hole, MA 02543 United States
AU: Farr, N
EM: nfarr@whoi.edu
AF: Woods Hole Oceanographic Institution, 266 Woods Hole Rd, Woods Hole, MA 02543 United States
AU: Camilli, R
EM: rcamilli@whoi.edu
AF: Woods Hole Oceanographic Institution, 266 Woods Hole Rd, Woods Hole, MA 02543 United States
AU: * Whelan, J
EM: jwhelan@whoi.edu
AF: Woods Hole Oceanographic Institution, 266 Woods Hole Rd, Woods Hole, MA 02543 United States
AU: Martens, C
EM: cmartens@unc.edu
AF: Univesity of North Carolina, Dept of Marince Sciences 12-4 Venable Hall, Chapel Hill, NC 27599 United States
AU: Goudreau, J
EM: jgoudreau@whoi.edu
AF: Woods Hole Oceanographic Institution, 266 Woods Hole Rd, Woods Hole, MA 02543 United States
AU: Mendlovitz, H
EM: mendlovitz@unc.edu
AF: Univesity of North Carolina, Dept of Marince Sciences 12-4 Venable Hall, Chapel Hill, NC 27599 United States
AU: Camilli, L
EM: zenwaverider@yahoo.com
AF: Moss Landing Marine Laboratories, 8272 Moss Landing Road, Moss Landing, CA 95039 United States
AB: Methane seeps are potentially a key contributor to the atmospheric methane reservoir 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. At less active, deep water seeps a large portion of the migrating gas is dissolved and oxidized before reaching the surface. However, in high-intensity, shallow water methane seeps the bubble density, speed and size are such that a significant fraction of the gas may reach the atmosphere. New types of in-situ chemical sensors are now available to quickly and reliably quantify dissolved methane throughout the water column. However, quantifying methane within the water column in the free gas phase (i.e., in bubbles) remains a challenging problem. 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, have a surface expression that introduces some noise, 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. Video-rate optical imagery from 3 cameras is used to generate precise measurements of the motion of bubbles. Lighting is provided by a distributed array of LED modules synchronized to the cameras. In order to conserve power and extend deployment times the system can be configured to be dormant until triggered by chemical sensors indicating high concentrations of methane. Plume characterization is based on the identification of the individual bubbles (and rejection of other particles). Additional image processing steps are then used to estimate each bubble's volume and velocity. The results are then integrated to produce an estimate of volumetric flux rate. This technique can also reveal fine scale variability in the spatial and temporal structure within the plume. The imaging package was field-tested over shallow, gas emitting coastal sediments on a fixed moorings together with an array of oxygen, methane and other chemical sensors. Preliminary results from the field plus flume tests with ground truthing suggest that vision-based sensing is a viable alternative approach for determining gas bubble fluxes.
DE: 1694 Instruments and techniques
DE: 4211 Benthic boundary layers
DE: 4262 Ocean observing systems
DE: 4825 Geochemistry
DE: 4894 Instruments, sensors, and techniques
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005