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Double, double methane and trouble: Quantifying natural and man-made methane seeps

Wang, B., S. A. Socolofsky, J. A. Breier, and J. S. Seewald (2016), Observations of bubbles in natural seep flares at MC 118 and GC 600 using in situ quantitative imaging, J. Geophys. Res. Oceans, 121, doi:10.1002/2015JC011452.

Dr. Binbin Wang and colleagues from Texas A&M and Woods Hole Oceanographic Institution deployed a newly developed, stereoscopic imaging device to observe the bubbles created at natural gas seeps on the sea floor. The team put the set of high frame-rate cameras on a Remotely Operated Vehicle to make accurate estimates of the bubble dissolution and transport rate. These measurements could help inform impact models that first responders use in the case of oil spills and gas leaks.

seep

Figure 1 – Images of two of the sample sites taken from the ROV. The yellow boxes indicate where the insets are focusing on. (Adapted from Wang et al., 2016)

The bubbles Dr. Wang observed are not like the air-filled ones we see on land. The ones he was looking at emerged from the ocean crust filled with methane, a potent greenhouse gas. When the bubbles dissolve, the gas trapped inside is released into the surrounding water. Small amounts of methane, such as those emitted from benthic seeps, are natural and can sometimes support bizarre, chemosynthetic communities. A large influx of methane, however, can have negative effects on both the climate and biological systems.

Consider the 2010 Deepwater Horizon disaster. The oil well blowout released over 4 million barrels of hydrocarbons into the surrounding waters, including approximately 1×1010 moles of methane. Scientists confirmed at the time that all of this methane dissolved into the water using above-surface atmospheric measurements. But estimates of how far from the spill the methane plume extended remain very uncertain.

Accurately modeling the spatial extent of a plume requires knowledge of both the dissolution rate, how quickly the bubbles break, and the transport rate, how quickly the bubbles move. This in turn has important implications for response-level and impact assessment models, two of the tools available to environmental regulators and policy makers. But building reliable dissolution and transport models requires knowing how the bubbles are behaving at the source. Fortunately, scientists haven’t been able to look at a methane seep from another large oil spill.

bubbles

Figure 2 – Some examples of the variability in the shape of the gas bubbles. Panels (b) and (c) are images of the same bubble taken the each of the stereoscopic cameras. Panels (d) and (e) are also a pair images from the two cameras (Adapted from Wang et al., 2016)

Dr. Wang and his team decided to study four natural methane seeps in the Gulf of Mexico as a proxy for a leak from a man-made system (Fig. 1). Recording the gas at the source is difficult; the ones observed for this paper were about 1000m below the surface. Dr. Wang made use of a Remotely Operated Vehicle, or ROV, loaded with scientific toys. They used an acoustic system to study the bubble streams at a large scale, a gas-tight chamber to capture bubbles and bring them to the surface, and a fancy bottle sampler to gather suspended particles near the seep.

The group also recorded images and video of the seeps using two high frame rate cameras in pressure housings. Dr. Wang oriented the cameras so they were pointing at the same place, a configuration called a “stereoscopic rig.” The images from both cameras were then combined to produce 3D reconstructions of the bubble field. The underlying principle is similar to that used in Hollywood to make Ironman pop out of the screen at you.

Instead of making CGI explosions, Dr. Wang used the 3D renderings to make measurements of bubble size and rise velocity. The team counted the bubbles from each vent to quantify the flow rate from the vents. Then, they computed the speed at which the bubbles rose in the water column by measuring the distance a given bubble travels between two frames. The group further looked at how irregularly shaped bubbles rose; non-spherical bubbles have different upward velocities (Fig. 2).

wang_graphs16

Figure 3 – An illustration of the zigzag rising pattern of a non-spherical bubble as measured by the stereoscopic cameras. (a) and (c) show the different components of the motion along the x and y axes as the bubble rises along the z-axis. The graph on the top right shows that same motion projected onto the x-y plane. (d) is a still of the bubble that made these motions. (Adapted from Wang et al., 2016)

Taken together, Dr. Wang was able to use these measurements to calculate the dissolution and transport rate of these methane bubbles. The researchers found that the majority of the bubbles were nonspherical and rose at rates about 40% lower than estimates from existing models. That is because the bubbles took a zigzag path rather then going straight up, effectively reducing the vertical component of their velocity (Fig. 3). The measured dissolution rate matched closely with theoretical values.

More in-depth (ha) studies of plume dynamics are needed before existing models are revised. And, unfortunately, similar measurements will likely have to be made at the site of another large, man-made gas leak to account for variations in the physics of the system. But Dr. Wang’s work represents an important first step toward improving how we inform the steps that are taken to respond to deep water drilling accidents.


 

Dr. Wang and his collaborators have made a number of videos from the ROV dive and of the bubbles rising as Supplemental Materials to their paper. You can check them out here.

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