Characterizing Variability in Pacific Northwest Methane Seeps Using a Fleet of Small AUVs

Searching for and Sampling Methane Seeps

To collect these data, this research team is developing new robotic sampling techniques using AUVs with advanced capabilities that eventually will be able to search for and intensively sample seeps without human involvement.

During this expedition, they used a combination of traditional (shipboard) and novel (AUVs) sampling tools to achieve their goals. Among the most important tools were the acoustic sonars. To make sure the data collected by the shipboard and AUV sonars were accurate in relation to each other, the team calibrated them with an artificial bubble source (a dive tank) in the controlled environment of Seattle’s Lake Washington before heading out to deeper waters.

Once at sea, with a storm on its way, the team went right to work, targeting an area with previously reported seeps. At night, researchers surveyed the area for seeps using a shipboard pole-mounted multibeam sonar and a CTD (conductivity, temperature, depth) rosette. Their survey showed a surprisingly large and intense seep that they nicknamed "Mondo."

Information about Mondo was shared with the dayshift, who then deployed the small-class AUVs to study it more closely. They used Virginia Tech 690 AUVs with 3D side-scan sonar to locate the seep and commercially available REMUS 100 AUVs with a high-resolution forward-looking sonar and custom sensors to perform more intensive in situ sampling (e.g., measure bubbles and dissolved methane). Together, the AUVs were able to locate the bubble plume and then sample the water in and around it. It's much easier to sample a seep in situ with an AUV than with a CTD rosette dangling from a ship. And, the close-up acoustic data collected by the AUVs is more accurate than those data collected by shipboard sonars, which may be hundreds of feet from a seep's source.

Mondo wasn't the only seep in the area, or the only one visited, but it had the strongest signal, which made it a good target for testing of the AUVs.

Next Steps

Back on shore, the team will synthesize the data and estimate seep emissions and analyze water samples collected with the shipboard CTD rosette. Outcomes from this expedition will inform ongoing development of the AUVs and their suite of tools, which will be further tested during additional lake tests and a second expedition planned for 2024 targeting deeper seeps.

Deploying the AUVs

For the second part of the expedition, the team brought the AUVs, targeting known seep sites on Heceta Bank — an area known for having abundant seepage — and hoping for the best. They found two sites with active seeps, and deployed the CTD and AUVs at the larger of the two, where vigorous bubbling was occurring at a shallow enough depth for the gas to potentially reach the atmosphere (approximately 40 meters/131 feet in spots). Once again they used Virginia Tech 690 AUVs and REMUS 100 AUVs. They used the 690 AUVs to locate the seeps and then the REMUS 100s to sample and further characterize them.

What Comes Next

With the two expeditions now behind them, the team will wrap up the project by processing, analyzing, and interpreting the data from this year’s expedition and comparing it to the seep data they collected during last year’s expedition. Preliminarily, both sites are in roughly 90 meters (295 feet) of water and consist of a cluster of seeps. However, last year's cluster had one large seep (nicknamed “Mondo”); this year’s did not have any large seeps.

Ultimately, this project will improve our understanding of how methane from these seeps impacts the ocean and atmosphere, both positively and negatively. It will also deliver a new and proven way to explore and characterize — and potentially discover — methane seeps.