Characterizing Variability in Pacific Northwest Methane Seeps Using a Fleet of Small AUVs (RC 101)
August 11-13, 2023
Research Vessel Rachel Carson
Off the Coast of Washington
From August 11 to 13, 2023, a team of researchers visited known methane seeps off the coast of Washington to test their new approach to mapping and characterizing methane seeps over wide areas. With easily deployable and relatively inexpensive autonomous underwater vehicles (AUVs) equipped with imaging sonars and custom sensors, they successfully located and mapped seeps and measured associated bubbles and dissolved methane right at their source and up through the water column.
Why Explore Methane Seeps?
Methane is a potent greenhouse gas, and it's released into the ocean from the seafloor as gas bubbles via methane seeps. Despite the prevalence of seeps along continental margins, data is limited, and their impacts on the ocean and atmosphere, both positive and negative, are largely unknown. To improve our understanding of these emissions, how they influence life on the seafloor and in the water column as well as their role in the global carbon cycle, high-resolution, close-up acoustic data focused at the seafloor source and co-located physical and chemical data from a variety of seeps, at a variety of depths, are needed.
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.
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.
Education theme pages provide the best of what the NOAA Ocean Exploration website has to offer to help educators in the classroom incorporate this project into their curriculum. Each theme page includes expedition features, lessons, multimedia, career information, and associated past projects and expeditions.
- Exploration Tools: Autonomous Underwater Vehicles | Sonar | CTD | Research Vessel Rachel Carson
- Ocean Fact: What is the difference between cold seeps and hydrothermal vents?
- Expedition: Gradients of Blue Economic Seep Resources
- News: Seafloor Mapping Data Reveals Large Number of Gas Seeps Off U.S. West Coast
- Additional project information
- University of Washington Applied Physics Laboratory
- Virginia Tech Bradley Department of Electrical and Computer Engineering
- Oregon State University/NOAA Cooperative Institute for Marine Ecosystem and Resources Studies
- University of Washington/NOAA Cooperative Institute for Climate, Ocean, and Ecosystem Studies
- NOAA Pacific Marine Environmental Laboratory
NOAA Ocean Exploration
Funding for this expedition was provided by NOAA Ocean Exploration via its Ocean Exploration Fiscal Year 2022 Funding Opportunity.
Published November 2, 2023