Optical Sensing of Ocean Gasses Using Hollow Core Fibers

Measuring Gas in the Ocean

Beneath the ocean surface, liquid appears to rule the seascape. Organisms are highly influenced by the water pressing down on them, to the extent that their bodies are built to thrive even under extreme pressure, cold, and darkness. Studying the seafloor can reveal pockets of another state of matter, gas, in unexpected places.

Methane gas leaks from crevices in either steady or intermittent plumes and supports chemosynthetic organisms, or creatures that create energy from chemicals. Dissolved oxygen persists near the seafloor. Across the ocean floor and throughout the water column, gas plays a major role in sustaining life.

Though scientists recognize the importance of gasses at the bottom of the ocean (in habitat creation, climate regulation, and more), detecting gasses at a wide range of concentrations can be challenging. In order to fully understand the scope of biogeochemical processes and how gasses contribute to these cycles in the ocean, gas-detecting instruments need to be able to measure very small concentrations. In the past, this has required the development of large, costly instruments that require physical samples and associated analysis to occur after instrument recovery.

As the future of ocean science and exploration moves toward incorporating in situ testing (i.e., testing at the site where the sample is taken) that allows more timely information-gathering and the ability to guide sampling in real-time, there is a growing need for low-cost, highly sensitive, smaller instruments that can be attached to autonomously operated vehicles.

Finding Methane in the Deep Ocean

An interdisciplinary team of biologists, geologists, and engineers from the Woods Hole Oceanographic Institution addressed this need by developing and testing a novel in situ methane sensor. The team designed a sensor using a hollow core optical fiber, a technique that drastically reduces the amount of gas needed for a measurement while still maintaining high sensitivity to gas concentrations.

During field tests in the Gulf of California, the team secured the instrument in separate trials to an autonomous underwater vehicle (AUV), a conductivity, temperature, and depth (CTD) rosette, and a remotely operated vehicle (ROV). They then evaluated their instrument's methane-sensing capabilities by taking measurements at a hydrothermal vent outlet, the base of a collection of tube worms, and other biological and geological sites.

The team found that their sensor detected trace concentrations of methane as low as 5 parts per million and functioned at depths of up to 2,000 meters (6,562 feet). The instrument was able to track methane signals, which enabled the team to determine that concentrations of the gas increased significantly when approaching vent sites. With the methane sensor attached to either an AUV or ROV, the team detected and mapped methane where bubble activity was low and tracked plumes away from a gas source. Using the sensor coupled with a CTD, the scientists also created continuous methane depth profiles near vent sites.

By implementing a high-precision methane sensor into existing underwater technology, the project team was able to make chemical maps, measure black smoker vents and diffuse flows, and assess methane concentrations while keeping costs reasonable. The sensor is also smaller, more sensitive, and faster than existing standard methane sensors on the market.

Since field testing the device, the project team has been invited to join more ocean exploration expeditions to further develop their technology. In the future, the team expects to advance to commercializing the sensor, which will provide other groups and scientists the opportunity to use their technology worldwide.

Published November 15, 2023

Principal Investigators: Dr. Anna Michel and Dr. Jason Kapit, Woods Hole Oceanographic Institution

Funding for this project was provided by NOAA Ocean Exploration via its Ocean Exploration Fiscal Year 2018 Funding Opportunity.