At a time when ocean noise is receiving increased global attention, researchers have developed an effective method to use an underwater robotic glider to measure sound levels over broad areas of the sea.
"Healthy marine ecosystems need to have noise levels within particular ranges," explains Joe Haxel, a senior researcher in the College of Earth, Ocean, and Atmospheric Sciences at Oregon State University, and part of the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Lab Acoustics program.
"An analogy for humans would be the difference between living in the country and living in the city or somewhere that's really loud."
Haxel is lead author of a paper published this week in the peer-reviewed journal PLOS ONE which describes the research with the underwater robotic gliders.
Ocean sound was recently listed as an essential ocean variable by the Global Ocean Observing System, a UNESCO program, due to its importance for marine life and seagoing humans and because it is used to monitor and locate everything from earthquakes and tsunamis to nuclear explosions.
Traditionally, scientists have measured ocean sound by attaching hydrophones to a fixed mooring in the water, which means they only get data from that single location.
Ocean sound can also be measured from a research ship, but they are expensive to operate. They also create a lot of noise themselves, which disturbs marine animals and fish that are sensitive to sound.
Attaching a hydrophone to an autonomous glider solves both problems because they are relatively quiet and can cover hundreds of miles over several weeks.
Gliders equipped with hydrophones can conduct repeated surveys of a region of concern for acoustic habitat degradation and provide real-time measurements of changing noise levels.
They have also been used successfully to measure noise from an underwater volcano and to predict surface wind speeds.
An additional benefit of gliders is that they can be outfitted with other sensors and instruments to provide important measurements such as temperature, salinity and depth.
For the research, Haxel and his team attached the hydrophone to a five-foot glider weighing 120 pounds.
The glider travelled for 18 days between Gray's Harbor, Washington, and Brookings, Oregon, a distance of 285 miles, operating along the North American continental shelf break, which on average is 30 miles off the coast and is where the ocean depth begins to drop more steeply. The shelf break is also know to be a key migratory path for many marine animals.
Once the scientists retrieved the hydrophone data, their main challenge was fine-tuning their algorithms to filter out the noise the glider creates when operating.
The researchers were then able to cross-reference the data collected during the 18-day trip with historical data from hydrophones attached to moorings along that route.