Researchers with Oregon State University and the National Oceanic and Atmospheric Administration have documented 1,000 underwater seeps of methane gas along the Pacific Northwest coast between Northern California and the Canadian border over the past two years.

A large concentration was found in the Astoria canyon west of the mouth of the Columbia River.

Next week, a new expedition begins in search of more.

Tectonic activity causes fissures in the sea floor that lead to seeps of hydrocarbon-rich fluids such as hydrogen sulfide and methane, a main component in natural gas. Methane released through the seeps often crystallizes into methane hydrate, an opaque or translucent ice. The releasing methane also takes the form of bubbling gas and can contribute to ocean acidification.

Surveying for methane offshore is a relatively new field, said Susan Merle, an oceanographer who works out of Oregon State’s Hatfield Marine Science Center in Newport, although geologists have long known there should be methane deposits offshore.

The study of methane seeps has both environmental and economic implications. The U.S. Geological Survey has estimated that global methane hydrate deposits could hold twice the natural gas energy as fossil fuels currently available.

Some of the first discoveries of methane seeps in the Cascadia Subduction Zone, a fault line running from Vancouver Island in British Columbia to Cape Mendocino in Northern California, were documented in the late 1980s. Fishermen sometimes spot the seeps as anomalies on their fish-finders — acoustic reflections from bubbling methane gas.

But only around 100 seep sites had been identified until a couple of years ago, when scientists began surveying with new multibeam sonar technology aboard the research vessel Nautilus, owned and operated by the Ocean Exploration Trust.

The new sonar allows researchers to rapidly locate and map the methane seeps. Researchers have mapped nearly 40 percent of the ocean floor along the Cascadia Subduction Zone with the Nautilus’ sonar, Merle said, and hope to cover more on this expedition. An onboard remotely operated submersible — Hercules — is deployed to collect samples of gas, methane hydrate, water, fauna and rocks near the seeps, which often support a diverse array of life.

In their survey, researchers found that much of the methane below an ocean depth of 1,600 feet was solidified hydrates, while higher up it usually appears as a gas in bubble streams. One theory is that warming seas turn the hydrates into carbon dioxide gases and contribute to ocean acidification, Merle said.

Leading the upcoming expedition is Tamara Baumberger, a researcher with the Cooperative Institute for Marine Resources Studies operated by Oregon State and NOAA at the Hatfield Marine Science Center. On the previous expedition, Baumberger sampled some of the bubbles and found chemical signatures pointing to the source of the methane.

“When methane is in seawater, it often is oxidized into carbon dioxide by microbial activity, which can keep much of it from reaching the atmosphere,” Baumberger said in a news release about the previous expedition. “The downside, of course, is that the newly formed carbon dioxide is also a problem and it can both reach the atmosphere and increase ocean acidification.”

Methane released into shallow water reaches the atmosphere quicker because bacteria in the water has less time to oxidize the gas, Baumberger said. Part of the new expedition is trying to map and study more shallow-water methane seeps.

Whether methane could be economically extracted from the ocean floor remains a question. Several countries have explored mining the hydrates as an alternative energy source. But the deposits are widely distributed, said Robert Embley, an Oregon State geophysicist and lead scientist on the previous expedition.

“One concern is what would happen during a major Cascadia Subduction Zone earthquake,” Embley said in a release. “It would add more permeability to the sea floor, add more pathways for the methane to escape, and increase the potential for its release to the atmosphere.”

The Nautilus leaves San Francisco next week and will survey all along the Cascadia Subduction Zone through the end of the month, including just south of the Astoria canyon. Video of the dives by Hercules will be made available at nautiluslive.org