If you’ve been following the news lately then you’ve likely heard about the large-scale methane leak currently plaguing the city of Los Angeles. The months-long environmental disaster stemmed from an underground pipeline rupture at a natural gas storage facility, prompting thousands of residents to evacuate their homes and California Gov. Jerry Brown to issue a state of emergency in January.
But some of the largest leaks occur with no public outcry, despite their potential effects on climate. Methane gas leaks can also occur underwater, though these “marine seeps” are much more difficult to monitor than land-based leaks. Methane is a potent greenhouse gas that occurs naturally and through human-caused processes. While CO2 is a more abundant greenhouse gas, methane is much more effective at trapping heat in the atmosphere, making it an important gas to monitor for climate change.
Scientists Sean Wiggins and John Hildebrand at Scripps Institution of Oceanography at UC San Diego are now exploring the use of acoustic recording technology to better understand and monitor methane emission sounds from the seafloor. Marine seeps produce sound—described by Wiggins as “musical rain on a tin roof”—as a result of gas escaping from vents in the seabed, pinching off into bubbles. The tone and sound levels of these emissions are related to bubble size, bubble plume density, and distance to the bubble plumes, providing important information about temporal variations of underwater methane release.
A recent study published in the journal Marine and Petroleum Geology details an experiment in which the Scripps researchers, in collaboration with Ira Leifer at UC Santa Barbara and Peter Linke at the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, utilized long-term passive acoustic monitoring (or PAM) to continuously record seep sounds over a seven-month period within the blowout crater at an abandoned well site in the central North Sea.
The crater, initially measuring 60 meters (197 feet) wide and 20 meters (66 feet) deep in a water depth of 100 meters (328 feet), was formed by a shallow gas blowout explosion from an oil exploration drill site in 1990, and has been spewing methane ever since. A 2011 survey estimated that approximately 90 liters (3 cubic feet) of gas was escaping from the seabed every second at the crater site.
The researchers collected two acoustic data sets in 2011 and 2012 from the crater site through the use of a High Frequency Acoustic Recording Package (HARP) data logger and a hydrophone (an underwater microphone) attached to a benthic lander—an observational platform equipped with a multitude of sensors that sits on the seafloor to record physical, chemical, and biological activity.
The use of a HARP recording package to monitor methane seeps is novel, and this study is the first to showcase how this new technique can be used to help scientists better understand the dynamic nature of marine seepage systems.
HARP technology was developed by scientists at the Scripps Whale Acoustics Lab to passively monitor sounds from marine mammals such as whales, dolphins, and seals, but it can also be used to study other sounds in the ocean such as those from ships, sonar, petroleum exploration, explosions, wind, and earthquakes.
Methane bubbles coming out of marine seeps have unique sound characteristics that allow them to be distinguished from other sound sources in the ocean. In the case of this study, scientists knew there were marine seeps with methane emissions in the blowout crater, and they determined that putting the HARP in the crater was the best way to record the seep sounds.
“These measurements would not have been possible without the long-duration and high-fidelity autonomous recording capabilities of the HARP, and showed us that methane emissions are not always stable and can change by large amounts episodically,” said Wiggins, a project scientist at Scripps and lead author of the study.
Over the course of this long-term study, the scientists recorded a large eruptive event in December 2011 (listen here), the first such observation of its kind. When the instruments were retrieved in 2012, the team observed a new gash in a nearby crater wall, from which a “very intense” major bubble plume had escaped. This major eruption also resulted in a new rift in the seafloor, causing the benthic lander to fall six meters deeper into the crater, and a persistent increase in sound levels was recorded thereafter.
“This study shows that passive acoustic monitoring is a viable technique for long-term monitoring of variable methane emissions from the seafloor, including daily trends associated with pressure changes due to tides and transient events like eruptions,” said Wiggins.
The researchers also found the crater site’s sound levels to be positively correlated with ocean tides, meaning that when tides were high, the sound levels of the flowing methane bubbles were higher due to an increase in the overriding water pressure. This could be due to tidal flows or other processes that are poorly understood.
Wiggins said that the next step in this research would be to develop a relationship between the sound levels measured and the amount of methane escaping the seafloor, which would allow for better methane emission estimates at seafloor sites. Climate change researchers could then use these estimates, along with models of how methane is transported through the water column to the atmosphere, to develop more accurate models of greenhouse gas inputs.
While the Scripps-led study focused on monitoring methane sounds, a number of other research teams have studied different aspects of the North Sea blowout site, including methane emission levels, ocean currents and methane transport within ocean layers, and the use of 3D sonar mapping of the bubble plume to measure methane concentrations. These studies also appear in the December 2015 issue of Marine and Petroleum Geology, a special issue presenting a comprehensive investigation of the crater site more than 25 years after the initial blowout.
– Brittany Hook