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Autonomous Hydrophone Array (AHA)

Looking for new volcanic eruptions on the global mid-ocean ridge and understanding when and where eruptions take place is one of the most exciting developments in marine geology and geophysics. Volcanoes, earthquakes and other seismic events are the primary forces that shape the Earth and rearrange the tectonic plates that make up the continents and ocean.

On land, a network of strategically placed seismic monitoring stations lets us locate earthquakes when they occur. But in the middle of the ocean, there are few places to put recorders unless they are specially designed to work in the ocean or on the ocean floor. Some monitoring stations on ocean islands do help, but scientists for many years have sought better ways to monitor seismicity in the worlds’ ocean basins. Since August of 1991, the Acoustic Monitoring Project of the National Oceanic and Atmospheric Administration’s (NOAA) VENTS Program has carried out continuous monitoring of mid-ocean ridge systems in the eastern Pacific using the U.S. Navy SOund SUrveillance System (SOSUS) network.

SOSUS was installed by the US Navy beginning in the mid-1950s for classified antisubmarine warfare and surveillance during the Cold War. SOSUS consists of groups of hydrophones that “hear” and record sound waves generated by seismic events, submarines, or whales, for example. The hydrophones are connected by undersea communication cables to facilities on shore. Individual hydrophone arrays are installed primarily on continental slopes and seamounts at locations that are unobstructed by seafloor features that would block sound wave transmission across large areas of the ocean.

In October of 1990, the Navy gave NOAA access to SOSUS arrays in the North Pacific to monitor oceanic phenomena of environmental or scientific interest. Data collection systems developed by NOAA’s VENTS Program began operating in August of 1991. Acoustic signals from the north Pacific Ocean are monitored and recorded at NOAA’s Pacific Marine Environmental Laboratory (PMEL) facility in Newport, Oregon. This is the nation’s primary facility for continuously monitoring low-level seismicity in the northeast Pacific and for detecting volcanic activity along the mid-ocean ridge, as it is happening.

The system demonstrated that it could successfully listen to eruptions taking place on the Juan de Fuca and Gorda Ridges using SOSUS. To expand the system's listening capabilities, our colleagues at the NOAA’s Hatfield Marine Center, in Newport, Oregon, under the direction of Dr. Chris Fox, figured out how to put self-recording hydrophones into the ocean at six locations in the Eastern Pacific Ocean, so that they can “hear” when eruptions on the mid-ocean ridge crest take place.

“Haru” phones are placed in a special part of the upper ocean between 600 m and 1200 m depth, called the SOFAR (SOund Fixing And Ranging) channel. This is the same sound channel that the SOSUS array uses to listen for submarines. The SOFAR channel acts as a pipeline for sounds in the ocean, allowing sound to travel long distances and still be detected. Since the mid-1940s when the SOFAR channel was discovered, scientists, engineers and the military from different countries have used this underwater pipeline to help them listen for submarines and whales and to study how sound travels in the ocean. Now ocean scientists are using it to study volcanic and tectonic processes on the mid-ocean ridge crest.

New hydrophone arrays have been developed to detect and monitor previously "unheard" volcanic and earthquake activity in the oceans.

New hydrophone arrays have been developed to detect and monitor previously "unheard" volcanic and earthquake activity in the oceans. They are deployed in the SOFAR channel, a layer of water in the ocean that channels sound waves generated by seismic events and allows them to be transmitted and detected thousands of miles away. (Illustration by Jack Cook, Woods Hole Oceanographic Institution)

An array of six hydrophones (yellow stars) recorded 7,785 seismic events (red dots) between 1999 and 2002

An array of six hydrophones (yellow stars) recorded 7,785 seismic events (red dots) between 1999 and 2002, mostly along the Mid-Atlantic Ridge and the Puerto Rico and Caribbean Trenches. (Courtesy Deborah Smith, Woods Hole Oceanographic Institution)