Topics: Autonomous Hydrophone Array (AHA) - Monitoring Volcanic and Tectonic Processes on the Mid-Ocean Ridge
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
Administrations (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.
Matt Fowler, a NOAA engineer, works on a hydrophone, or Haruphone, before it is deployed in the ocean.
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 NOAAs VENTS Program began operating in August of
1991. Acoustic signals from the north Pacific Ocean are monitored and recorded
at NOAAs Pacific Marine Environmental Laboratory (PMEL) facility in Newport,
Oregon. This is the nations 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 NOAAs 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.
In May of 1996, this array of autonomous hydrophones began long-term monitoring
of portions of the mid-ocean ridge in the central equatorial Pacific (the East
Pacific Rise and Galápagos Ridge). The Autonomous Hydrophone Array (AHA)
consists of six hydrophones spaced many hundreds of kilometers apart. These hydrophones
have been nicknamed Haruphones after Haru Matsumoto, the NOAA
engineer who developed them.
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
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