 | |  |
|
| Enlarge ImageAn eruption that covered 9 square miles of seafloor with lava at the East Pacific Rise trapped three ocean-bottom seismometers. Scientsts attempted to recover them with the remotely operated vehicle Jason in April 2007. They successfully recovered two and will learn in the weeks ahead if the data they contain are still viable. (Photo courtesy of the National Deep Submergence Facility, ROV Jason, Woods Hole Oceanographic Institution, and the National Science Foundation.) |
|
|  |
|
| Enlarge ImageRecovering the instruments called for the dexterity of the WHOI-operated robotic vehicle Jason, whose mechanical arms and hands were used to knock loose hardened lava and extricate the instruments. The arms are powered and controlled through a 6-mile-long tether that delivers video of the live action on the seafloor up to pilots and scientists on the ship. (Illustration by E. Paul Oberlander, Woods Hole Oceanographic Institution) |
|
|  |
|
| Enlarge ImageWHOI marine geologist Dan Fornari holds chunk of recently erupted seafloor lava that came up with an ocean-bottom seismometer that was caught in an undersea eruption and stuck to the seafloor. Singed but intact, the instrument was recovered and may still contain rare data on seafloor eruptions. (Courtesy of Dan Fornari, Woods Hole Oceanographic Institution) |
|
|
Related Multimedia |
|
Jason to the Rescue
The robotic undersea vehicle frees instruments glued to the seafloor by hardened lava.
The National Deep Subergence Facility, ROV Jason, Woods Hole Oceanographic Institution, and the National Science Foundation. Movie by Tim Silva, Woods Hole Oceanographic Institution. | » View Video (Quicktime)
|
|
|
|
|
Related Links |
|
» 9 North OSC Expedition Web site
Learn what else researchers did, in addition to recovering the instruments, during the spring 2007 expedition at the East Pacific Rise.
|
» ROV Jason/Medea
Jason is a remote-controlled deep-diving vehicle that gives shipboard scientists immediate, real-time access to the sea floor.
|
|
The two earthquake-monitoring instruments—each the size and weight of a
small refrigerator—were glued to the ocean bottom by erupting lava that
had flowed and hardened around them. If scientists could pry them
loose, the payoff could be huge.
The instruments,
called ocean-bottom seismometers (or OBSs for short), appear rather
bulky, but they are designed for a sensitive job. Sitting on the
seafloor, they convert ground motions—from the tiniest flutters to
major earthquakes—into electrical signals that are digitally recorded.
These particular seismometers, stuck 1.6 miles (2,500 meters) deep on
the Pacific ocean floor, may also have also been stuck at a particular
moment in time, recording something scientists rarely have a chance to
pinpoint: the precise timing of an undersea volcanic eruption. Seafloor
eruptions happen daily along a 40,000-mile-long volcanic mountain chain
called the mid-ocean ridge that encircles the Earth. Fresh lava
frequently intrudes into the ocean crust beneath the ridge and erupts to
pave and repave the planet with new crust and reshape the face of the
planet. But scientists can’t see this fundamental
process through miles of water (as they can by observing volcanoes on
land) to learn exactly how this type of volcanism occurs in the cold,
liquid environment under enormous pressure at the bottom of the sea. Did
the eruption happen all at once or over a longer time period? Seismic
spikes—recorded on Jan. 22, 2006, by two other seismometers that were
not engulfed in lava and recovered a year ago—suggest the former. But
geologists and chemists, using fluid temperature records and rock
samples from the eruption site to date the lava flows, believed the
eruptions occurred over several months in late 2005 into early 2006. “It’s a good, old-fashioned, friendly science debate,” said marine geophysicist Maya Tolstoy from the Lamont-Doherty Earth Observatory of Columbia University. “We have two types of data, saying two different things. The recovery of the instruments could resolve the issue.” The seismometers survived the eruption because the temperature of water in the deep ocean is nearly freezing—about 35°F (2°C). This rapidly quenches hot lava as it emerges from the seafloor. “You couldn’t throw an instrument like this into a lava flow on land and expect it to survive!” Tolstoy noted. In the hands of Jason
In 2005, scientists deployed a dozen OBSs over a few square miles
of seafloor on the East Pacific Rise, part of the mid-ocean ridge located about 450 miles south of
Acapulco, Mexico. Each was numbered (“OBS 201” to “OBS 212”), and each
was equipped with a sound source that chirped in the dark depths to
help researchers locate them.Researchers returned in
April 2006, thinking they would simply recover the instruments and send
down others. Instead they realized an eruption had occurred. They
quickly found and recovered the four seismometers not engulfed in lava,
two with useable data. Five others, from which they received no
signals, were presumed buried in lava and lost. That meant three remained on the seafloor with potentially useful data. Two
months later, scientists returned and found two of them, OBS 206 and OBS
210, partially buried in the lava flow. They tried to nudge and pull
them loose using a camera-equipped sled towed by a cable behind their
ship—without success. Freeing them from the lava flow called for the
dexterity of the WHOI-operated robotic vehicle Jason. The 1,200-pound vehicle is controlled through a 6-mile-long tether that delivers video from the seafloor up to pilots and scientists who sit in a command center on the ship and watch the live action below on a half dozen television screens. The tether also transmits commands from the pilots back to Jason, along with electrical power to operate two mechanical arms with metal claws. In April 2007, researchers returned with Jason,
hoping its mechanical hands (and pilots) could extricate the
instruments, without further damaging them, or having them float away. Where are you, OBS 212?
Finding OBS 212, however, was not straightforward. Researchers
had a good idea of where they had left it in 2005, but they knew that
the erupting lava flow had likely moved the instrument. “When
we landed at the site of OBS 212, it was nowhere to be seen,” said Adam
Soule, a volcanologist at WHOI. They made some “spaghetti tracks,” with
no luck, and then became more systematic. The pilots controlling Jason started running parallel lines about 130 feet (40 meters) long, like a lawnmower moving in a careful pattern over grass. “We
ran this sort of survey for about two hours and were quite discouraged
that we hadn't found the OBS,” Soule said. They considered giving up,
but one of the pilots suggested using acoustic transducers on the ship
to zero in on the instrument. The transducers
transmitted sound, in the form of high-pitched “pings” at specific frequencies, to receivers on
the OBS, asking it to reply. By analyzing the time it took for the
pings to reach and return from the instrument, the pilots and
scientists were able to home in on the OBS. When the pings’ travel time
between ship and instrument became shorter, they knew they were getting
closer. “Finally, off in the distance I saw four
faint lights in the water, oriented in a square pattern,” Soule said.
“Manmade objects tend to really jump out at you on the seafloor.” The
lights turned out to be strips of reflective tape on the corners of the
instruments’ yellow, plastic enclosures. “It was
fortunate that the top of the instrument was pointed toward us, or we
would have never seen this pattern, and it would have taken much more
time to find,” Soule said. A two-fisted rescue
Once OBS 212 was located, “the devil was in the details when
trying to get it out,” said WHOI marine geologist Dan Fornari. “We had
to see how it was situated in the lava flow. Then we had to figure out
how to remove it.”Typically both of Jason’s
robotic arms are controlled remotely by one pilot. But in this case,
the two pilots on the job, Tito Collasius and Bob Waters, decided that
the situation called for a tag-team effort. One of their first tasks
was removing large chunks of lava stuck to the back and sides of the
instrument, which had glued it to the seafloor. “One
guy used one arm to knock off chunks of lava,” Collasius said. “The
other guy was working to attach a float” —made from a snap hook and
scrap chunks of syntactic foam found on the ship—“in case the
instrument broke free and tried to float away.” Within
45 minutes, the instrument was free and rising to the sea surface,
where ship’s crew located it floating near the ship and recovered
it (see "Jason to the Rescue"). Waiting and hoping
OBS 206 and OBS 210 had different stories, one sweet, one sour. OBS 206 was wedged into the lava flow and partly buried, Fornari said. A large scorch mark was clearly visible. Jason secured it with a float and wiggled it from the lava. Within 30 minutes, the instrument was on the ship. OBS 210, however, was stuck too tightly between collapsed lava flows. “We worked for several hours trying to break ... the lava crusts around it and digging out
the rubble, to no avail,” Fornari said.“After digging and pulling
it as best we could without damaging the vehicle, we determined it was
not going to come out. But we gave it our best shot.” When
the ship docked in San Diego on April 27, Fornari ferried the two
recovered instruments to the Scripps Institution of Oceanography. Engineers
will spend the weeks ahead determining if they can recover any data,
which could speak volumes about the volcanic processes occurring at the
mid-ocean ridge and how they shape our planet. Tolstoy was optimistic
about finding some data.“Even though the instruments
are pretty singed-looking in places,” she said, “the damage is quite
minimal given what they went through.” —Amy E. Nevala
The
2007 expedition to the East Pacific Rise on R/V Atlantis (Cruise AT15-17, with Chief Scientist Emily Klein of Duke University) was supported by the National Science Foundation
Ridge 2000 Program.
Posted: May 15, 2007 [top] |
