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We almost immediately began deploying the 10 "transponders" that will allow scientists to know the position of the DSL 120 side scan sonar, theArgo II picture-taking robotic sled, and the manned submarine Alvin, when one vehicle after the other descends into the depths along the study track in the next weeks. These transponders are themselves sound-emitting sonar devices packaged inside round yellow floats. Each glass-lined float looks like two oversized industrial hard hats bolted together brim to brim.
Donned in real hard hats, industrial-strength work vests and gloves, expedition co-principal investigator Emily Klein, and Duke doctoral student Dan Curewitz, helped launch the first transponder in the glow of a picture-perfect sunset. When first lowered into the warm equatorial water, the float dragged behind the ship - cruising at only a few knots - by an 800-foot-long wire tether.
When the transponder was positioned over the target - as determined by satellite Global Positioning (GPS) - a heavy weight at the end of the tether also got thrown into the ocean. The weight dragged the float downward, and the transponder began a 1 1/2 hour long trip to the ocean floor.
All the transponders were dropped by about midnight on a trajectory intended to place them above Hess Deep's rim on the north side of the chasm. At the expedition's conclusion, when the devices' work is done, special sound frequencies will sever the floats from the anchored tether so that they ascend back to the surface for collection and reuse.
These transponders are needed, said Hess Deep chief scientist Jeff Karson, because the usual maps of the ocean floor are not precise enough. These charts are made using other kinds of "multi-beam" arrays of sonar devices implanted on the bottom of ship hulls, including Atlantis's. Each separate sonar beam follows its own path to the bottom. When each path bounces back to the ship, they combine into an audio signal that sounds like a bird chirp.
The problem is that the sonar data gets "homogenized," added Karson as he pointed at a chart of Hess Deep spread out on a table in Atlantis's computer lab. That's because the movement of sound waves vary by the depth they travel. To make a bottom map, computers must thus "reconcile" the certainty of the ship's position at the surface - determined via GPS - with the uncertainty of this sonar-provided information.
So these charts include "lots of artifacts, especially in deep fracture scars," Karson said. Compounding these difficulties, charts are usually based on sonar data from several different cruises, using different software. He called the results "electronic patchwork, forced together."
Transponder signals are much more precise, especially since sonar returns from different transponder stations can be triangulated to better fix the positions of the DSL 120, Argo II or Alvin as they prowl Hess Deep's north face.
After dropping the transponders, Atlantis spent all Tuesday morning retracing its route and circling over each device to get as precise an idea as possible where they had come to rest.
At 2:10 p.m., another hardhat-wearing team began launching the DSL 120, so-named because it sends high frequency 120,000 cycles per second sonar beams out from both sides, pointing downward so that the signals create a path of reflected sound energy on the terrain below.
Housed in a silver-white shell shaped something like a large candy bar, this "fish" - as researchers call towed devices - was lifted by crane from the side of Atlantis's aft deck in a hot sun. After being lowered into the water, it was dragged along behind the ship by a 6/10 inch thick cable While its shell is neutrally buoyant, attached weights made DSL 120 slowly sink as the cable was played out by a winch. Over the next several hours, it will descend to its planned operating depth, about 300 feet above the side of the Hess Deep cliff face and about two miles below the surface.
All that time, the cable will serve as DSL's umbilical link to the mother ship, the fiberoptic fibers providing communications while the copper's 40,000 pound test strength secures the link from breaking.
Tuesday night will begin a round the clock series of watches during which the entire scientific staff will take turns monitoring what DSL 120 sends back: Hess Deep 1999's first major scientific operation.
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