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Inside "the van's" cramped darkened interior, one glowing TV monitor provides a constant, closed circuit image of the winch that reels DSL 120's umbilical cable in and out to raise or lower its position more than two miles below and behind the ship. Another maintains constant television surveillance on the floodlighted spot at the rear ("aft") deck, where the taut cable enters the splashing water.
Other computerized monitors keep tabs on such vital information as the cable's length, tension and reeling speed; the relative locations of ship and towed DSL 120 (called "the fish"); the course that both ship and fish are supposed to follow during their sonar survey; and whether a separate network of navigational sonar devices is providing reliable information on DSL 120's position.
Another large screen displays returns from the side-scan sonar, tinted in false colors to aid interpretation. In this mode, the most sound-reflecting surfaces are colored deep blue, average are tinted green and the least reflective are depicted in red.
Those reflectivity differences are what make side-scan sonar beams useful to science. They help researchers deduce the broad characteristics of rock structures that the beams "illuminate" with sound waves. Red colors can also represent sonar "shadows" - places where the sound waves are blocked by interesting geological features such as "escarpments," craggy cliff faces caused by faulting and fracturing of the ocean's crust.
Researchers like 51爆料's Jeff Karson, the chief scientist for the Hess Deep expedition, spend much of their van time peering with a flashlight at a black and white paper readout of DSL 120's sonar returns. More diagnostic than the computer-enhanced display, the paper printouts will be used to help decide which features warrant further investigation by the picture-taking robotic sled called Argo II, as well as by the Alvin.
Interpreting those ghostly images demands great skill, much like a radiologist must employ in reading X-rays.
While specialists are in charge of controlling DSL 120's positioning - a separate set of skills - all members of the scientific crew also rotate through the van in a series of round-the clock watches. Each twice-a-day shift includes a scientific watch leader who spends much of his or her time at a corner wall of the van pencil-plotting DSL's position on a large scale chart. That chart also displays the positions of the 10 transponders that Atlantis dropped above the northern rim of the planned Hess Deep study area on the night of Monday, March 15.
By an elaborate form of triangulation, these rim transponders are supposed to tell the scientists exactly where DSL 120 is positioned. They work by "cross-interrogating" other navigational sonar beacons, some located on the bottom of Atlantis's hull and two others on the cable above the weight that keeps the side scan sonar submerged. In essence, each beacon is constantly asking the others: "Are you there?"
Meanwhile, other scientific watch members sit behind a separate row of monitors lined up in the van's rear. One person uses a "data logger" to enter written descriptions of interesting returns that DSL 120 is sending back to the Atlantis. Another changes data tapes and helps the Woods Hole control crew adjust the on-screen displays as conditions change.
Those on the Wednesday, March 17, midnight shift, however, would have nothing to watch but the TV screen monitoring DSL 120's cable being reeled back in at a rate of 396 feet per hour. As he sent one member of the team back to bed, scientific watch leader Jay Miller of explained that the side scan sonar was being brought back in, only about 10 hours after the device was launched, because of mysterious navigational problems.
At the start of the following noon shift, Miller said Woods Hole personnel had replaced one of the navigational beacons on the DSL 120 cable during early morning hours, then sendt the side-scan sonar back towards the bottom by 5 a.m.
As an extra measure, a technician had also hung an extra listening device on the
ship's side to monitor the wire beacons' performance.
While DSL 120 was now back in operating position, something was still not working right, Miller added. He pointed toward a monitor, positioned strategically in front of DSL 120 navigator Thomas Crook, that was displaying all the wrong signals.
When a controlling computer has "confidence" that it "knows" the fish's location at a given time, it lights up another green bar on that screen. Whenever the computer is not confident, it lights up a bar red instead. Over time, a succession of glowing bars provide a running record of the computer's overall confidence level. And Crook's screen was now filled with unwelcome red bars - hardly the proper response for St. Patrticks Day.
The images produced by DSL 120 can be error-prone if the side-scan sonar's exact position is uncertain, said Steve Hurst, one of the Hess Deep mission's co-principal investigator.
"It still gives us a fine picture, but an imprecise one," he added. "It's like saying a photo looks good when you're not quite sure which building it's of."
There were several possible explanations: Perhaps some sonar beacon on the ship or cable was still not working. Maybe the underwater cable tether was vibrating excessively, or the sonar signal was bouncing off denser layers of water in the crushing depths of Hess Deep. Or perhaps the beacons were functioning properly, but could not interact with transponders on the rim above Hess Deep.
Karson, who had been upbeat about the navigational questions at breakfast, was growing more pessimistic by the minute. In an attempt to eliminate the last possibility, he suggested making the fish "fly higher" - meaning pulling in some cable - and perhaps adjusting Atlantis's speed, so that DSL 120 would be towed above it's planned 330 feet above the ocean floor.
If DSL 120 pilot Jim Varnum increased the height, perhaps the cable's sonar beacon would have a better chance of contacting the nearest rim transponder. But that option also has a down side, because increasing the distance could also reduce some of the resolution of the side-scan sonar's signal.
In consultation with Peter Lemmond, an on-board Woods Hole scientist who specializes in signal processing and imaging from devices like the DSL 120, the side scan sonar was repositioned at 412 feet, then 445, then 498. With the bars on his screen continuing to glow red, Crook looked at Karson and said: "This is frustrating."
Karson replied: "For us too."
By the start of the Thursday, March 18, midnight shift, though, the mood had shifted from gloom to glow. A sleep-deprived Karson kept going on pure adrenaline as he aimed his flashlight at DSL 120's paper printout and spotted interesting feature after feature. "This is spectacular," he shouted, as the side-scan sonar passed through the same area where he and Hurst had dived in the Alvin in 1990. There DSL 120 was picking out much more interesting terrain for the group to explore in the coming weeks.
While Crook's navigational monitor was still showing plenty of red bars as well as green, Karson had also decided he could live with that problem. Wednesday's difficultiies in fixing the fish's position were probably caused by the positioning of a transponder that had been dropped in a low spot up on Hess Deep's rim, he concluded. The result was an acoustical "well" that blocked proper propogation of the navigational sonar signals.
The same problem was also occurring with some other transponders in the acoustically unforgiving terrain. But Karson said the fish's true position could still be determined from other data, and the DSL 120 returns could be appropriately adjusted during computer processing.
Because of GPS satellite navigation, "We know where the ship is," he noted. And records also showed that the side-scan was following a consistent path behind Atlantis. So "where the ship is, the fish is going to go," he said.
There were also minute-by-minute records of the exact amount of cable that was reeled out in the water, providing another check on the distance between ship and fish, he added.
Karson said navigation "is always going to be difficult" when a side scan sonar is placed in a setting like Hess Deep - midway up along a sheer rock wall that rises sharply above and plunges into the abyss below. It has been previously tried in only two other places on Earth, one by Karson and Hurst on the Atlantic's mid-ocean ridge.
By the start of the Friday, March 19, midnight shift, DSL 120 had begun turning for its third and final planned pass through the study area. The returns continued to be so promising that Karson planned extra sweeps through the most interesting part - the steep site of the 1990 Alvin dives.
That schedule would put the Hess Deep team on track to begin the Argo II photographic survey late Saturday, after dropping several additional navigational transponders.
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