Research Voyages : Feature Voyage


Deep Sea exploration with the RV Southern Surveyor
11 January – 1 February 2008

Voyage 01/08 11Jan-1Feb 08 reports from the ship >view

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Seeing Underwater With the Autonomous Benthic Explorer (ABE) >more

Report No 2: Surviving the "Zone of Death"

21 January 2008
By Ron Thresher

Underway data Follow the RV Southern Surveyor on this voyage: ship track and underway data.  

The last few days have seen our first spell of good weather (defined as winds more or less consistently less than 20 knots) and we have used the time to do three ABE deployments.

The first launch, in mid-day and good weather, was beautiful, with the vehicle slipping easily into the water after being released from the crane that hoisted it over the side. Deployment takes about 8 people, most holding lines to keep the vehicle from swinging about while in mid-air. The conditions were good, and deployment went smoothly, despite the ship's crew having no previous experience with the vehicle. Its recovery, however, was much more complicated. In normal operations, the ABE is recovered by bringing the ship about a hundred meters from it, after ABE surfaces (which takes about an hour from 1000 m). Then the ABE's motors are used to swim it to the stationary ship, after which a line is slipped over a cleat on the top of the ABE, more lines are secured fore and aft on the vehicle, and ABE is hoisted out of the water, swung onto the deck, and lowered into its cradle. Not quite so simple on the Southern Ocean. Winds had been increasing throughout the day, as had the height of the waves. By the time recovery was started, the winds were gusting to more than 35 knots and a 4-6 m swell and steep chop was buffeting the ship. We tried the standard approach, but quickly gave it up. ABE's motors, though adequate in the relatively quiet water 1000 m below the ship, could make no progress in the heavy seas, and so the effort was abandoned. For the next attempt, the ship's master very slowly moved Southern Surveyor next to and up-wind of the ABE, providing it a small pocket of sheltered water in which the recovery could be attempted. As the ABE drew into the still pocket, its motors kicked on and it was steered towards the waiting crewmen, who successfully got a line on it, and then several more. The rocking ship yanked the vehicle in and out of the water, and as it was lifted clear, it began swinging wildly back and forth, pulling lines free and hammering against the hull. Quickly, ABE was dropped back into the water, to stop the swing, and a second attempt made, this time with even more lines and people holding them. This time success, as the ABE's gyrations were slowed, and the vehicle dropped solidly into its cradle, into which it was quickly lashed.

ABE launch

It took most of the next morning to repair the damage to the vehicle, most of which fortunately was superficial. A few replacement hull plates and a few braces straightened out, and it was ready to go in again.

We've now done three missions with the vehicle. The first, described above, was a brief dive to test our systems. The second, intended as a long working mission along the top ridge of the Sisters seamounts, quickly ran into problems. As ABE tried to swim through the valley between the first two peaks, it suddenly stopped moving, prompting fears it had tangled in lost fishing gear (the Sister's Hills have been heavily trawled in the past). If fishing ropes were tangled in the ABE's propellers, our study would be over. ABE would be stuck on the bottom, forever, unless another deep diving vehicle could be sent down to recover it. After a tense 40 minutes and some manoeuvring by the vehicle (turn left, turn right, all done by the ABE's on-board computer following its emergency programming), the vehicle again began to move, prompting a lot of relief and speculations ranging from amorous giant squids to complex bottom topography that might have confused ABE's navigation systems. The truth proved more interesting. Having cleared the valley once, ABE had to traverse it again on its return trip. Hitting the "zone of death", ABE was suddenly shoved more than 100 m off-course, to the far sides of the pinnacles, apparently by a previously unknown strong current running through the valley. This current had been what stopped the vehicle the first time, as its weak electric motors could not make progress against the strong currents. Hopelessly off its programmed course, ABE nonetheless spent the next eight hours photographing the far side of the hills, before surfacing and being recovered.

Location as at 20 January 08
Location of the Southern Surveyor, 11:42 pm 20 January 2008

The final mission was more "textbook". Sent down to a deep pinnacle and ridge from 1250-1850 m depth, ABE spent a more or less uneventful 23 hours on the bottom photographing and surveying the ridge and pinnacle.

Results so far? The second dive, on the shallow (850 m) peaks of the Sisters seamounts, was disappointing. The mission confirmed that the tops of the pinnacles are barren rock, with the occasional piece of broken coral lying on the bottom, a few live corals in cracks and crevices, and the occasional orange roughy swimming by. Several years of fish trawling has apparently largely denuded the tops of the seamounts. It had been suggested that we could find healthy reefs in the valleys between the pinnacles, which were thought to have been lightly fished, but even there coral was very sparse. The third mission, to the deepest part of the Sisters, was also informative. Unlike the pinnacles of the shallower seamounts, the deeper ones were blanketed by extensive fields of deep-sea coral, from about 1250 to 1400 m. Mixed into the corals was a range of echinoderms, hydroids, seastars and crinoids, as well as the occasional fish and crustacean. Below this zone, life was much sparser, with large areas of apparently barren rock and sediment slides along steep cliff faces. We see the occasional stalked crinoid, urchin or seastar, but none of the reef present only a few hundred meters farther up the slope. However, even deeper, at 1850 m, we again found coral. A steep cliff face about 50 meters high ran along the base of the ridge. At its bottom were a few solitary corals, relatively sparse, some fossil, but also a few apparently live, hundreds of meters deeper than we'd expected.

We are now en route to our second survey area, a series of underwater ridges and mountains near the Tasman Fracture Zone. The next area will test how deep the coral, and other reef animals, can be found, as we hope to reach depths of 3 km on the next set of ABE missions.

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