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Author Topic: Underwater search operations  (Read 8527 times)

Greg Daspit

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Underwater search operations
« on: June 01, 2013, 11:11:04 AM »

What are the best methods to search the 200' debris field, shallow area and the sonar target area?
How long can a diver stay down at the shallow depths, and the 200' depth?
What would be the best search pattern for a slope where debris may have covered the top of objects and but not the sides?
How do you deal with the sharks?
What measures are taken to prevent avalanches from the ROV cable, subs and divers?
What are the advantages and disadvantages of Subs, ROVs and divers for each area?
What targets get searched first in order to prevent avalanches on others?
Andrew, can you let us know what your observations and search methods were from previous dives?
3971R
 
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Ric Gillespie

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Re: Underwater search operations
« Reply #1 on: June 01, 2013, 11:51:58 AM »

We don't put divers down deeper than 100 feet.  Any investigations below that depth will have to be done with a submersible of some kind - probably an ROV.

You avoid landslides by not disturbing the reef slope.
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Andrew M McKenna

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Re: Underwater search operations
« Reply #2 on: June 01, 2013, 12:12:58 PM »

Greg

In 2001, we surveyed the reef via scuba from the landing channel to near the NE tip of the island from essentially the surf line where the reef flat ends down to about 60 to 80 ft.  There were 3 of us in the water and the buddy system dictated that we always work together, so there was no opportunity to split up into 2 teams which may not have been optimal.  Part of the reason behind this was that for the lagoon operations, one of us had to drive the skiff, so there were only two in the water at a time.  A 4th diver would have been superfluous at that point.  A trip to Niku always requires compromises when it come resources desired vs resources available, and how to allocate them.

Work in the very shallow water started at about 20 ft depth, and involved literally swimming up the channels in the coral to the surf line, and back out again.  At the time, we were imagining that there could be wreckage caught in these channels, and I think we swam most of them, if not all of them pretty methodically from the NC to the North, including past the area we now think the Bevington Object was located.

For the deeper water, we swam parallel to the shore, 3 abreast some 20-40 ft apart down the reef such that the each of us was at a different depth, but the same height above the reef.  We then traded places periodically so that none of us would have a disproportionate consumption of air out of our tank.  Visibility was always excellent on the reef, often 50-60 ft or more. (4 ft in the lagoon!)

While not perfect, I feel pretty confident that we covered the area in a reasonable search, and that if there had been any significant wreckage we would have seen it.  A single landing gear, covered in coral, however, could have been missed for sure, but let's not get into a discussion of probability of detection here as that has been extensively covered elsewhere in the Forum  :)

In 2001, we made the decision to stick with recreational diving depths as a safety measure.  Niku is at least 24 hours away from possible emergency extraction which would have to involve the US Coast Guard landing at Canton, and there is no re-compression chamber available within any distance that would make a difference.  Niku is not a place to get the bends.

During a previous expedition, divers chose to dive deeper and used a technique to dive deep quickly, then drift back up to the shallows for decompression.  My understanding is that some of these guys got down into the 150 / 200 ft depths, but would have been able to spend no significant time to search for anything down there.  Maybe they had the experience to do this (I suspect that some of them had special training paid for by tax dollars), but it is not something that I would attempt or recommend anyone attempt at Niku without the proper support.  At the time, the risks involved were not well understood by TIGHAR, but they are now, and I doubt that any dive profile below recreational depths will ever be condoned during a future TIGHAR expedition except if performed by professional divers with the right training and equipment, probably including a re-compression chamber available.

So, to attempt to answer your questions, here goes

Searching a 200 ft depth debris field would best be accomplished by a small ROV such as the unit we used in 2010, or the larger unit used in 2012, unless we can get ahold of a small submarine  :)

I haven't looked at my dive tables recently, but rule of thumb might be 60 minutes at 60 ft before requiring a decompression stop.  At 90 ft, you are down to 25 minutes, 140 ft is down to 8 minutes, and 200 ft is off the charts for recreational diving.  Not that it can't be done, but it requires particular attention to decompression depths and time, special gasses are sometimes used, and is really not in the realm for recreational divers.  The risks go up, particularly given the location.

Best search pattern is to fly over the terrain in some sort of methodical method doing your best to scan and examine whatever you think is interesting, using the Mark 1 Eyeball.  I would think video could come in handy in the future underwater ops so we can put more eyeballs on the problem after the fact. 

Sharks are not really an issue, they do their thing, divers do theirs.  Once you are in their realm, you are just another big fish.  I got ready to poke one in the nose a couple of times, but they always veered away before it came to blows.  Avoid bleeding.  See Mark Smith's encounter with a shark at Niku on Youtube.

I don't see avalanche issues as a problem.  Dive ops should avoid touching the bottom as much as possible, so dragging ROV cables and etc around on the bottom is not going to happen.  The natural energy exerted by the ocean is probably more than anything we could apply, so if avalanches are imminent, they'd already have been triggered by ocean forces. 

Subs would allow humans to be looking at deeper depths than recreational diving, but are complicated machines requiring support specialists and maintenance.  There is also a risk of having humans at depths that could be life threatening.
ROVs eliminate that risk, but don't allow the freedom and field of view that a sub would allow.
Subs and ROVs can't operate in the very shallow water without risk to the machines which tend to be really expensive.  Walt is working on a simple and cheaper ROV that might work better in the shallow waters.

Targets to search first are the ones we think we've identified already - the debris field and the anomaly, and move out from there in some logical way.  I don't see divers and ROVs in the water at the same time in the same place, so having the two tangled or affected by each other is not likely.

I'm by no means the definitive expert in these matters, and I'm not the TIGHAR dive master, so I expect there will be more opinions expressed.

I hope that helps.

Andrew
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Greg Daspit

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Re: Underwater search operations
« Reply #3 on: June 01, 2013, 01:30:35 PM »

Thanks Andrew,
Those details help alot. I see the logic in the approach (as usual for TIGHAR).
3971R
 
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Ric Gillespie

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Re: Underwater search operations
« Reply #4 on: June 08, 2013, 12:04:47 PM »

I agree with Andrew's description and assessment of the scuba situation at Niku.

During a previous expedition, divers chose to dive deeper and used a technique to dive deep quickly, then drift back up to the shallows for decompression.  My understanding is that some of these guys got down into the 150 / 200 ft depths, but would have been able to spend no significant time to search for anything down there.  Maybe they had the experience to do this (I suspect that some of them had special training paid for by tax dollars), but it is not something that I would attempt or recommend anyone attempt at Niku without the proper support.  At the time, the risks involved were not well understood by TIGHAR, but they are now, and I doubt that any dive profile below recreational depths will ever be condoned during a future TIGHAR expedition except if performed by professional divers with the right training and equipment, probably including a re-compression chamber available.

That was the very first trip in 1989.  One of the dive team members was an Air Force physician with special training in hyperbaric medicine but going to the depths they went to - mostly for the purpose of collecting valuable "black coral" - was totally irresponsible.  I didn't learn they had gone that deep until after we got home and I saw the dive logs.  There is a learning curve to mounting expeditions to places like Niku. We do lots of things differently now.
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