mlml logo

nsf logo Powered by Blogger.
Saturday, October 20, 2007: Everybody dives!
With our dive tomatoes sitting 200 m apart, it was easy to drill a single hole halfway between them to for the third transducer and have both divers and the VideoRay in the water at the same time. Since we are still working in front of the station, where the ice is 8 years old and 7 m thick, anything that reduces the number of holes we have to drill makes things much easier.

Here's our Tucker, placing one tomato over a dive hole.
(continued...)

We started with the hole drilling, taking advantage of the enthusiasm of our tender for the day, Elizabeth. Elizabeth had volunteered to help the divers, and we asked her to also help us drill the transducer hole. In an ideal deployment, we have an isosceles right triangle with the main hole (that the ROV goes through) at the 90 degree angle. The length of the legs is constrained to 30 m by the length of the transducer cables, but the longer they are, in theory, the better the accuracy of the navigation. However, this time, the distance between the tomatoes forced our triangle into a squashed configuration so that the angle was greater than 90 degrees. You’ll see the consequences of this later.
Bryan's-eye view from the lab window of our deployment configuration (badly drawn by Stacy, who still has not mastered the basics of perspective nor any graphics program).

Then, to make things even more confusing, we deployed the ROV through the north tomato dive hole instead of the central hole. We did this because we were using the VideoRay, which won’t fit in the smaller 15 cm hole that we drill, and testing the navigation with it. We couldn’t get the heater in the tomato going because we were missing the hose that connects “Mr. Heater” to the propane canister –another example that you have to check EVERYTHING and not assume that just because you request a heater and a fuel tank that you will also be issued the thing that goes between them… Even Mamback, our friendly propane guru from the carps shop, did not have the right part. So it was a chilly start. We warmed up though by taking turns chipping and dipping – the ice out of the dive hole, that is. About 8 cm of ice formed since the hole was drilled, less than 24 hours ago! And since the tomato sits well off the ground on its skis, and there is a lot of snow on top of the ice here, we had a 1.5 m drop from where we were and the ice we were trying to remove. Add that the tomato is somewhat small and this led to very interesting “crab moves” as we tried to slam the 7 kg chipper bar hard enough into the ice to break it but not so hard that it slipped out of our hands and got lost in the water below.
Mindy and Elizabeth demonstrate proper crab posture for chipping and dipping.

After finally getting the hole clear of ice we put in the down line with 2 cameras (still and video), a core rack, and a quadrat and slate on it, as well as the usual flashers and flags for relocation, and pony bottle for emergencies. The we put in the transducer, and then the VideoRay, and finally, the divers, Nick and me. After we were all gone and there was a moment of peace in the tomato, Mindy and Elizabeth rigged an ingenious carabiner clip system to help them hoist our 27 kg (60 lb) tanks and 18 kg weight belts out of the hole, and put in the ladder so we could climb out.

Finally those pesky divers get out of the way and leave the tenders in peace. This is an image taken by the VideoRay of Stacy dropping down through the dive hole, just before she got her legs tangled in the tether...

Underwater, Nick was taking imagery of the larger animals and I was collecting samples of the tiny infaunal animals that comprise the community, to allow us to fully describe the ecosystem. Once again, the still camera refused to work though the video worked fine. And it was great having the ROV in the water too – its lights helped illuminate what we were working on, we could grab it and direct it at something we wanted a picture of, and of course, Marcus and Bob who were operating it out of the Tucker had their own list of goals – to map the positions of sponges and trash on the surrounding seafloor.

Another VideoRay image of Stacy collecting core samples (and making a little duststorm of mud).

Here’s where the configuration of the transducers became interesting. The navigation system works by sending a signal from transducer 1, which causes the transponder that is attached to the ROV to send out another signal that is received by transducers 1, 2 and 3. The transducers are all hooked into the same clock, and the speed of sound is constant (presuming the water conditions are constant) so by the tiny differences in when each transducer hears the ping, the system can calculate how far away each transducer is from the transponder. The problem is when you have a really squished triangle, the error can be just enough that you can’t tell if you are on the right side or the left side of the baseline. Then you have to use other information, like depth, to confirm where you are. Either that, or there were some spooky mirror images on the seafloor. But in the end, we were navigating right back to mapped targets with 1 m accuracy, so we were quite happy…though we’ll keep working toward the 20 cm accuracy target!

Some of the sponges (white) and plastic trash (yellow) that the VideoRay was mapping.





Comments:
Post a Comment!

Subscribe to Post Comments [Atom]



<< Home
Feeds
Subscribe to Posts [Atom]

Archives
This material is based on work supported by the National Science Foundation under Grant No. ANT-0619622 (http://www.nsf.gov). Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
Webmaster