Wednesday, November 4, 2009

If it wasn’t for bad luck…

Leaving it’s mark(er). When we come back here in future – and we surely will – this little marker on the seafloor will provide a “reality check” between one cruise’s navigation systems and the next – as well as a lasting testimony of some of the challenges faced, and the contributions made to deep submergence research by “the little ship that can” and all who sailed with her to get us this far along our path.
Position at 14h00 EDT – Too rough for Nereus, CTD undergoing repairs
Lat: 18° 31’N Long: 081° 44’W
Water depth: ~4850m

… we wouldn’t have any luck at all, it seems. When you’re out at sea you sometimes just have to take a few knocks and keep going. Where that becomes tough is when the knocks keep coming from various directions. Here’s yesterday’s tales of woe:-

1. Deceived by bathymetry. On Monday night we thought we had the source of a low-temperature site narrowed down because the source, based on depth, should have been on top of one of two small hills: the only places shallow enough to be responsible. Trouble was, when we landed with Nereus, the depths on our map were out by about 150m or so. This made our search area (everywhere shallower than 2150m) about 10 times bigger than anticipated. We searched hard for 10 hours and got lots of information about where the Eh signals were getting stronger and weaker in the water column, but we eventually got to the point where we couldn’t double back on our fiber. The final piece of evidence showed the source was still to the south of us, but all we could do was go north – and we were out of battery power anyway. So we left one of our markers at the seafloor to show where we left off – anchored, instead of the usual dive-weights, with a coffee mug of the RV Cape Hatteras (a ship’s tradition we were happy to help continue) and headed back up.

2. Deceived by sensors. Something else we found out during that last Nereus dive. When you are driving across the seafloor and then stop, our temperature sensor gets warmed up and, if you stay put long enough, the warming effect (because you aren’t constantly driving forward into fresh cold water, can also cause small signals on the Eh electrode. We found this out by stopping to look at some outcrops of old lava covered by sediment on this last dive. Nothing hydrothermal in site, but when we moved off, our sensor data had recorded anomalies just as big as the ones Nereus had collected at the end of Leg 1 while making a map close above the seafloor. There, the biggest anomalies coincided with when the vehicle – in autonomous mode, was having to climb the steepest terrain: across the very top of the volcanic hill and when approaching the base of, and rising up , some steep cliffs. Both locations looked attractive as potential hydrothermal sites – so the same data set had fooled three of us, each working independently to select where we should dive with the ROV to make sure we didn’t make a mistake. So how did we get that wrong? Well, steep terrain like that is also exactly where Nereus, in autonomous mode, would have been most likely to have to slow down and take time to adjust its dive plane before climbing vertically up and over the seafloor. And that slowing down process, we now know, could be just as capable of generating anomalies in temperature and Eh: signals we had mistaken for a hillside covered in weak diffuse hydrothermal flow.

3. Heart-broken by the weather. Even though we had spent two ROV dives – and had been preparing for a third - chasing after false signals in our sensor data, we still could have been in good shape for the rest of the cruise. Whatever else happened, we still knew of one high-temperature site that should be present in our search area. We don’t think it is likely to be the source of most of the plume signals we have been detecting but it is less than a mile from the deeper hill we had dived on previously. We found it rather by accident on an early Nereus dive when our compass went wrong, so we don’t have the greatest navigation data to show exactly where it is. But even so, we believe we know where to go to within 100m on the seabed because between 35 and 36 minutes past midnight, on an evening dating from about 2 weeks ago, Nereus passed through a column of hot, smokey water while driving around just 120m above the seabed. So last night we went back through what data we could salvage from that work to decide exactly where Nereus should dive next as an ROV. We got up this morning (most of the Nereus team were up at 5am) to prepare everything for launch, and then waited for the latest weather forecast. Which told us that the weather was due to turn worse today and might continue to get worse still for the rest of the week. Given the problems we had at the end of Leg 1 we had no choice. We had to postpone today’s dive until further notice and hope that, for the first time in a while, the weather forecasters will be proven wrong

4. Let down by our trusty CTD. You wonder whether your time’s up when even your best friends turn against you. Given that the sun is shining, the sea is blue, and there is a fresh breeze blowing, even the fact that we still couldn’t launch Nereus this morning didn’t have me too downhearted. There was still much to learn and interesting samples to be taken with the CTD that has been a trusty work-horse since the cruise began. But overnight last night and into today, even that has begun to get its own ideas about how many more times it wants to be deployed. Since 8am this morning we checked through everywhere we have already been in our area and where is left that the big vent-site we are after has left to hide. With another 12 hours of work we reckon we would be ready to lower the CTD right onto the site. Even if we couldn’t launch Nereus again in our last 3 days, that would still be pretty cool to achieve. But maybe the CTD has become self-aware and doesn’t want to risk getting its feet scorched? Because on 3 attempts since 10m, it has thrown up technical problems when we’ve tried to put it to work. Poor Steve is doing his best but it must be frustrating – Chris and Daniel have volunteered their help from the Nereus team (they are frustrated in a different way because they don’t have any ROV driving to do) but there is a limit to what any of the rest of us can do but sit here and wait.

As I said to Andy just before lunch: “A weaker person might begin to feel discouraged at this point.” But I’m probably just flattering myself there – weaker should probably be replaced by “smarter” or, at the very least, “less stubborn”. But we still have 60 hours left (psychological trick: sounds bigger than just 2 and a half days) so hope springs eternal - no point coming all this way and then not doing as much as you possibly can, right up to the end.

No comments:

Post a Comment