Day 17 - mapping the seafloor
The sample area for this voyage, created using Multibeam mapping. Photo Amelia Connell/NIWA, map NIWA
Today we finished our sampling at Matatara knoll with three beam trawl tows. It interests me how the same sampling method can result in such different catches depending on the sample site. At the 1000m sample site we got a small, clean catch of animals, at the 1200m deep site we caught about 1 tonne of mud, and then at the 1500m deep site we, again, had a small catch of animals.
We did however get good verification of what we saw on the underwater video (DTIS) when we surveyed these sites. At the 1200m deep site we saw a lot of dark sandy sediment patches on the video, in the beam trawl, amongst the mud, were patches of dark sandy sediment and dark rocks that are similar to pumice, just a bit denser and heavier.
After our final beam trawl at Matatara Knoll we headed off to our last slope sample site, about five hours away. The travel time allowed us to catch up on our cataloguing and jar labelling. Labelling can be quite challenging at times, for the sole fact that we can’t always find the jars that we have labels for. With two shifts and five people on each it is inevitable that some get put somewhere else, even though we have a system for where unlabelled jars are supposed to go. We then have to hunt for them, which can take some time. My shift ended with the specimen jars from one tow still unaccounted for. It turns out that I was just not looking for them properly and they were exactly where they were supposed to be all along.
While we have been heading to our next sampling location the ship has still been working. Have you ever wondered how maps of the seafloor are made? Well, we have been making one as we steam along. The ship has a very sophisticated sonar attached to the outside of the hull, at the bottom of the ship, which sends and receives sound waves. Instead of sending out just one beam of sound it sends out 400 simultaneously, forming a fan of sound from the ship to the seafloor – see image below. This is why it gets the name the ‘Multibeam’.
The ship and the Multibeam system surveying the seafloor. Image Amelia Connell
Sending out lots of beams of sound, instead of just one, allows us to cover a much wider area of seafloor. In deeper water the area covered is much wider than in shallow water. At the moment we’re mapping a strip of seabed about 6 km wide, as we steam along at 9 knots.
The sound waves that bounce back from the seafloor are received by the ships transducer. Multiple computers then process this information to generate detailed maps of the seafloor topography. Multibeaming mapping takes time to do – it is a bit like mowing a large lawn, to mow the entire area you have to mow lots of strips side by side. So too with Multibeaming, you have to work your way back and forth over an area until you have complete coverage.
The inside of the multibeam lab, full of lots of electronics and computers which process all the data and generate the maps. Photo Amelia Connell/NIWA