Now that the DVLA is all tucked away and listening under the ice, we need to play it some music! …well, we need to play it something anyway. We have two sound sources, the J-15 and the HLF-5, and we are lowering one down in the water and transmitting sound on and off for about a day and then lowering the other one down into the water and transmitting. We did this first at 50 km from the DVLA and then 100 km from the DVLA and now we are on our way to a spot 200 km away. We are going to see how far we can get before our time is up on this research cruise.
One reason that we are doing this is because we want to know how far sound travels in the Arctic Ocean and how what we receive on the hydrophones changes depending on the distance that we are transmitting over. Because the sound travels from the source through a bunch of water to the hydrophone receiver, the sound that we receive on the hydrophone can tell us a lot about the water through which the sound traveled.
Sound travels a lot farther and faster underwater than it does in air, but we also have some tricks up our sleeve to actually “hear” the sound at these long ranges. The J-15 and the HLF-5 sources are sending coded signals called m-sequences over and over again. The J-15 sends signals at around 75 Hz and 125 Hz and the HLF-5 sends signals at 250 Hz, which is about a middle C on the piano – actually a pretty flat middle C for the musicians out there! After we receive the sound, we decode the signals and separate all of the different repetitions of the signal. We then add all the different repetitions of the signal together to get a bigger signal. This makes it easier to pick the signal out from the background noise. This way we can get a signal to appear louder without actually playing the source louder. We call this signal processing gain.
Saturday, August 8, 2015
The Pink Hard Hat Makes a ComebackWe have now finished our biggest job out here, which was to deploy a mooring called the Distributed Vertical Line Array (DVLA). This DVLA is an basically a long wire with hydrophones clamped onto it, which is anchored to the bottom of the ocean and held upright by a buoy that sits below the water surface. Since it was a big day out on the deck of the Sikuliaq, with cranes operating and heavy equipment being moved around, I had to bust out my signature pink hardhat. Deploying an array like this is a big job and there were a lot of people involved as you can see from all the orange float coats out on deck (they are nice and warm and have built-in flotation).
|Photo credit: Bruce Thayer|
|Photo credit: Scott Carey|
|Photo credit: Scott Carey|
We deployed a similar mooring in the Philippine Sea Experiment, but because we are now surrounded by ice, we had to do things a little bit differently up here in the Arctic. In the Philippine Sea, we first put the buoy over and then strung out a bunch of wire rope and instruments that all floated at the surface in a big line strung out behind the ship until we and finally dropped the anchor and the whole thing followed the anchor down as it sank. Well, we can’t string out several kilometers of wire rope and instruments up here because we are surrounded by ice! This time, we had to put the anchor in first and slowly lower the wire in after it, clamping on all the instruments as we went, and then we finished up with the buoy. The DVLA was already mostly vertical and in the correct orientation so it just sunk straight down when we dropped the buoy. Basically we did it just backwards of the way that we did it in the Philippine Sea.
The water depth was about 3850 meters (approximately 2.4 miles) and the buoy is about 50 meters below the surface, so the mooring itself was about 3800 meters long. We attached 60 hydrophone modules (see HM post from PhilSea) as well as 24 MicroCAT instruments. The CAT in MicroCAT stands for Conductivity And Temperature, which is what the instruments measure, and what we what we need in order to calculate the speed of sound in the ocean. The way sound travels in the Arctic Ocean is largely dependent on the temperature and salinity changes that result from interactions with the ice, which happens near the surface of the ocean (we’ll talk more about that later). The hydrophones and other oceanographic instruments were concentrated towards the top of the mooring to capture all this excitement in the upper ocean.
Tuesday, July 28, 2015
Sikuliaq (“Si-COOL-i-ak” - Emphasis on the COOL!)
|R/V Sikuliaq breaking ice in July 2015|
The Sikuliaq is operated by the University of Alaska Fairbanks on behalf of the National Science Foundation, and her home port is Seward, Alaska. The Sikuliaq is a brand new ship that just left the shipyard last year, so it has all new state-of-the-art equipment.
At 261 feet long, the Sikuliaq is a pretty big ship as far as research vessels go. The main level of the ship is mostly outdoor deck space, which is great for doing moorings and storing big equipment. There is also a main lab, a wet lab, a computer lab, an analytical lab (for chemists and biologists) and a Baltic room, which is kind of like a big garage. They have a great heater set-up in there for when you’re cold on deck and need to warm up quick!
On the next deck up are the galley (kitchen), mess (dining room) and the lounge, as well as the science berthing. There is berthing space for 24 scientists, and since there are only 12 of us in the science party on this cruise, it is nice and roomy. The next two floors are berthing spaces for the crew, and then the bridge is up top. The bridge is where the captain and the mates drive the ship, and it has the best view for watching the ice break.
Also, because the Sikuliaq operates primarily in the icy Arctic, it even has a sauna! I haven’t tested it out yet, but I will. Oh yes I will.
Monday, July 27, 2015
Feeling HARPyThe main event of this research cruise is putting out an array of hydrophone (underwater microphone) receivers and transmitting sound to them from a ship-based source, which we will get to in a few days, but there are a lot of other things going on as well! Today we are steaming to the location of a HARP, which stands for High-frequency Acoustic Recording Package. This instrument package has been out here for a year recording the sounds of the Arctic, also called the Arctic soundscape.
The HARP is on a small mooring with an anchor at the bottom, then 4 meters of chain, then acoustic releases, then another 4 meters and then the HARP package, which consists of 3 tubes containing the recording electronics, the disks, and the batteries. The hydrophone itself is attached by a rope and sits above the recording package.
The HARP records at a rate of 200,000 samples per second (200 kHz). The HARP could record for 10 months continuously at this rate, but scientists are really interested in knowing how the soundscape changes through the cycle of an entire year, so the HARP is programmed to record at a duty cycle of 2/3, which means it samples 2/3 of the time, and lasts for the entire year.
HARPs have been deployed up here in the Arctic since 2006 and have recorded sounds from bowhead whales, beluga whales, ringed seals, bearded seals, ribbon seals, and once even a walrus. They also record passing ships, wind and rain sounds, and lots and lots of ice breaking.
Friday, July 24, 2015
This time we are heading waaaaay up North to the Arctic Ocean. A lot of melting has been going on in the Arctic in recent years and scientists are interested in finding out how fast the ice is melting and how this melting affects the physical oceanography of the region, as well as the rest of the globe. On this expedition, we will be exploring how sound travels underwater and under ice and how acoustic data can help us learn about the changing Arctic.
For those of you who have never been to Nome (not many people have!), it is a city in Alaska with a population of about 3800 people. It is located at approximate latitude 64.5 degrees North and longitude165.4 degrees West, and is actually closer to Siberia than it is to the mainland United States. Because it is so far north, and because it is summertime right now, it is light outside almost all day and all night. The sun doesn’t even set until after midnight! July is actually the warmest month in Nome, and the average high temperature is 58 degrees F (break out your swim suits!). Nome is a gold rush town (any fans of the reality series Bering Sea Gold out there?) and is also the end-point of the famous Iditarod dog sled race.