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.