Sound is used by humans and marine life to communicate underwater. This is partly because electro-magnetic waves (including light) do not penetrate very far through water. Sunlight is more or less gone below a few 10s of meters, and the rest of the ocean depths are in complete darkness. On the other hand, sound travels long distances in the ocean. Whales take advantage of this property of seawater to communicate with each other across whole ocean basins.
Oceanographers also take advantage of long-distance sound propagation in the ocean. For example, the RAFOS floats that we will be launching during this cruise and which were discussed in an earlier post listen for the moored sound sources over distances of 100s or 1000s of kilometers. In this case, relatively low frequencies are used (about 260 Hz) because lower frequencies travel longer distances.
Last night we started testing the acoustic releases that will be used to retrieve the deep-water current meter moorings that are also being deployed as part of the OSNAP project. We are going to be deploying about 20 of these moorings during this cruise leg. Each one will have attached to it a number of self-recording sensors to measure temperature, salinity and current speed and direction over the next year. Considering the huge expense to get these moorings into the water, it is extremely important that we get them back after their one year in the water so we can retrieve the data.
The acoustic release is an instrument that is attached to the mooring near the very bottom—just above the mooring’s gigantic anchor. When we come back a year from now, we will transmit a special signal that only that acoustic release will understand (at a frequency around 12 kHz) which will tell it to “let go” of the anchor. At that point, all the flotation on the mooring which have been holding it upright will cause the mooring wire and the attached sensors to float to the sea surface where it can be recovered by the ship. If the acoustic release doesn’t work right, then we don’t get the mooring or data back. So before being deployed on an actual mooring, each release is lowered down deep on the CTD package, and we practice talking to it to make sure it hears our signals, talks back to us saying “I hear you”, and that the release mechanism operates on demand. We tested three releases last night, and they all worked as expected. An underwater microphone (called a hydrophone, of course) is used to listen for the signals from the release. I recorded the pings from the release and you can hear them on this audio file. The first and loudest sound in each set is the direct signal from the release, which is suspended at 2500 meters water depth. The “spray” of sounds after the main signal are echos of the main signal that are bouncing off the sea floor in various locations.
At the same time we were doing these tests, a school of dolphins appeared right next to the ship! I don’t know for sure, but I suspect that they could hear the pinging sounds and had come by to investigate.
» Listen to the sound of an acoustic mooring release being tested on the R/V Knorr.
The first and loudest ping that is heard in each group is coming directly from an acoustic mooring release that is attached to the CTD package suspended underwater at a depth of 2500 meters. This is how the acoustic release is tested before it is actually deployed at the base of a mooring. It is crucial to make sure this piece of equipment works properly because it is used to get the mooring, and all the attached sensors, back to the sea surface and on to the ship after one or two years.