Sunday, April 4, 2010

An ocean acoustic tomography experiment

Have you ever seen an unborn baby on an ultrasound? To get that picture, a transducer placed on the mother’s belly sends out sound waves that bounce off the baby, producing an acoustic image of the baby inside. Acoustics can also be used in the ocean to get an image of things we cannot see with our eyes. Fish finders send out sound from a boat into the water, looking for the sound to bounce back from fish below. Whales and dolphins also use echolocation to find their way underwater by making sounds in the water and listening for it to bounce back to them. During our experiment, we will use sound in much the same way to get an image of the sea floor.

Acoustics is a great tool for looking for things in the ocean, whether that be the ocean bottom itself or things swimming around in the water, but it can also be used to look at the water itself. The speed that sound travels in the ocean depends on a number of things like the temperature of the water, the salinity of the water, and how fast and in what direction the water is moving. By taking extremely precise measurements (on the order of micro-seconds!) of the time it takes for the sound to travel from a sound source to a listening receiver, we can learn a lot about the ocean that the sound has traveled through. Some things in particular that we can look at are the temperature of the water and the speed and direction of ocean currents. This is called ocean acoustic tomography. To learn more about how sound is used underwater, check out the Discovery of Sound in the Sea website.

In this experiment, funded by the Office of Naval Research (ONR) and led by Dr. Peter Worcester, we will put out six acoustic sources, arranged a few hundred km apart in a pentagon shape, that will all transmit to a vertical line array (VLA) receiver which will have 150 hydrophones, or underwater microphones, that will be listening to the sources. Because the sources are positioned around the array, the sound transmitted from each source will travel through a different part of the ocean to reach the receiving array, giving us measurements of six different paths or “slices” of the ocean. The sources and receivers will stay in position for 1 year and we will go and pick them up and collect our data. The sources transmit for about 2 minutes at a time a few times a day every few days, allowing us to see how the ocean is changing throughout the year.