Sunday, April 11, 2010

Anatomy of an Acoustics Mooring

Taking measurements in the ocean is a lot different than taking measurements on land. If you want to take a temperature measurement on land, you don't have to worry about your sensor floating away! Oceanographic moorings anchor instruments to the bottom so they can stay in the same place for long periods of time. We want to make acoustic measurements for a year, and we need our sound sources and receivers to stay put, so we install, or deploy, acoustic moorings.

An acoustic source mooring, like the one shown on the left, consists of an anchor at the bottom and a buoy at the top with the source suspended in between. It is all connected by shackles and chains, and, most of all, jacketed wire rope. You can think of wire rope as the skeleton of the mooring. Everything between the anchor and the buoy is attached to it.

The buoy at the top is a subsurface buoy, meaning that it does not bob around in the waves at the surface, but it sits about 180 meters below the ocean surface so that passing ships will not hit it. The buoy provides flotation so that the wire will be pulled tight and the mooring will stand up straight.

Because the mooring is over 5 km long, we need some additional buoyancy to help the buoy keep the mooring as straight as possible. For this we use glass spheres that are filled with air so they float. These glass spheres are encased in hard yellow plastic shells so there is a way to attach them to the rest of the mooring and to protect them so they don't crack.

In the mooring drawing we show here, the sound source is located 1050 meters below the surface. Above the source is an array of four hydrophones. These four hydrophones will listen to the other sources that we deploy and the other source moorings will listen to this source. We also have an ADCP (Acoustic Doppler Current Profiler) on this mooring, which sends out much higher frequency sound in the upward direction, which will be used to measure the strength and direction of the currents passing over it. A picture of the ADCP and some of the other mooring elements is shown on the right. Pictures of the glass balls and anchors can be seen in our earlier post describing the loading of the ship.

These moorings will stay in the water for a whole year collecting data, but what happens when that year is over and we want to get our hands on the data? We have to go back to the mooring location and pop up the mooring and bring it back onto the ship. It would be pretty difficult to bring a 2-ton anchor back on board so, well, we don't. Just above the anchor we have 2 acoustic releases. These instruments have a hook (where the red arrows are pointing in the picture on right) that stays locked on a chain that is attached to the anchor. When we go back to pick up the moorings, we send out a special coded acoustic signal from the ship, and when the acoustic release "hears" it, it unhooks the chain to the anchor and floats up to the surface with the rest of the mooring, leaving the anchor on the bottom.