Quarterdeck Volume 6, Number 1, May 1998
Cetacean sound off
Underwater mikes detect whales kilometers away
All whales and dolphins studied to date are very vocal, producing a wide variety of species-specific moans, whistles and clicks. GulfCet scientists use underwater microphones to help them detect and identify whales and dolphins by listening to them.
These underwater microphones, or hydrophones, are variably spaced along a cable, which is extended into the water about 15 meters. The ship tows these "acoustic arrays," while three tape recorders record underwater sounds-both man-made and animal. Acoustic signal processing software displays real-time spectrograms on the computer screen, allowing us to monitor the sounds made by whales and dolphins. We also can monitor underwater sounds produced by the ship and human activity.
The GulfCet I array had a detection range for sperm whale sounds of 11 kilometers on each side of the towing vessel. The large sample size has reduced the population estimate variance and increased the precision of the population estimate.
The GulfCet II array was smaller, shorter, and considerably more sensitive than our previous array, resulting in a greater detection range. Plus, because we could tow it at faster speeds, the GulfCet II array increases the area we can sample during each cruise. Also, because GulfCet II focused on sperm whales (see page 9), the array was designed to optimize its directional characteristics at the most predominant frequecy of sperm whale pulses (five kilohertz).
Also attached to the array were two time-depth recorders-one just behind the first hydrophone and another just aft of the last hydrophone. These recorders provide data on the depth of the array and the water temperature at that depth. This information is used to calculate sound velocity at the towed array depth, and we use that data to determine how far away the cetacean was from the hydrophone when we recorded it. The recorders also provide additional information that can be used in habitat characterization, such as the water temperature at a depth of 15 meters throughout the cruise.
A cable containing hydrophones is unwound into the water and towed by the ship. On board, scientists listen for the clicks and moans of cetaceans.
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Last updated May 1, 1998