Abstract
Underwater acoustic vertical arrays can localize sounds by measuring the vertical elevation angles of various multipath arrivals generated by reflections from the ocean surface and bottom. This information, along with measurements of the relative arrival times of the multipath, can be sufficient for obtaining the range and depth of an acoustic source. At ranges beyond a few kilometers ray refraction effects add additional multipath possibilities; in particular, the existence of a sound-speed minimum in deeper waters permits purely refracted ray arrivals to be detected and distinguished on an array, greatly extending the tracking range for short-aperture systems. Here, two experimental vertical array deployments are presented. The first is a simple two-element system, deployed using longline fishing gear off Sitka, AK. By tracking a tagged sperm whale, this system demonstrated an ability to localize this species out to 35 km range, and provide estimates of the detection range of these animals as a function of sea state. The second deployment—a field trial of an 128-element, mid-frequency vertical array system off Southern California—illustrates how multi-element array gain can further extend the detection and tracking ranges of sperm and humpback whales in deep-water environments. [Work supported by NPRB, NOAA, and ONR.]
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