The use of acoustic multipath for localization of sperm whales

Abstract

Sperm whales produce impulsive sounds, or ‘‘clicks,’’ that are ideally suited for acoustic tracking. If a widely distributed passive acoustic array is available, then the relative time delays from a click’s direct path can be used to derive an animal’s location. There are many interesting circumstances, however, where deploying a wide-baseline 3D array geometry is either unavailable or impractical. In such situations acoustic reflections of clicks from the ocean surface and bottom can be used to effectively convert a few hydrophones into a wide-aperture array that permits localization in range, depth, and azimuth. In addition acoustic multipath, particularly surface reflections, can also simplify automated detection and tracking procedures, and permit a single hydrophone to be used to distinguish between multiple whales calling simultaneously. Reviewed here is recent research on how multipath effects detected on one or two hydrophones have been used to recover dive profiles of sperm whales in the Gulf of Alaska and the Gulf of Mexico. This information, in turn, has been used to explore whether the animals exploit multipath for their own purposes, to estimate signal directionality, and to observe how sperm whales remove fish from longlines. [Work sponsored by ONR, the Minerals Management Service, and the North Pacific Research Board.]