Three-dimensional localization of diving sperm whales using a short-aperture towed horizontal array

Abstract

Three-dimensional sperm whale localizations are typically obtained via recording their ‘‘clicks’’ on widely distributed hydrophones. Here, an alternative 3D localization method is investigated, using data collected by an 8-m aperture, five-element horizontal towed array deployed from the NOAA ship Gordon Gunter in the Gulf of Mexico, as part of a recent sperm whale pilot study. During the night of 3 July 2000, the Gunter towed the array at a steady 1-kn speed and 60-m depth through a pod of sperm whales, in 1000-m-deep water. During that time, surface and bottom reflections from a single sperm whale click were often recorded. By measuring the bearings and relative arrival times of the direct arrival and reflections, the whale location, array depth, and array tilt could be computed. The latter results were checked against measurements obtained from a time-depth recorder attached to the array. Assuming that the speed of the ship remained fixed and the prevailing currents did not change, subsequent positions could then be obtained using only a surface reflection. This method may provide a convenient way for observing dive profiles of animals within a few kilometers of a survey ship. [Work supported by Minerals Management Service, National Marine Fisheries Service, and ONR.]