A simple and easily implemented observation method to observe sperm whales that swim in groups and repeatedly dive to depths of more than 1,000 m, is desirable in order to understand their underwater behavior. In this paper, we propose a passive acoustic method of classification and localization of individual whales, which does not disturb them, by analyzing click sound data captured by two hydrophones suspended from two boats. We classify the whales based upon the time delay between the direct clicks and their surface reflections on each hydrophone, and also the time delay between the direct clicks observed at the hydrophones. The former time delays identify two hyperboloids of potential whale locations, since the reflection on the sea surface is equivalent to the signal received at a virtual hydrophone. A third hyperboloid is similarly obtained from the latter time delay. The three hyperboloids intersect at two points when the depths of two hydrophones and the distance between the two boats, which correspond to baselines of hyperbolic positioning, are known. The whale depths and locations can be determined with symmetric ambiguity about the baseline between the boats. One of the advantages of this method is that the depth of the whales can be calculated independently from the horizontal distance between the two hydrophones. An analysis has been carried out using data collected off the Islands of Ogasawara in the summer of 2003. This method effectively distinguished six whales in conformity with visual investigation

Automatic classification of diving sperm whales by analysis of click time delay using two hydrophones

A simple method using only two hydrophones, each loosely deployed from separate mobile platforms, has been developed to simultaneously track several vocalizing sperm whales. The separation distance of several hundred meters between the hydrophones implies that a particular whale could present vastly different beam orientations towards them, thus precluding use of any specific relationship between the click signal levels at the two locations for the purpose of whale localization. This method utilizes time‐of‐arrival of the direct clicks and their surface reflections and matches them at the two hydrophone locations. Whales are segregated on the basis of the set of observed time delays. Click parameters such as interpulse interval and average click frequency are proposed to be used as secondary data for the purpose of track refinement. Depth profile of the whales is obtained independent of the separation distance between the two hydrophones. However, knowledge of the separation distance between the hydrophones provides the 3D coordinates of the whales within a left–right ambiguity. Whale tracks have been obtained using actual data collected off Ogasawara Islands in Japan. These results demonstrate the utility of this method for studying the bioacoustics and behavior of deep diving sperm whales.

Junya Ura

Papers

Passive tracking of multiple diving sperm whales using single hydrophones at two mobile locations