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

Abstract

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