Precise sound source localisation of dolphin biosonar pulse transmissions

Dolphins rely on auditory perception for survival and their biological sonars (or biosonars) equip them for life in marine environments. Dolphins probe their habitats using sequences of short underwater sound pulses (or clicks) for echolocation that enable them to navigate and avoid collisions with natural objects in the environment, and to detect and discriminate between prey, predators, and companions. In May 2009, sequences of biosonar click transmissions, which were emitted by Tursiops aduncus dolphins while swimming freely in their natural habitat, were recorded by a linear array of three widely spaced hydrophones located close to the sea floor in Jervis Bay (35° 07' S, 150° 42' E), New South Wales. In this paper, the hydrophone array output data for two sequences are processed for a 7.5 s observation period, where the arrival time differences of the biosonar signals at the hydrophones are measured to within 0.1 μs. The active biosonar of an individual dolphin is observed to operate in two modes - a regular mode, where the time interval between consecutive click transmissions is long (13-59 ms), followed by a burst mode, where there is a rapid succession of pulses with a short time interval (2-9 ms) between buzz signal transmissions. The source position of each click is localized by using the method of passive ranging by wavefront curvature. This nonintrusive acoustic source localisation method requires the differences in the arrival time data of the received pulses to estimate the range of the source from the centre of the array, as well as its bearing angle with respect to the longitudinal axis of the array. During the observation period, a dolphin is detected near the array. Initially, it is at a range of 28 m at the beginning of the regular click phase, and then it closes to a range of 13 m by the end of the buzz phase. Also, there is a rapid source bearing rate change during the buzz phase, with the bearing changing by 9° in 0.7 s. The instantaneous range and bearing of the moving source change continuously and vary by less than 13 cm and 0.07° respectively between interclick transmissions, and 2 cm and 0.06° between consecutive buzz signals. In addition, the arrival times at the hydrophones of click signals from a far dolphin are extracted from the data. For the 17 click transmissions that formed a sequence, the interclick time interval varies from 34 to 58 ms. The passive ranging by wavefront curvature method estimates the range of the far dolphin as 239±1 m with a bearing of 30.05° ± 0.01°. With this source localisation method, the precision of the source bearing estimates is within 0.01°, independent of the source range. The precision of the source range estimates varies from 1 cm for the near dolphin buzz sequence to 1 m for the far dolphin click sequence.