Tracking of Fin Whales Using a Power Detector, Source Wavelet Extraction, and Cross-Correlation on Recordings Close to Triplets of Hydrophones

Whale signals originating in the vicinity of a triplet of underwater hydrophones, at a 2 km distance from each other, are recorded at the three sensors. They offer the opportunity to test simple models of propagation applied in the immediate neighborhood of the triplet, by comparing the arrival times and amplitudes of direct and reflected paths between the whale and the three hydrophones. Examples of recordings of individual fin whales passing by hydrophone triplets, based on the characteristics of their vocalizations around 20 Hz, are presented. Two types of calls are observed and their source wavelets extracted. Time segments are delimited around each call using a power detector. The time of arrival of the direct wave to the sensor and the Time Differences of Arrivals (TDOA) between sensors are obtained by correlation of the extracted source wavelets within the time segments. In addition to direct arrival, multiple reflections and the delays between the reflection and the direct arrival are automatically picked. A grid-search method of tracking the calls is presented based on the TDOA between three hydrophones and reflection delay times. Estimates of the depth of vocalization of the whale are made assuming a simple straight ray propagation model. The amplitude ratios between two hydrophones follow the spherical amplitude decay law of one over distance when the cetacean is in the immediate vicinity of the triplet, in a circle of radius 1.5 km sharing its center with the triplet’s center.