A physical system for 24/7 observation of the Indus River dolphins through Bio-acoustic Tracking (funded by WWF-Pakistan as part of IRDCP)
Group with: M.A. Rana, T. Ahmed, A. Zia. Supervisors: A. Muhammad, S. Mazhar.
Problem
Over the last few decades, a rapid decline in the population of the Indus River dolphins (Platanista gangetica minor) – one of the few river dolphins in the world – has brought them on the verge of extinction, most probably because dolphins wander into the irrigation canals during high flood season, but in winter, (when the canals are closed for cleanup) they get trapped in small desiccating pools and die because of inaccessibility to food.
For their conservation, we need to develop a robust and reliable system for tracking and analyzing the activities of these dolphins, so that the stranded dolphins can be timely rescued, and any other reasons for the decline in their population can be determined.
For their conservation, we need to develop a robust and reliable system for tracking and analyzing the activities of these dolphins, so that the stranded dolphins can be timely rescued, and any other reasons for the decline in their population can be determined.
Approach
Lacking a functional eyesight, the Indus River dolphins perceive their environment through echolocation (the use of sound waves to detect objects). To achieve this, they produce sound waves in the form of trains of pulses (or clicks) – click trains – ranging from 15kHz to 250kHz in frequency, and sense the reflections back. We can sense those clicks through underwater-sound-sensing devices, called hydrophones. These clicks are then extracted from the stream of incoming sensory data using an appropriate algorithm. We extracted the clicks through amplitude thresholding as well as through variance-based impulsive event-detection. To calculate the direction of arrival (DoA) of a dolphin's signal, we used two hydrophones. The time-delay of signal reception between the two hydrophones can be used to find the DoA, using the same technique as that used for multilateration.
Calculating time delay
To calculate the time delay, the signal from each hydrophone is segmented such that each segment contains only a single click. Then two segment containing the same click received by the two hydrophones are cross-correlated with each other and the time delay is calculated as
td = tmax – to ,
where tmax is the time instant at which the maximum of the cross-correlation function occurs, and to is the time instant at the center of the time-axis of the resulting cross-correlation plot.
Direction of Arrival (DoA) Estimation
Once the time delay is calculated, we can calculate the angular position of the dolphin as shown in Fig 3, which constitutes 1DoF tracking trhough DoA estimation. But, even with two hydrophones even 1DoF tracking is ambiguous; two different positions of the dolphin can cause the same td and hence causing only one DoA to be calculated. The second dolphin position causing this ambiguity is shown in Fig 4.
We conducted two visits to Sukkur for recording the dolphin signals.
Using the data we recorded from the two hydrophones, 1DoF DoAs of the dolphins detected in time spans of 3.6s were simulated, but with the ambiguity that has already been discussed, as shown in Fig 5.
To remove this ambiguity, another hydrophone should be placed ideally in a plane perpendicular to the plane of the previous two. This type of a hydrophone array allows the calculations of two angles, allowing the 1DoF DoA to be completely specified using triangulation.
Data Recorded
An Indus River dolphin's click train from the data we recorded is shown in Fig 6a, and typical click of the dolphin is shown in Fig 6b.
We analyzed the data in detail, exploring the temporal and spectral features of the dolphin signals, formulated the results and discussed them in one of our possible publications currently submitted to JASA.
Publication
Sikandar, M.U.B., Rana, M.A., Mazhar, S., Ahmed, T., Zia, A., Muhammad, A.. Echolocation Characteristics of the Indus River dolphins in the wild, (submitted).