Detection of leopard seal (Hydrurga leptonyx) vocalizations using the Envelope-Spectrogram Technique (TEST) in combination with a Hidden Markov model
AbstractThis paper describes a technique for the automated detection of leopard seal (Hydrurga leptonyx) vocalizations. Automatic detection of leopard seal calls within the Antarctic underwater soundscape is difficult because (a) the calls are frequently of low amplitude (b) the call duration is highly variable and (c) the frequency band overlaps with those of many other marine mammal vocalizations. However, humans easily distinguish leopard seal vocalizations from those of other marine mammals because of the calls' distinctive sound, which is a result of the pulsed structure of the leopard seal vocalizations. To exploit the unique temporal evolution of the pulse repetition rate (PRR) in high (HDT) and low (LDT) double trills, the Envelope-Spectrogram Technique (tEST) was developed. The extracted PRR feature allows detection of the target vocalizations even against a background of other marine mammal vocalizations. To handle the high variability of the calls' duration, the tEST algorithm was combined with a Hidden Markov Model (HMM) which is particularly well adapted to handle temporal variability. The developed HMM based detection algorithm worked rather reliably. The detection rate over a 4 day test period was high (72 %) although the signal to noise ratio (SNR) was low (< 10 dB). The number of false positive detections (12 %) was tolerable. Most of the false positives occurred during the period when R/V Polarstern was approaching the recording station and the SNR was temporarily < 0 dB. The detector worked 3 times faster than real-time and is therefore suitable for both off line biological research and time critical in-the-field applications, such as the detection of the presence of leopard seals in the context of human diver operations.
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