Subspace detectors for long-term analysis of blue whale vocal activity in the Indian Ocean

Abstract

The Comprehensive Test Ban Treaty Organization (CTBTO) deep water hydrophones around Diego Garcia island (H08) in the central Indian Ocean observe a number of blue whale calls. The hydrophones operational since 2002 are deployed in two groups of three (triads). The first one H08S is to the south, and the second H08N to north of the island. Not much is known about the distribution of whales in the region. The CTBTO observations are hence an opportunity to address this knowledge gap. This work builds subspace detectors to track the whales. The subspaces use the time-frequency signatures of the respective calls. There were however implementation challenges. The call-frequencies drifted across the 16 year dataset. The observed SNRs across hydrophones varied much due to fading. There were also interference issues due to multiple species vocalizing at the same time. This work extends the subspace approach to deal with these issues. The new approach tracks changing frequencies, fuses calls across each triad, and rejects interfering calls. Test results show that the new detectors are relatively robust to false alarm and improve detection rates. The work then applies the detectors to all the H08S and H08N recordings to track whale seasonality and suggest migration patterns.