Estimating source levels and depth distributions of calling bowhead whales using geoacoustic inversion

Abstract

Every year during open-water season in 2008–2014, up to 40 passive acoustic recorders (DASARs) were deployed in the Beaufort Sea. Over the eight-year period, more than 3.1 million bowhead calls were localized. A vertical array was also deployed near DASARs in some years, enabling the matched-field geoacoustic inversion of bowhead whale calls to estimate the local acoustic propagation environment. In turn, these environments were used to estimate the source level and calling depth distributions of nearly 50,000 calls between 2008 and 2014. The resulting source level distributions exhibited a stable mode near 160 dB re 1 uPa @ 1 m (rms), which proved robust to the year investigated, the whale call analysis method (manual or automated), and the type of acoustic propagation model (a simple empirically-derived “power-law” transmission loss model or a detailed waveguide propagation model). Depth distributions of calling whales were more sensitive to the environmental model used but always showed a mode at ~25 m...