Using directional frequency sonobouys to localize singing humpback whales.

Abstract

For many years, the navy has been using directional frequency analysis and recording (DIFAR) sonobouys to record and track ships. DIFAR sonobouys compute the acoustic particle velocity for two bimodal perpendicular hydrophone elements which, with an omnidirectional hydrophone and magnetic compass, gives a directional bearing to the sound. The omnidirectional hydrophone is also used to record sounds. DIFAR can be more accurate for low‐frequency whale calls than time of arrival methods, and only requires the three co‐located elements, which can function autonomously without a ship. In this study, we used two moored DIFAR sonobouys over a period of 4 days to record and track multiple singing humpback whales (Megaptera novaeangliae) off of Kaena Point, Oahu, HI. The distance between the two sonobouys was 1100 m. The digital acoustic signal was recoded at the sonobouy to preserve full dynamic range before multiplexing, and was analyzed in the laboratory using custom MATLAB programs. DIFAR localizations were correlated with visual observations from both the vessel and a 950 elevation land‐based location. Relative locations of singing humpbacks and durations of song were recorded. In addition, source levels for singing whales were also calculated.