Fin whale localization and environmental inversion using modal arrivals of the 20-Hz pulse

Abstract

Fin whale doublet calls, described as two 20-Hz pulses recorded at different interpulse intervals, have been attributed to the whale's calling behavior, however they could also result from acoustic mode propagation effects. Modes travel with their own frequency-dependent group velocities. The dispersion of these modes results in the sound being recorded as multiple arrivals on a receiver. Multiple modal arrivals of 20-Hz fin whale calls were recorded, with mode one arriving after mode two. The time delay between modal arrivals varied throughout the recording and is range dependent. The range dependency of this time delay was used to estimate the range of the whale from individual hydrophones by determining the modal group velocities that resulted in the observed delays. A pair of hydrophones was used to localize the whale for the duration of time when two modes were detected. The KRAKEN normal mode model was utilized in an inversion scheme to determine the compressional wave speed and depth-to-bedrock in the study area that supported the estimated modal group velocities. The inversion resulted in a depth-to-bedrock of 200 m and compressional wave speed of 1735 m/s, which were supported by measurements reported in literature for nearby regions. [Work supported by BOEM.]