Matched-field processing, geoacoustic inversion, and source signature recovery of blue whale vocalizations

TLDR

Matched‑field processing and global inversion techniques were applied to vocalizations from four whales recorded on a 48‑element tilted vertical array off the Channel Islands in 1996, and the resulting inversion results were subsequently used to perform straightforward matched‑field processing on additional calls. Global inversions of selected whale calls using as few as eight array elements revealed ocean‑bottom composition, array geometry, and animal positions, with sediment sound‑speed estimates agreeing with field samples, most whales migrated east‑to‑west while one stayed near its original spot, all vocalizing at 10–40 m depth, and three detailed acoustic sequences illustrated matched visual sightings, two‑whale tracking to 8 km, and high‑resolution dive profiles, including an unusual signal set with FM downsweeps suggestive of internal resonance that coincided with a sudden oceanographic shift.

Abstract

Matched-field processing (MFP) and global inversion techniques have been applied to vocalizations from four whales recorded on a 48-element tilted vertical array off the Channel Islands in 1996. Global inversions from selected whale calls using as few as eight elements extracted information about the surrounding ocean bottom composition, array shape, and the animal’s position. These inversion results were then used to conduct straightforward MFP on other calls. The sediment sound-speed inversion estimates are consistent with those derived from sediment samples collected in the area. In general, most animals swam from the east to west, but one animal remained within ∼500 m of its original position over 45 min. All whales vocalized between 10 and 40 m depth. Three acoustic sequences are discussed in detail: the first illustrating a match between an acoustic track and visual sighting, the second tracking two whales to ranges out to 8 km, and the final sequence demonstrating high-resolution dive profiles from an animal that changed its course to avoid the research platform FLIP (floating instrument platform). This last whale displayed an unusual diversity of signals that include three strong frequency-modulated (FM) downsweeps which contain possible signs of an internal resonance. The arrival of this same whale coincided with a sudden change in oceanographic conditions.