Estimating geo-position and depth of echo-locating beaked whales using an array of drifting recorders

An array of 8 drifting recorders were deployed in the Catalina Basin off Southern California to localize beaked whales. The drifting recorders with hydrophone pairs at 90-135 m were deployed along two parallel lines with ~1 km separation between recorders. The array was re-deployed daily at approximately the same location to maintain this array spacing. Cuvier’s beaked whales (Ziphius cavirostris) were detected on 26 occasions from their distinctive echo-location clicks. On 8 of these occasions, direct-path and surface-reflected signals were received on four or more drifting recorders, which allowed us to estimate location and depth of the whales. Average array tilt during these events was less than 0.2°, and always less than 0.6°. The same echolocation clicks were seldom received on more than two recorders, so we could not use methods that require TDOA measurements between recorders. We develop a novel method of 3-D localization using the vertical bearing angles estimated from the direct- and surface-reflected signals and use optimization methods to find the unique location and depth at which these angles converged. Detection ranges varied from 1.0 to 3.7 km (mean = 2.0, sd = 0.65), and depths of vocalizing animals varied from 696-1150 m (mean = 948, sd = 152).