Can acoustic multipath explain finback (B. physalus) 20-Hz doublets in shallow water?

Abstract

A broadband acoustic propagation code is combined with an empirically derived geoacoustic seafloor model for the purpose of interpreting finback doublet sequences recorded in the northern Gulf of California by Thompson et al. in 1987 [Thompson et al., J. Acoust. Soc. Am. 92, 3051–3057 (1992)]. The result is that for ranges less than 30 km, it is plausible to explain the appearance of the finback doublet in this environment in terms of multipath propagation through the water column and the seafloor. For a channel depth as shallow as 50 m, the detection range of the subbottom arrival may exceed that of the water-borne arrival. This result may have important implications for the accuracy of hyperbolic localization algorithms which assume a sound speed consistent with a water-borne propagation path. When doublet propagation is supported, the interpulse delay is demonstrated to be strongly range dependent. With prior knowledge of the geoacoustic environment, the interpulse delay may provide a practical tool for the real-time passive ranging of finbacks using only a single hydrophone or sensor array. Scientists might consider the impact the multipath-induced doublet has on long distance communication between whales based on temporal cues encoded in patterned 20-Hz pulse sequences.