Target Strength of Southern Resident Killer Whales ... - BioSonics, Inc

Code:46.2008-final-Feb9
the second observation segment yielded TS values ranging from −10 dB to −48 dB over a range of aspect
from head through broadside to tail. In observation segment three, TS observations ranged from −7 dB to
−40 dB for retreating whales insonified at the tail aspect. These TS measurements are relatively
comparable to the model for killer whales at 200 kHz, which predicts TS values within the range from
−15 dB to −35 dB. To our knowledge, no other field measurements of killer whales TS have been
reported in the literature.
The target strength model of killer whale TS is based on TS measurement of a bottlenose dolphin at 67
kHz and a three-layer reflection model used to extrapolate modeled killer whales TS values from 67 kHz
to 200 kHz. The TS of an adult killer whale was modeled for an insonifying frequency of 67 kHz by
assuming that a killer whale shares similar morphological structure with a bottlenose dolphin and that the
ratio of lung length to total length is the same across dolphin species and sizes of animals within species.
The TS of killer whales at higher frequencies - 200 kHz were modeled using a simplified three-layer
model with correction based on the estimated differences in acoustic reflection coefficients as a function
of blubber thickness at different frequencies. Interestingly, the averaged measurement data of three
different Lime Kiln observation segments show that the TS of killer whales are in the range of −11 dB
(broadside) to −43 dB (tail aspect). The modeling result gives range from −18 dB (broadside) to −38 dB
(tail aspect) at 200 kHz frequency. Both the modeling and measurements TS results are surprisingly low.
In Au’s 1996 measurement of the bottlenose dolphin, he found an unexpected decrease in TS at the
broadside aspect as the frequency increased from 23 to 45 kHz. Au explained the TS decrease might be
due to the anechoic properties of dolphin skin or blubber layer because, based on the formula provided in
Urick (1983), the TS for most underwater objects increases or remains constant with frequency.
However, the data used by Urick (1983) are mostly for simple geometric shapes of uniform composition
and are assumed to have a large impedance difference from water. None of these assumptions hold for the
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