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

Code:46.2008-final-Feb9
measurement system were manipulated to obtain the TS directivity (i.e., aspect dependent TS) of the
dolphin. Three different signals were used to measure the animal’s TS (two frequency-modulated pulses
and a broadband click with a peak frequency of 67 kHz). The target strength estimates at each aspect
were averages from echoes of 20 pings acquired for each angle of insonification of the whole dolphin or
of specific portions of the dolphin (Au, 1996). The ratio of the lung length to the full body length of this
dolphin was about 0.286. Assuming that dolphins and killer whales share a similar morphological
structure and that lung size is proportional, a 7.5 m long killer whale (mature female) would have a lung
length of about 2.143 m. Given the difference in TS of a 0.6-m-diameter sphere and 2.143-m-diameter
sphere is about 10.77 dB (Urick, 1983) we estimate that a 7.5 m long killer whale will have a TS at 67
kHz about 10.77 dB higher than that for a 2.2 m long bottlenose dolphin.
5) Three-Layer Model To Extrapolate Target Strength Modeling of Killer Whales at 200 kHz
Miller and Potter (2001) suggested the difference in TS estimates between gray and right whales was
due to the difference in the thickness of their blubber layers. They also suggested an approach to estimate
TS at other frequencies. Their method requires estimates of blubber thickness, its density and the speed
and attenuation of sound through the blubber.
The mathematic formula for three-layer reflection coefficients can be found in Medwin and Clay
(1997), Miller and Potter (2001), and Brekhovskikh and Lysanov (2003). The TS of killer whales at
higher frequencies such as 200 kHz can be estimated based on the plane wave reflection coefficient as a
function of frequency for the elements of a three-layer model (skin, blubber layer, and lung) for a whale
(Miller and Potter, 2001). Miller and Potter (2001) estimated the reflection coefficient for different
thickness of blubber using a compressional sound speed of 1700 m/s, density of blubber of 1200 kg/m3,
and attenuation of sound through blubber of 1 neper/m. In contrast, Jaffe et al. (2007) measured the
attenuation of 100 kHz sound through sirenia (manatee) blubber to be about 5.57 dB/(10MHzmm),
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