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

Code:46.2008-final-Feb9
insonifying beam incidence are consistent with our interpretation of whale motion based on analysis of
the change in range with time of echoes from the whales.
D. Target Strength As Function of Aspect Angle
The TS observations of three individual of killer whales as a function of the angle of incidence of the
acoustic system beam are shown in the polar plots of Figure 6 for the three observation segments. In the
first observation segment, most of the TS measurements are for head-on aspect because the whales were
approaching the transducer. The TS observations for this segment vary from about −45 dB at an
insonifying incident angle of 20° (near head) to −22 dB at an aspect angle of 80° (near broadside aspect)
(Figure 6a). In the second observation segment (Figure 6b), the highest TS observations, about −10 dB,
were at 90° incidence (broadside); however, there are also a few observations of high TS at 0° (head
aspect) and 180° (tail aspect). In the third observation segment (Figure 6c), the TS measurements were
primarily from incident angles larger than 90°, more toward tail aspect, as expected for whales that were
moving away from the sonar system. The highest TS value observed was −7 dB at broadside aspect. The
data also show a lobe in the directivity at about 110°, with TS values up to −13 dB. The lowest TS values
observed were near −40 dB.
E. Target Strength Modeling of Killer Whales
The TS directivity estimates at 67 kHz for killer whales along with the TS directivity for bottlenose
dolphins measured by Au (1996) are shown in Figure 7. The estimated TS of killer whales is modeled on
the measurements of bottlenose dolphin reported by Au (1996) with a 10.77 dB offset to account for the
difference in the size of the lungs in the two dolphin species. The TS of an adult killer whale 7.5 m in
length at a frequency of 67 kHz is approximately −8 dB at the broadside aspect and −28 dB from the tailon
aspect. This estimate is applicable for 7.5-m killer whales swimming close to the surface; however, the
TS of aquatic mammals vary with water depth because of changes in lung volume with increasing
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