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

Code:46.2008-final-Feb9
body of whales, which are complex, composed of layers with very different physical and acoustic
properties. Even when the three-layer model is simplified by not considering the skin and muscle, it
might still provide a better estimate of acoustic reflectance than the two-layer model with large
impedance mismatch used by Urick. These considerations might explain the unexpectedly low TS
measurements Au’s 1996 obtained for bottlenose dolphin and might also help explain the relatively low
killer whale at 200-kHz frequency.
As with any other experiment, unavoidable measurement errors are always a factor affecting
estimates. For example, the pulse length of the Biosonics DT-X system is 400 μs (user selectable from
0.1 ms to 1.0 ms), which is likely too short to insonify an entire whale; this is most certainly the case
when the whale is approaching or swimming away from the sonar. In that case, the TS of killer whale at
tail or head aspects would be underestimated. The TS of an underwater object is likely to be less at short
ranges than at long ranges. One reason is that a very directional active sonar signal fails to insonify the
entire target. Another reason is the target reflection as echo usually acts differently from point source,
depending on the size and shape of the underwater object.
The trajectory of whale motion in Figure 1 and the whales’ speed as a function of time in Figure 3
show that during the 3-min sonar observation period, the whales first were approaching the transducer for
about 20 s and then moved closest to the transducer for about 45 s. In the final 20 s, the whales moved
away with increased speed. Similar behavior was observed also in studies by both Love (1971) and
Lucifredi and Stein (2007) with center frequencies of 20 kHz and 23 kHz for humpback whales and gray
whales. They observed that the whales approached the transmit site, slowed down or even stopped to
listen for a while, and then moved on with increased speed. It is generally considered safe to use
echosounders at frequencies above 180 kHz in the vicinity of marine mammals because this is out of the
hearing range of most marine mammals. However, the recent study of a signal leakage of active sonar
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