Blue whale calling in the Rottnest trench-2000, Western ... - ANP

strong downward refraction below the base, where the sound speed dropped sharply. This effect
can be illustrated in a ray plot. This type of analysis treats the sound waves leaving a source as a
series of closely spaced rays, then calculates the trajectory and refraction of each ray as it passes
through layers of water with different sound speeds, taking into account reflections off the
surface or bottom An example of the ray trajectories near the bluey logger site for a 200 m deep
Leeuwin current (as for Figure 4, bottom plot), a source at 40 m depth (the commonly reported
depth of a calling blue whale, Thode et al 2000, D'Spain et al 1995), in a constant 450 m water
depth, out to 20 km, are shown on Figure 15. For simplicity the bottom reflected rays are not
shown. This figure shows strong downward refraction just below the Leeuwin current such that
of the rays plotted, none are evident below 100 m depth from 11 km outwards. In reality signal
energy would occur in this shadow zone due to sound energy reflecting off the seabed or
entering the seabed and later being refracted or reflected out of it back into the water.
Figure 15: Ray trace for a 20 Hz source at 40 m depth in a constant 450 m water depth using the sound speed profile
of Figure 4-bottom (200 m deep Leeuwin current) extrapolated as for Figure 5. Strong downward refraction is
evident. Made using the 'Bellhop' program (Porter, 1994), with only surface reflections included.
The interaction of sound waves with the seafloor is vitally important for the propagation of
sound in shallow water (continental shelf and slope, waters). Sound energy may reflect off the
seafloor, be scattered off the seafloor or may enter the seafloor, travel through it, then be
reflected or refracted back into the water column. There are several mathematical approaches for
calculating how impinging sound waves interact and refract in the sea floor. These are
implemented in specialised programs which also include the surface reflection interaction and
water column sound speed variations. Each of these programs has its own set of limitations and
peculiarities necessary for successfully running and interpreting the outputs. All require detailed
geological information on the seafloor properties. No geological information on the seafloor
characteristics appropriate for sound propagation studies were available to the author for the
trench region. Based on sound propagation modelling by Hoffman et al 2000 and Jones et al
2000, a set of sediment properties for the shelf above the 200 m contour, west of Rottnest was
extrapolated to encompass the whole trench region. The parameters used are given in Table 2.
These parameters were used for all water depths < 200 m. For water depths > 200 m the depth of
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