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

• a water column sound speed structure based on that shown in Figure 4-bottom (ie. a 200 &
75 m thick Leeuwin current) and extrapolated to that shown on Figure 5, and input into RAM
at 25 m depth steps;
• the seafloor parameters listed in Table 2 with the medium sand layer thickness constant at 0.5
m for water depths ≤ 200 m then linearly increased to 50 m thick at 2500 m water depth;
• the eight bathymetry paths shown in Figure 17, with the bathymetry calculated at a 250 m
interval and input into RAM in 500 m steps;
• the transmission loss calculated in 100 m steps out to 37-50 km (depending on which heading
was used);
• using the principle of reciprocity, the RAM receiver placed at the actual source depth of 20 or
40 m, and a RAM source placed at the actual receiver depth of 475 m (the bathymetry
program interpolated depth at the bluey logger site)
• a frequency of 24 Hz.
The output of RAM gave transmission loss (dB) with range, at the specified frequency, for a
fixed receiver depth equivalent to the bluey logger location, a source depth of 20 or 40 m and for
eight profiles on 45 o headings from the logger location. To then convert the transmission loss
estimates to a blue whale component received level at a specified range, required an estimate of
the appropriate component source level. This is dealt with below.
3.3) Blue whale source levels
Three estimates of the source level (theoretical level of the noise source assuming it to be a point
and the measure made at 1 m range from the point) of blue whale calls are given in the literature.
Cummings and Thompson (1971) present an estimate of an average source level in the 14-222
Hz band (or encompassing all the signal energy) as 188 dB re 1μPa at 1 m, for blue whales
recorded off the Chilean coast. They do not state the type of measurement so it is assumed to be
the mean squared pressure or squared rms value of the signal, since this is the most common
measurement type reported. In terms of absolute pressure this is mathematically equivalent to:
p
2
1
=
T
Te
2
∫ ps+
n
T0
( t)
dt
where:
2
p = the mean squared pressure or the square of the rms pressure, reduced to an equivalent
value at 1 m range from the source
T = the signal duration
ps+n = the blue whale pressure signal plus the noise
In all calculations carried out in this analysis the mean squared pressure was calculated by using
the portion of the signal which encompassed 95% of the signal energy (thus standardising the
way T was calculated) and by subtracting the background noise component from the signal of
interest (ie replacing
p + above with
2
s n
p ).
2
s
Cummings and Thompson (1971) estimated the source level using spherical spreading and visual
estimates of range from calling whales.
D'Spain et al (1995) used complex localisation techniques from recordings made with vertical
arrays of hydrophones to estimate the 3D location of calling blue whales off the Californian
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