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

source level of the type II signal in each block was, the transmission loss estimates at 24 Hz for
all combinations of:
• the single range estimate (Table 3),
• the eight bathymetry paths;
• a 200 m and 75 m thick Leeuwin current;
• and a source depth of 20 and 40 m
were added to the appropriate mean squared pressure of the type II signal to give 224 'possible'
source levels. Note that some are erroneous values since eight bathymetry tracks, two source
depths and two sound speed profiles were used, yet for each block only one combination is
correct. Propagation of only the 20-26 Hz signal was considered, as this spectral section of the
type II signal was consistently 10 dB or greater than the input from other frequencies, hence
contributed all the effective signal energy (section 3.1.3). The frequency distribution of these
'possible' source levels was plotted as per Figure 23 and the distribution bin with the highest
count taken as an estimate of the type II source level, or 183 dB re 1μPa mean squared pressure
at one metre.
This was considered a crude estimate but since there was no valid way of finding the true source
level for each call without determining which bathymetry path the animal was along, and that an
error in the range estimate of only a few hundred metres could potentially give a large difference
in transmission loss, it was considered reasonable. The value is higher than the source level
estimate of D'Spain et al (1995) of 169 dB re 1μPa msp, or Thode et al (2000) of 180 dB re 1μPa
msp, but less than that of Cummings and Thompson (1971) of 188 dB re 1μPa msp, but these
estimates are from different blue whale stocks using different methods.
The type II component source level estimate of 183 dB re 1μPa mean squared pressure at one
metre has been used below in range estimation.
Table 3: Measured relative arrival times and range estimates with predicted arrival times, for seven sets of blue
whale calls.
block path 2 / path 3 (where
available) travel time,
relative to path 1
(ms)
horizontal
range estimate
(m)
predicted path 2 / path 3 /
path 4 (where available)
arrival time - relative to path
1 (ms)
10 150 3550 150 / 418 / 777 121.4
24 120 4750 120 / 331 / 625 122.7
33 170 / 430 3050 171 / 471 123.8
43 100 6200 100 / 269 / 506 121.6
335 210 / 550 2350 212 / 571 128.8
337 223 / 596 2200 223 / 596 125.7
338 254 1850 256 / 657 128.8
26
measured type II
component level
(msp - dB re
m Pa)
3.4) Transmission of blue whale calls to bluey site
All propagation calculations concentrated on the type II component only, as this was the
component with the highest source level and which appeared best at all ranges in the
spectrogram plots. Using the transmission loss and source level estimate as derived above, a plot
of the resulting received level for the type II component about the bluey logger location, for the
bottomed receiver, source at 40 m depth and a 75 m deep Leeuwin current is shown on Figure
24 (using the eight 45 o transmission loss tracks and interpolated between).