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

Figure 25: Output of RAM for 200 m thick Leeuwin current, 24 Hz, 450 m receiver, 40 m source and bathymetry
tracks 5 (solid line) an 6 (dotted line).
To give a statistical probability that a received level was within some defined range of the bluey
logger the following steps were taken for each of the eight bathymetry tracks (Figure 16):
• threshold received levels were chosen from 100 to 130 dB re 1μPa in 2.5 dB increments
( t i );
• for each threshold level the ranges at which the RAM predicted received level equalled or
exceeded the threshold level were found ( r n );
• for each threshold level the number of RAM range points equalling or exceeding the
threshold level ( r n ) in 250 m bin increments from 0 to the bathymetry profile length (37-50
km) were found ( n i );
• for each threshold level the cumulative sum vector of i n was calculated (ie ci
∑ni
where
28
x
=
x was the total number of bins or 250 m steps in the bathymetry path length) ;
• and the 95% point of this vector c i found to give the range within which 95% of all
components for the given threshold and bathymetry path occurred.
The resulting curves giving the 95% probability of detection range for the type II component on
the eight path lengths for the bluey logger site and a 200 m deep Leeuwin current, are shown on
Figure 26. Although for several of the paths the detection range for the lower threshold levels is
dependant on the total path length used (ie. < 107.5 dB re 1μPa for > 47 km path 5), it is clear
that the propagation of these signals to the bluey logger site is excellent. This is notable on paths
4, 5, and 6. Paths 4-6 face the opposite side of the trench on headings 135, 180 and 215 degrees
from the bluey site respectively. Paths 1-4 all run up into shallow water thus the transmission of
the 18-26 Hz signal would be expected to at some point drop rapidly, which it does.
The same propagation modelling exercise as described above was carried out for: a Leeuwin
current of 75 m thickness, source depth of 40 m: and a Leeuwin current of 200 m thickness and
source depth of 20 m. The resulting 95% probability of detection ranges are shown on Figure 27
and Figure 28 respectively.
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