Appendices 5-13 - Nautilus Cares - Nautilus Minerals

for computing a is given in Fisher and Simmons (1977), and yields a value of
7.
3
-6
x 10 dB/m at 250 Hz. Figure 12 shows that in this case absorption is likely to
become significant at ranges in excess of 100 km.
Received level (dB re 1uPa)
150
140
130
120
110
100
90
80
70
60
50
10 0
10 1
10 2
Range (km)
Figure 12. Extrapolated transmission loss vs. range assuming cylindrical spreading without (blue) and with
(red) absorption.
* There is only a minor difference between results obtained using the two different
seabed models. This appears to be because the interaction of the sound with the
relatively steep basalt downhill slopes results in the rays being flattened so that by
the time they hit the softer seabed a lot of the energy is at grazing angles less than
the critical angle (~ 20 degrees) and is therefore strongly reflected. The most
noticeable difference between the results for the two seabed models can be seen in
the range-depth plots of Figure 7 and Figure 8. It is apparent that in the case of the
basalt seabed (model A), there is energy travelling at steep angles that fills in the
'holes' in the model B received level plots. Conversely, the softer sediment in
model B rapidly attenuates this energy.
10 3
10 4
21