Appendices 5-13 - Nautilus Cares - Nautilus Minerals

* Noise levels attenuate rapidly with range out to about 2 km, after which they drop
off more slowly. This is due to the increasingly important contribution of seabed
reflected sound at longer ranges.
* Results for different azimuths are similar, indicating that the overall seabed
topography has a relatively minor effect. However, some bathymetry profiles
result in distinct focussing effects, resulting in localised increases in received
sound levels.
* At very short range the reduction in sound level with increasing range corresponds
to spherical spreading (see Table 3 and Figure 10), but this approximation
becomes successively worse as the range increases and the contributions of the
seabed and sea surface reflections become more important.
* At ranges beyond 2 km the rate of reduction of maximum received level
corresponds well to cylindrical spreading (see Figure 10). This is to be expected
given that the sea surface and basalt seabed are almost perfect reflectors at small
grazing angles, and even the sandy silt seabed has a high reflection coefficient at
these angles (see Figure 5). In addition, the sound speed profile in Figure 6 will
result in sound being refracted away from the boundaries, towards the depth of the
sound speed minimum at around 1000 m, further reducing boundary losses.
Seawater absorption losses are very small at these frequencies and ranges, so the
only significant loss mechanism is spreading in the horizontal plane. It would
therefore be reasonable to use the following formula, based on cylindrical
spreading and a best-fit effective source level, to predict the maximum received
level at longer ranges:
RL 172 -10log10
+ r,
) (1)
where RL is the received level (dB re 1 "Pa), and r is the horizontal range in
metres. Note that this formula is only valid for ranges greater than 2 km and small
enough that absorption isn't a significant factor. Absorption increases strongly
with increasing frequency and therefore acts as a low-pass filter. For a reasonably
narrow band source spectrum like the one considered here its effects can be
included approximately by modifying (1) as follows:
+ r,
ar
RL 172 -10log
-
(2)
) 10
where a is the absorption coefficient at the centre frequency in dB/m. A formula
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