Appendices 5-13 - Nautilus Cares - Nautilus Minerals

Abstract
This report presents the results of numerical modelling of the production and underwater
propagation of thruster cavitation noise from a surface vessel carrying out a subsea
mining operation. Modelling indicates that received levels will drop rapidly with distance
from the vessel out to a horizontal distance of approximately 2 km, after which they will
decay more slowly in accordance with cylindrical spreading.
The noise is likely to be audible to marine animals at ranges in excess of 600 km in
directions unimpeded by land or shallow water depths (to the west in this case), however
levels will be insufficient to produce physiological effects such as temporary threshold
shifts at any range, except perhaps in the immediate vicinity of the vessel (within tens of
metres). To put this audibility range in context it is important to note that long range
sound propagation in the ocean is common for both natural and man-made sounds. For
example, low frequency sounds produced by the nearby North Su subsea volcano are
likely to be audible for thousands of kilometres, and other seismic events such as
earthquakes can produce sounds that are well above ambient noise levels at ranges of
thousands to tens of thousands of kilometres. In the case of man-made sources, sounds
produced by airgun arrays during seismic surveys have been recorded by the authors at
ranges of several thousand kilometres.
Marine mammals are likely to avoid the vessel at ranges of approximately 15 km, and
may suffer signal masking effects at similar ranges. However, some animals may also be
attracted to the vessel, and the avoidance range is likely to reduce over time as animals
habituate to the vessel's presence.
An analysis of the production and propagation of noise from the subsea mining machine
and the potential impacts of these noise levels on marine mammals has not been carried
out due to a lack of information about the machine's likely source characteristics. The
noise produced by this device needs to be considered in the context of the operation being
carried out approximately 1 km from an active subsea volcano, which is itself likely to be
a major source of low frequency underwater sound.
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