Appendices 5-13 - Nautilus Cares - Nautilus Minerals

corresponding to the eight directions indicated in Figure 3. These profiles are plotted in
Figure 4.
Attempts to run the elastic parabolic equation model, RAMS, using these bathymetry
profiles and a basalt seabed model were unsuccessful, with the model becoming unstable
at higher frequencies. An equivalent fluid model was therefore devised, with parameters
chosen to provide as close a match as possible to the reflection coefficient for basalt over
the grazing angle range 0! to 70!, which enabled the more stable fluid parabolic equation
model RAMGeo , written by Mike Collins from the US Naval Research Laboratory, to be
used for propagation modelling.
The geoacoustic parameters of the equivalent fluid are also given in Table 2, and a
comparison between reflection coefficients for the basalt seabed and the equivalent fluid
seabed is shown in Figure 5. There is very good agreement between the two seabed
models for all grazing angles less than 70!, but the fluid model underestimates the
reflection coefficient at steeper angles. This will lead to a slight underestimation of noise
levels in the lower half of the water column at very short range (< 1 km).
The sound speed profile used for the modelling was obtained from the nearest grid point
of the World Ocean Atlas (2005) and is shown in Figure 6.
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