Coupling of Acoustic Modes by Continental-Shelf Internal Solitary Waves
Jim Preisig and Tim Duda
Sound transmission from an acoustic source at 18 meters depth
at range position 0 (red circle) to a array of receivers at
range 33 km (line of red circles) can be simulated computationally.
If the simulation is repeated many times with a packet of
non-linear internal solitary gravity waves at many positions between
the source and receiver, the acoustic field strength vs. depth
and packet position shows strong fluctuations (lower color image).
The fluctuations are caused by strong echange of energy between acoustic normal
modes (mode coupling). The modes are natural structures in the
sound caused by the surface and seafloor boundaries, analogous to
guitar-string modes and many other familiar vibration patterns.
Acoustic Intensity Fluctuations Caused by a Moving Packet
The packet position can be converted to time using our knowledge of internal
wave dynamics, so that the acoustic signals would show the
same fluctuations over time for the case of a propagating packet.
The lower figure corresponds to about 12 hours of time for a packet
moving at 0.8 m/s.