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.

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.