Sound propagation effects of near-seabed internal waves in shallow water

Abstract

Computational modeling is used to examine effects of near-seabed nonlinear internal waves. Waves of this type have been observed many times on the South New-England Shelf and elsewhere. The waves appear to be common when the dominant vertical density gradient zone is near the seabed. Many observations have been with profilers that alias the waves, or with instruments fixed to the seafloor that provide snapshots of passing waves. We have recently observed these features in greater detail using acoustic remote sensing from a ship, and we are now in a position to model their effects on sound propagation. Results are presented for many frequencies and many source/receiver/wave geometries. An important question remains about along-crest coherence of the waves, which would govern ducting and other anisotropic propagation effects. These waves are not as well studied as waves linked to a near-surface pycnocline, which can be studied remotely with EM via surface signature. The strong density gradients found in many shallow water areas allow the internal waves to have high frequencies, creating rapidly changing acoustics thereby shortening acoustic coherence time. The related strong sound-speed gradients, found both near the surface and near the seabed, are responsible for the important acoustic effects of the waves.