Modeling and Measurement of Acoustic Coherence Degradation due to Volume Randomness

Our long-term goal is to understand how shallow water internal waves affect acoustic propagation. Another goal is to develop an understanding of the dynamics of shallow water internal waves and to relate this understanding to acoustics. Similarities and differences with open ocean internal waves are of interest. At least two internal wave processes are present in shallow water: a background continuum of internal waves analogous to the deep water field, and deterministic internal wave trains known as solibores that can dominate the displacement field over significant time periods. These deterministic wave trains are present in many shallow water areas of the world's oceans. This work is directed toward developing models for use in explaining how these two processes, in particular their horizontal properties, can impact the coherence of acoustic signals measured along horizontal receiving arrays. An acoustic propagation experiment was conducted by APL in the mid-Atlantic Bight at the same time as internal waves were monitored by a team from Oregon State University. These measurements were components of the Synthetic Aperture Sonar Primer of the 1996 Coastal Mixing and Optics Experiment. Our approach consists of developing a statistical internal wave model analogous to that of the deep water internal wave model of Garrett and Munk. Our model is used to describe an internal wave continuum that we conjecture exists as a background to the episodic solibore events associated with the baroclinic tide. The solibore events are modeled in a deterministic manner as a mode 1 process displacing isopycnals. This two-component model is used in acoustic fluctuation theory to predict phase fluctuations in acoustics signals propagated over two volume refracted paths: a lower path propagating near the bottom and an upper path that samples the shallower and more energetic, near surface field.