Statistics of Acoustic Pulse Signals Through Nonlinear Internal Waves on the Continental Shelf of the Northeastern South China Sea
Abstract
A component of the Office of Naval Research (ONR) funded Windy Islands Soliton Experiment (WISE) was conducted from 13 - 15 April 2005 on the continental shelf in the northeast portion of the South China Sea to study the effects of nonlinear internal waves on the transmission of a 400-Hz signal. To capture the ocean variability along the acoustic path, a series of environmental moorings were deployed that sampled the water column. Significant variability in the sound-speed field was observed to be induced by nonlinear internal tides with a broad (~ 10 km) horizontal scale (referred to as the "long-wave pattern") and narrow (< 1 km), high-frequency, nonlinear internal depression and elevation waves superimposed on the internal tides. Through the use of an empirical sound-speed field and a coupled, normal-mode acoustic propagation model, the phenomenology of the nonlinear internal wave field upon the observed intensity pattern was examined. Analysis of the observed and modeled acoustic intensity time-series indicates that the long-wave pattern dictates, to a large degree, the temporal changes in the vertical structure of the sound intensity level. Furthermore, both measurement and model results show that when the thermocline was rapidly displaced by the nonlinear internal waves, sound intensity fluctuations reached their maximum. Modeling results suggest that these maximums are due to the scattering of acoustic energy into both higher and lower acoustic modes along the edges of the elevation/depression waves where strong horizontal sound-speed gradients were present. An additional goal of this paper is to propose and validate an extended statistical theory that relates the observed statistics of the acoustic intensity to the number of resolvable arrivals. The number of resolvable arrivals depends on signal bandwidth and the criteria of well separateness and was found to vary significantly as the nonlinear internal waves evolve along the transmission path.
Document Details
- Document Type
- Technical Report
- Publication Date
- Mar 01, 2008
- Accession Number
- ADA479987
Entities
People
- Justin M. Reeves
Organizations
- Naval Postgraduate School