Statistics of acoustic pulse signals through nonlinear internal waves on the continental shelf of the northeastern South China Sea

Abstract

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. The theory is found to be pertinent when the temporal length of the window for calculating statistics was expanded sufficiently in order to collect a sample population with the following characteristics: 1) the standard deviation of the estimated number of arrivals is small, and 2) sufficient internal wave events are captured to ensure the phase distribution of the arrivals in the sample population is uniform.