RESUMO
A specific mechanism of mode coupling in a waveguide propagation is studied when two range-dependent eigenvalues approach each other. This phenomenon is analogous to the so-called quasi-crossing of states in atomic physics (Landau-Zener theory). It is considered for the sound wave propagation in a coastal wedge in the presence of a sound-speed profile. The change in mode composition and the corresponding spatial variability of the sound field are analyzed by using modes coupling equations and the parabolic equation with a field decomposition over adiabatic modes, respectively.
RESUMO
Anomalously large, transient fluctuations of acoustical noise intensity, up to 4-5 orders of magnitude above the background, were observed with single-hydrophone receiver units (SHRUs) and on the L-shaped horizontal and vertical line array of hydrophones (HVLA) in the Shallow Water 2006 experiment on the continental shelf off New Jersey. Here, temporal and spatial properties of these noise bursts are investigated. As tidally generated nonlinear internal waves (NIWs) move across the site of the experiment from the shelf break toward the coast, they form trains of localized, soliton-like waves with up to 25-35 m displacement of isopycnal surfaces. The NIW trains consecutively cross the positions of five SHRUs and HVLA that are located about 5-8 km from each other along a line perpendicular to the coast. The noise bursts were observed when a NIW train passed through locations of the corresponding acoustic receivers. Turbulence of the water flow, saltation, and bedload of marine sediments were the dominant causes of the acoustic noise bursts caused by NIWs at different frequency bands. On near-bottom hydrophones, the most energetic part of the observed noise bursts is attributed to collisions of suspended sediment particles with each other, the sensor, and the seafloor.