Intensity fluctuations of midfrequency sound signals passing through moving nonlinear internal waves

Abstract

The fluctuations of intensity of broadband pulses in the midfrequency range (2–4.5kHz) propagating in shallow water in the presence of intense internal waves moving approximately along the acoustic track are considered. These pulses were received by two separate single hydrophones placed at different distances from the source (∼4 and ∼12km) and in different directions. It is shown that the frequency spectra of the fluctuations for these hydrophones have different predominating frequencies corresponding with the directions of the acoustic track. Comparisons of experimental results with theoretical estimates demonstrate good consistency.

Passive Time Reversal Acoustic Communications Through Shallow-Water Internal Waves

“…At this and other depths, the calculated ray cycle was about 600 m for the depressed waveguide and 800 m for the not depressed case. Another separate effort on SW06 showed similar results for the ray cycle calculation [32]. The average number of bottom bounces was about 20-30 times for the 19.8-km acoustic track.…”

Horizontal coherence of low-frequency fixed-path sound in a continental shelf region with internal-wave activity

“…The refraction can cause sound convergence within internal-wave ducts or divergence from antiducts (Katsnelson and Pereselkov, 2000;Badiey et al 2005: Frank et al, 2005, and fluctuating sound radiation from terminating ducts (Lin et al, 2009). Further effects observed as a result of horizontal refraction are sharp intensity fluctuations and frequency-dependent fluctuations (Badiey et al, 2007;Katsnelson et al, 2008;Katsnelson et al, 2009;Badiey et al, 2011), plus rapidly varying modal arrivals, including nulling of modes that would be prominent in the absence of the waves (Lin et al, 2009). Both the across-wave and along-wave situations cause rapid changes in the nature of received acoustic signals.…”

Horizontal Lloyd mirror patterns from straight and curved nonlinear internal waves

“…The lead wave is spatially coherent and selfhealing to perturbations, so it does not change significantly in height or width. 22 Consequently, the wave preserves its shape as it moves relative to the acoustic source, at the center of the region, and calculations are illustrated for various NIW locations [choices of t in Eq. (4)].…”

“…The most significant acoustic activity usually occurs near the lead internal wave in an internal wave train (typically the lead wave is the largest). 5,22 Refraction and reflection effects determine variations of the acoustic field. These mechanisms are controlled by the angle between the directions of acoustic propagation and internal wavefronts.…”

“…These reflection coefficients have the same form as Eq. (22). For l isospeed layers the reflection coefficients are recursively defined as 26,27 …”