Measurement and modeling of three-dimensional sound intensity variations due to shallow-water internal waves

Abstract

Broadband acoustic data ͑30-160 Hz͒ from the SWARM'95 experiment are analyzed to investigate acoustic signal variability in the presence of ocean internal waves. Temporal variations in the intensity of the received signals were observed over periods of 10 to 15 min. These fluctuations are synchronous in depth and are dependent upon the water column variability. They can be explained by significant horizontal refraction taking place when the orientation of the acoustic track is nearly parallel to the fronts of the internal waves. Analyses based on the equations of vertical modes and horizontal rays and on a parabolic equation in the horizontal plane are carried out and show interesting frequency-dependent behavior of the intensity. Good agreement is obtained between theoretical calculations and experimental data.

Short-Range Acoustic Propagation Variability on a Shelf Area with Strong Nonlinear Internal Waves

“…Oscillations of I in the waves are also of order 10 dB. These results are very similar to those of a prior ~30-160 Hz experiment at 15-km range [12].…”

“…Sound propagating perpendicular to the crests of waves of this type would exhibit strong coupling of the vertical acoustic normal modes [16]. In this experiment, signals from S1 measured at V1 and V2 would be expected to have undergone refraction of normal modes but no mode coupling [12]. The amount of refraction of each mode depends on the phase velocity gradient (or modal wave number k gradient).…”

“…Prior experiments have measured strong acoustic fluctuations after a few tens of kilometers of propagation through waters populated with waves of these types [8][9][10][11][12]. After such long propagation distances many intervening bathymetric, internal wave, and frontal features can cause multiple scattering effects.…”

Passive Time Reversal Acoustic Communications Through Shallow-Water Internal Waves

“…There have been a number of experimental and modeling efforts to investigate acoustic fluctuations in the presence of internal waves. For example, internal waves were reported causing intensity fluctuations of acoustic pulses during the 1995 Shallow Water Acoustics in Random Media (SWARM-95) Experiment [6] and these fluctuations were later modeled [7], [8]. The coherence properties of acoustic pulses in an internal wave field have also been studied [9]- [12].…”

Three-dimensional acoustic propagation through shallow water internal, surface gravity and bottom sediment waves

“…These effects were first described for internal waves theoretically in [9] and numerically in [10,8]. The first experimental observations of these strong intensity fluctuations (6-10 dB) compared to the "no waves" case, with an along wave geometry for internal waves passing over a fixed source and receiver, were made during the SWARM95 experiment [8] and described in [50]. Later, the SW06 experiment was specifically configured for measuring three-dimensional acoustic effects, and the intensity fluctuations in these directions were commonly observed in presence of internal wave trains [12].…”