Acoustic normal mode fluctuation statistics in the 1995 SWARM internal wave scattering experiment

Abstract

In order to understand the fluctuations imposed upon low frequency (50 to 500 Hz) acoustic signals due to coastal internal waves, a large multilaboratory, multidisciplinary experiment was performed in the Mid-Atlantic Bight in the summer of 1995. This experiment featured the most complete set of environmental measurements (especially physical oceanography and geology) made to date in support of a coastal acoustics study. This support enabled the correlation of acoustic fluctuations to clearly observed ocean processes, especially those associated with the internal wave field. More specifically, a 16 element WHOI vertical line array (WVLA) was moored in 70 m of water off the New Jersey coast. Tomography sources of 224 Hz and 400 Hz were moored 32 km directly shoreward of this array, such that an acoustic path was constructed that was anti-parallel to the primary, onshore propagation direction for shelf generated internal wave solitons. These nonlinear internal waves, produced in packets as the tide shifts from ebb to flood, produce strong semidiurnal effects on the acoustic signals at our measurement location. Specifically, the internal waves in the acoustic waveguide cause significant coupling of energy between the propagating acoustic modes, resulting in broadband fluctuations in modal intensity, travel-time, and temporal coherence. The strong correlations between the environmental parameters and the internal wave field include an interesting sensitivity of the spread of an acoustic pulse to solitons near the receiver.

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

“…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.…”

Investigation of Mode Filtering as a Preprocessing Method for Shallow-Water Acoustic Communications

“…Specifically, acoustic normal-mode filtering and mode fluctuations in shallow-water experiments were considered in [10]. The problems in undersized sparse vertical arrays were investigated in [8].…”

Passive Time Reversal Acoustic Communications Through Shallow-Water 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]. Both coherence and intensity fluctuations of the acoustic channel can affect the performance of coherent underwater acoustic communications.…”