Modeling mode arrivals in the 1995 SWARM experiment acoustic transmissions

Abstract

As part of the Shallow Water Acoustics in a Random Medium (SWARM) experiment, a 16 element WHOI vertical line array (WVLA) was moored in 70 m of water off the New Jersey coast. A 400-Hz acoustic tomography source was moored some 32-km shoreward of this array, such that an acoustic path was created that was anti-parallel to the primary propagation direction for shelf-generated internal wave solitons. The presence of these soliton internal waves in the acoustic waveguide causes significant coupling of energy between propagating acoustic modes, creating fluctuations in modal intensities and modal peak arrival times, as well as time spreading of the pulses. Two methods by which acoustic propagation and scattering in soliton-filled waveguides can be modeled are presented here in order to understand and explain the scattering observed in the SWARM field data. The first method utilizes the Preisig and Duda [IEEE J. Ocean. Eng. 22, 256-269 (1997)] Sudden Interface Approximation (SIA) to represent the solitons. The second method, which is computationally slower, uses a finely meshed, "propagated" thermistor record to simulate the solitons in the SWARM experiment waveguide. Both numerical methods are found to generate scattering characteristics that are similar to the SWARM field data.

Analysis and modeling of broadband airgun data influenced by nonlinear internal waves

“…The airgun source used along this track produced signals with energy primarily in a low-frequency ͑10-180 Hz͒ band. This band contrasts with previous studies that focus on CW signals above 200 Hz and with other broadband simulations 12,13,18 at higher frequencies. Observed time variations in acoustic measurements, the presence of high quality environmental data, and the passage of a strong and coherent train of nonlinear internal waves provide an exceptional opportunity for testing the applicability of the resonance mechanism.…”

“…More recently, the role of effective internal wave wavenumbers has been shown for continuous wave ͑CW͒ transmissions at 240 Hz, 6 above 450 Hz, 11 and for broadband pulses centered at 224 Hz and above. 12,13 Nonetheless, we are not aware of combinations of a well-sampled ocean environment, research-quality acoustic data, and a comprehensive modeling study that demonstrates the occur-rence and effective operation of the resonance mechanism. Deterministic and random internal wave fields were found by simulations to have distinct effects on acoustic propagation 14 in an environment like SWARM.…”

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

“…The work by Zhou et al (1991) on coupled-mode propagation through idealized nonlinear internal-wave packets motivated more detailed analysis of mode coupling and mode interference in unitary nonlinear internal waves (Preisig and Duda, 1997). Additional publications covered temporal fluctuations caused by moving nonlinear wave packets (Duda and Preisig, 1999;Rouseff et al, 2002) and fields of waves Headrick et al, 2000b). Follow-on work expanded into analysis of bias effects (net attenuation or amplification) caused by coupled-mode propagation in the presence of two different mode-stripping conditions (Duda, 2004), and into analysis of acoustic field spatial coherence under similar conditions (Duda, 2006;Finette and Oba, 2003;Mignerey and Orr, 2004).…”

Effect of shallow water internal waves on ocean acoustic striation patterns

“…Consequently, the rate at which the modes attenuate as they propagate in range tends to increase with mode number. The bottom attenuation 0.04 dB/A used in the simulations was based on the average value observed in the 1995 Shallow Water Acoustics in a Random Medium Experiment [26,27]. To test the sensitivity of the beta distribution to this parameter, the simulations were repeated using the same internal wave field but with increased bottom attenuation.…”