Frequency dependence and intensity fluctuations due to shallow water internal waves (original) (raw)
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The Journal of the Acoustical Society of America, 2005
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.
An overview of the 1995 SWARM shallow-water internal wave acoustic scattering experiment
IEEE Journal of Oceanic Engineering, 1997
An overview is given of the July-August 1995 SWARM shallow-water internal wave acoustic scattering experiment. This experiment studied both acoustic propagation through and scattering by the linear and nonlinear internal waves found on the Mid-Atlantic Bight continental shelf, as well as the physical oceanography of the internal wavefield. In order that our goal of explaining the nature of the acoustic scattering should not be hindered by incomplete environmental knowledge, numerous instruments, both ship-deployed and moored, measured the acoustics, geophysics, and oceanography. In this paper, we show some of the results from the first year's analysis of the environmental and acoustic data. The environmental measurements, which are a key input to the analyses of the acoustic data, are given slightly more emphasis at this point in time. Some of the more interesting oceanographic, geophysical, and acoustical results we present here are: evidence for the dominance of the lee-wave mechanism for soliton production, evidence for the "solibore internal tide," the "dnoidal wave" description of solitons, the inversion of chirp sonar data for bottom properties, propagation loss extraction from air-gun data, and the intensity and travel-time fluctuations seen in propagating acoustic normal modes. Directions for future research are outlined.
Acoustic normal mode fluctuation statistics in the 1995 SWARM internal wave scattering experiment
Journal of The Acoustical Society of America, 2000
In order to understand the fluctuations imposed upon low frequency ͑50 to 500 Hz͒ 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 geology͒ 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 ͑WVLA͒ 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.
Fluctuations of Broadband Acoustic Signals in Shallow Water
The long-term goal of this project is to obtain quantitative understanding of the physical mechanisms governing broadband (50 Hz to 50 kHz) acoustic propagation, reflection, refraction, and scattering in shallow water and coastal regions in the presence of temporal and spatial ocean variability. OBJECTIVES The scientific objective of this research is to understand acoustic wave propagation in a dynamic environment in two frequency bands: Low (50 Hz to 500 Hz) and Mid-to-High (500 Hz to 50 kHz). The goal for the low frequency band is to assess the effects of internal waves on acoustic wave propagation, with an emphasis on the mechanisms that cause significant temporal and spatial acoustic intensity fluctuations. The goal for the mid-to-high frequency band is to assess the effects of water column and dynamic sea surface variability, as well as source/receiver motion on acoustic wave propagation for underwater acoustic communications, tomography, and other applications.
Temporal sound field fluctuations in the presence of internal solitary waves in shallow water
The Journal of the Acoustical Society of America, 2009
Temporal variations of intensity fluctuations are presented from the SWARM95 experiment. It is hypothesized that specific features of these fluctuations can be explained by mode coupling due to the presence of an internal soliton moving approximately along the acoustic track. Estimates are presented in conjunction with theoretical consideration of the shallow water waveguide.
The Journal of the Acoustical Society of America, 2008
Fluctuations of the low frequency sound field in the presence of an internal solitary wave (ISW) packet during the Shallow Water '06 (SW06) experiment are analyzed. Acoustic, environmental, and on-board ship radar image data were collected simultaneously before, during, and after a strong ISW packet passed through the acoustic track. Preliminary analysis of the acoustic wave temporal intensity fluctuations agrees with previously observed phenomena and the existing theory of the horizontal refraction mechanism, which causes fluctuations when the acoustic track is nearly parallel to the front of the internal waves.
Temporal and azimuthal dependence of sound propagation in shallow water with internal waves
IEEE Journal of Oceanic Engineering, 2002
The short time scale (minutes) and azimuthal dependence of sound wave propagation in shallow water regions due to internal waves is examined. Results from the shallow water acoustics in random media (SWARM-95) experiment are presented that reflect these dependencies. Time-dependent internal waves are modeled using the dnoidal solution to the nonlinear internal wave equations, so that the effects of both temporal and spatial variability can be assessed. A full wave parabolic equation (PE) model is used to simulate broadband acoustic propagation. It is shown that the short term temporal variability and the azimuthal dependence of the sound field are strongly correlated to the internal wave field.
Broadband shot signal data are analyzedjrom the SWARM'95 experiment to investigate acoustic variability in presence oj internal solitons. A 10 to 15 minute temporal variations in the intensity ojthe received signals were observed. These temporal variations are azimuthally dependent on variability oj water column in the presence oj internal solitary waves. These jluctuations should be explained by significant horizontal rejraction (3D-effects) taking place when orientation oj acoustic track is close to direction oj the wave jront oj interna! solitons. Analysis on the base oj both equation oj vertical modes/horizontal rays and PE in horizontal plane is carried out, good agreement between theoretical calculations and experimental data is obtained.
Shallow Water Internal Waves and Associated Acoustic Intensity Fluctuations
Defence Science Journal, 2006
Physical oceanographic and acoustic data were simultaneously collected from the coastal waters of the Arabian Sea. Acoustic transmissions were carried out from an anchored vessel using 620 Hz transducer and received by an array of hydrophones moored at ~5 km away from the anchorage. Thermal structure in this region was characterised by a tri-layer structure, ie, a strong thermocline (> 0.4 o C/m) sandwiched between an upper (< 10 m) and bottom (> 25 m) homogeneous layer. High-resolution (sampled at 10 s interval) temperature data from moored sensors revealed intense internal wave activity. The maximum value of Brunt-Vaisala frequency, which is the maximum frequency limit of internal waves in the thermocline, suggests that the upper frequency limit of the internal wave, which can be generated during this period, is 23 cph (2.6 min). High and low frequency waves caused variations of ~3 o C and ~5 o C respectively in the temperature field. But the low frequency internal waves were found to contain maximum energy compared to the high frequency waves. Fluctuations of 8-12 dB were noticed in the measured acoustic intensity values in the presence of low frequency internal waves. Simulation studies carried out using parabolic equation model using 620 Hz source indicated well-defined ducted propagation with minimum transmission loss, when the source was kept within the homogeneous layer. The presence of tri-layer thermal structure, ie, a strong gradient layer sandwiched between an upper and bottom homogeneous layer, caused surface and bottom channel propagation in this region.