Siegmann William L.

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William L.

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  • Article
    Measurement and modeling of three-dimensional sound intensity variations due to shallow-water internal waves
    (Acoustical Society of America, 2005-02) Badiey, Mohsen ; Katsnelson, Boris G. ; Lynch, James F. ; Pereselkov, Serguey ; Siegmann, William L.
    Broadband acoustic data (30–160 Hz) 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.
  • Article
    Experimental evidence of three-dimensional acoustic propagation caused by nonlinear internal waves
    (Acoustical Society of America, 2005-08) Frank, Scott D. ; Badiey, Mohsen ; Lynch, James F. ; Siegmann, William L.
    The 1995 SWARM experiment collected high quality environmental and acoustic data. One goal was to investigate nonlinear internal wave effects on acoustic signals. This study continues an investigation of broadband airgun data from the two southwest propagation tracks. One notable feature of the experiment is that a packet of nonlinear internal waves crossed these tracks at two different incidence angles. Observed variations for the lower angle track were modeled using two-dimensional parabolic equation calculations in a previous study. The higher incidence angle is close to critical for total internal reflection, suggesting that acoustic horizontal refraction occurs as nonlinear internal waves traverse this track. Three-dimensional adiabatic mode parabolic equation calculations reproduce principal features of observed acoustic intensity variations. The correspondence between data and simulation results provides strong evidence of the actual occurrence of horizontal refraction due to nonlinear internal waves.
  • Article
    Analysis and modeling of broadband airgun data influenced by nonlinear internal waves
    (Acoustical Society of America, 2004-12) Frank, Scott D. ; Badiey, Mohsen ; Lynch, James F. ; Siegmann, William L.
    To investigate acoustic effects of nonlinear internal waves, the two southwest tracks of the SWARM 95 experiment are considered. An airgun source produced broadband acoustic signals while a packet of large nonlinear internal waves passed between the source and two vertical linear arrays. The broadband data and its frequency range (10–180 Hz) distinguish this study from previous work. Models are developed for the internal wave environment, the geoacoustic parameters, and the airgun source signature. Parabolic equation simulations demonstrate that observed variations in intensity and wavelet time–frequency plots can be attributed to nonlinear internal waves. Empirical tests are provided of the internal wave-acoustic resonance condition that is the apparent theoretical mechanism responsible for the variations. Peaks of the effective internal wave spectrum are shown to coincide with differences in dominant acoustic wavenumbers comprising the airgun signal. The robustness of these relationships is investigated by simulations for a variety of geoacoustic and nonlinear internal wave model parameters.