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The inversion and statistics of surface forward scattered underwater sound with relation to wave shape

  • Author(s): Walstead, Sean Philip
  • et al.

The rough and time-varying nature of the sea surface scatters sound in complex ways. The focus of this doctoral research is to better understand the interaction of sound with the ocean surface and exploit that understanding to inversely learn about surface wave structure. An inversion technique is developed that estimates surface wave shape from the details of forward scattered sound. This technique can improve the remote sensing of surface waves, which enhance greenhouse gas exchange and momentum between the atmosphere and ocean. The work also has implications for the performance of underwater acoustic communications systems. In the laboratory, the resolution of acoustically inverted surface waves exceeds that of a nearby camera. Two physical length scales over which information can be determined acoustically about the surface are confirmed. An outer length scale, the Fresnel zone surrounding each specular reflection point, is the only region where optimized surfaces converge to a resolution set by the inner length scale, a quarter-wavelength of the acoustic pulse. Ocean forward scattered sound is inverted for wave shape during three periods: low wind, mix of wind and swell, and stormy. The power spectral density of the inverted surface wave field during low wind saturates around a frequency of 8 Hz while an upward looking SONAR saturates at 1 Hz. The improved high frequency resolution provided by the scattering inversion indicates that it is possible to remotely gain information about high frequency components of ocean waves. The inability of the inversion algorithm to determine physically realistic surface waves in periods of high wind indicates that bubbles and out of plane scattering become important in those operating scenarios. Experimentally observed phenomena such as arrival time spread, Doppler spread, and multiple distinct arrivals are described in terms of Fresnel zone coherence for 50 kHz - 2000 kHz very high frequency (VHF) sound. Products of this research are improved models for high intensity surface-scattered arrivals and new models for the second-order statistics relating scattering intensity, delay time, and Doppler spread

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