UACE2017 Proceedings: Frequency-difference beamforming in shallow water environments

• Session:
  Signal and image processing
• Paper:
  Frequency-difference beamforming in shallow water environments
• Author(s):
  Alexander Douglass, Heechun Song, David Dowling
• Abstract:
  Conventional beamforming is a signal processing technique that can provide unambiguous ray arrival directions from high signal-to-noise ratio recordings from a transducer array, provided the spacing of the array elements does not exceed half of a signal wavelength. When the array element spacing is significantly larger than a wavelength, spatial aliasing may cause conventional methods to fail. Frequency-difference beamforming [Abadi et al. 2012] was developed to overcome the effects of spatial aliasing by utilizing a surrogate field at a below-band frequency generated from a quadratic product of recorded complex field amplitudes at different frequencies. This below-band frequency (the difference frequency) can be chosen specifically for the needs of the user and is limited only by the frequency resolution and the recorded signal's bandwidth. In this presentation, the performance of frequency-difference beamforming is presented for three shallow ocean scenarios involving realistic multipath propagation. First, the performance of frequency-difference beamforming is presented in a simulated ocean channel that is 106 m deep and downward refracting, with a 56 m long, 16 element vertical receiver array placed 3 km away from an 8-element source array. Here, frequency difference beamforming is applied to 11 kHz to 33 kHz frequency sweep signals and compared with conventional beamforming applied to similar signals one decade lower in frequency. Second, frequency-difference beamforming results are presented for vertical array measurements made in a nominally-identical ocean sound channel. Lastly, frequency-difference beamforming is investigated for horizontal arrays in simulated environments with range- and position-dependent sound speeds and depths, and varying receiver position uncertainty. In all three cases, it was found that frequency difference beamforming could be successful when conventional in-band beamforming was not. [Sponsored by ONR and NAVSEA]
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Contact details

• Contact person:
  Mr Alexander Douglass
• e-mail:
  
• Affiliation:
  University of Michigan
• Country:
  United States