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Journal of the Acoustical Society of America / Volume 125 / Issue 2 / UNDERWATER SOUND [30]

Green's function approximation from cross-correlations of 20–100 Hz noise during a tropical storm

J. Acoust. Soc. Am. Volume 125, Issue 2, pp. 723-734 (2009); (12 pages)

Laura A. Brooks and Peter Gerstoft

Marine Physical Laboratory, Scripps Institution of Oceanography, La Jolla, California 92093

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Approximation of Green’s functions through cross-correlation of acoustic signals in the ocean, a method referred to as ocean acoustic interferometry, is potentially useful for estimating parameters in the ocean environment. Travel times of the main propagation paths between hydrophone pairs were estimated from interferometry of ocean noise data that were collected on three L-shaped arrays off the New Jersey coast while Tropical Storm Ernesto passed nearby. Examination of the individual noise spectra and their mutual coherence reveals that the coherently propagating noise is dominated by signals of less than 100 Hz. Several time and frequency noise normalization techniques were applied to the low frequency data in order to determine the effectiveness of each technique for ocean acoustic applications. Travel times corresponding to the envelope peaks of the noise cross-correlation time derivatives of data were extracted from all three arrays, and are shown to be in agreement with the expected direct, surface-reflected, and surface-bottom-reflected interarray hydrophone travel times. The extracted Green’s function depends on the propagating noise. The Green’s function paths that propagate horizontally are extracted from long distance shipping noise, and during the storm the more vertical paths are extracted from breaking waves.

© 2009 Acoustical Society of America

ACKNOWLEDGMENTS

Work supported by the Office of Naval Research under Grant No. N00014-05-1-0264, and by the Department of Energy National Energy Technology Laboratory via the Gulf of Mexico Hydrates Research Consortium, University of Mississippi. SWAMI data supplied by the Applied Research Laboratories, the University of Texas at Austin. Shark data supplied by Woods Hole Oceanographic Institute. L.A.B. is appreciative of support from a Fulbright Postgraduate Award in Science and Engineering, and from the Defence Science and Technology Organisation, Australia.

Article Outline

  1. INTRODUCTION
  2. THEORETICAL BACKGROUND
  3. EXPERIMENT
  4. ANALYSIS OF DATA PREPROCESSING METHODS
    1. Removal of main contamination
    2. Spectra and coherence
    3. Frequency domain normalization
      1. Normalization methods
      2. Application to data
    4. Time domain normalization
      1. Normalization methods
      2. Application to data
  5. GEOMETRIC COMPARISONS
  6. TEMPORAL VARIATIONS
  7. CONCLUSION

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KEYWORDS and PACS

Keywords

acoustic correlation, acoustic noise, acoustic wave interferometry, Green's function methods, ocean waves, oceanographic techniques, storms, underwater acoustic propagation

PACS

  • 43.30.Pc

    Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography

  • 43.60.Fg

    Acoustic array systems and processing, beam-forming

  • 43.30.Nb

    Noise in water; generation mechanisms and characteristics of the field

ARTICLE DATA

History
Received 08 May 2008
Accepted 04 Dec 2008
Revised 02 Oct 2008
Permalink
http://dx.doi.org/10.1121/1.3056563

PUBLICATION DATA

ISSN:

0001-4966 (print)  

Publisher:

$abstract.society.value is a Member of CrossRef Acoustical Society of America

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