Measurements of Spatial Coherence of Bottom-Interacting Sound in the Tagus Abyssal Plain

  • J. M. Berkson
  • R. L. Dicus
  • R. Field
  • G. B. Morris
  • R. S. Anderson
Part of the NATO Conference Series book series (NATOCS, volume 5)


Coherence of bottom-interacting sound in the Tagus Abyssal Plain was determined as a function of bearing and frequency (20 to 2000 Hz) for grazing angles between 11° and 13°. An acoustic experiment was performed in which SUS charges were dropped in a circular pattern of 28 km radius around a deep hydrophone. The bottom-interacting arrival was isolated and processed to remove the decorrelating effects of varying bubble-pulse periods. A spatial coherence function was calculated between shot pairs corresponding to 5, 9, and 13 km separation. For frequency bands of 20 to 500 Hz and 1200 to 2000 Hz, the spatial coherence of the bottom-interacting arrival is high. For the frequency band of 500 to 1200 Hz, the spatial coherence is lower and more variable. The high coherence values are consistent with the Eckart theory for scattering from an interface having rms roughness less than 0.1 m. The sharply tuned nature of the low coherence values in a discrete frequency region suggests that the effect is due to interference from sediment multi-paths rather than scattering from bottom roughness. Bandpass filtered impulse responses show that the energy of the sediment-refracted arrival predominates at low frequencies and the reflected arrival predominates at high frequencies. The interference effects occur in the middle frequency region where the two types of arrivals have nearly equal amounts of energy, and may be expected to depend on sediment properties, bottom topography, measurement geometry, and oceanographic environment.


Spatial Coherence Grazing Angle Ocean Bottom Coherence Spectrum Shot Number 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.


  1. 1.
    C. W. Horton, A review of reverberation, scattering, and echo structure, J. Acoust. Soc. Am., 51:1049 (1972).CrossRefGoogle Scholar
  2. 2.
    J. M. Berkson, Measurements of coherence of sound reflected from sediments, J. Acoust. Soc. Am., to be published.Google Scholar
  3. 3.
    C. S. Clay, Coherent reflection of sound from the ocean bottom, J. Geophys. Res., 71:2037 (1966).CrossRefGoogle Scholar
  4. 4.
    W. F. Ruddiman and L. K. Glover, “Geology and Geophysics of The Tagus Basin,” Technical Note TN6120–1–76, Naval Oceanographic Office, Washington, D.C. (1976).Google Scholar
  5. 5.
    R. L. Dicus, “Preliminary Investigation of the Ocean Bottom Impulse Response at Low Frequencies,” U.S. Naval Oceanographic Office Technical Note 6130–4–76 (1976).Google Scholar
  6. 6.
    C. S. Clay and W. K. Leong, Acoustic Estimates of the topography and roughness spectrum at the sea floor southwest of the Iberian Peninsula, in: “Physics of Sound in Marine Sediments,” p 373, L. Hampton, ed., Plenum, N.Y., (1974).Google Scholar
  7. 7.
    D. E. Kerr, “Propagation of Short Radio Waves, MIT Radiation Laboratory Series, Vol. 13,” McGraw-Hill, N.Y. (1951).Google Scholar
  8. 8.
    R. L. Dicus, “Synthetic deconvolution of explosive source acoustic signals in colored noise,”U.S. Naval Oceanographic Office Technical Note 6130–3–76 (1976).Google Scholar
  9. 9.
    V. A. Benignus, Estimation of the coherence spectra and its confidence interval using Fast Fourier Transform, IEEE Trans. Audio Electro-acoustics, AU-17:145 (1969).CrossRefGoogle Scholar
  10. 10.
    G. M. Jenkins and D. G. Watts, “Spectral Analysis and its Applications,” Holden-Day, San Francisco (1968).Google Scholar
  11. 11.
    A. F. Seybert and J. F. Hamilton, Time delay bias errors in estimating frequency response functions, J. Sound and Vibration, 60:1 (1978).CrossRefGoogle Scholar
  12. 12.
    G. C. Carter, C. H. Knapp, and A. H. Nuttall, Statistics of the estimate of the magnitude-coherence function, IEEE Trans. Audio Electro-acoustics, AV-21:388(1973).CrossRefGoogle Scholar
  13. 13.
    G. C. Carter, C. H. Knapp, and A. H. Nuttall, Estimation of the Magnitude-Squared Coherence Function via Overlapped Fast Fourier Transform Processing, IEEE Trans. Audio Electro-acoustics, AV-21:337(1973).CrossRefGoogle Scholar
  14. 14.
    M. R. Foster and N. J. Guinzy, The coefficient of coherence: Its estimation and use in geophysical data processing, Geophysics, 32:602 (1967).CrossRefGoogle Scholar
  15. 15.
    C. Eckart, The scattering of sound from the sea surface, J. Acoust. Soc. Am., 25:566 (1953).CrossRefGoogle Scholar
  16. 16.
    C. S. Clay, Personal communication. An exponent of 2 is incorrectly ommitted in the corresponding expressions in reference 3 (1979)Google Scholar
  17. 17.
    R. R. Goodman and A. Z. Robinson, Measurements of reflectivity by explosive signals, in: Physics of Sound in Marine Sediments, p.537, L. Hampton, ed., Plenum, N.Y. (1974)Google Scholar
  18. 18.
    G. J. Fryer, Reflectivity of the ocean bottom at low frequency, J. Acoust. Soc. Am., 63:35 (1978).CrossRefGoogle Scholar
  19. 19.
    J. S. Hanna, Short-range transmission loss and the evidence for bottom-refracted energy, J. Acoust. Soc. Am., 53:1686 (1973).CrossRefGoogle Scholar
  20. 20.
    R. E. Christensen, J. A. Frank, and W. H. Geddes, Low frequency propagation via shallow refracted paths through deep ocean unconsolidated sediments, J. Acoust. Soc. Ara., 57:1421 (1975).CrossRefGoogle Scholar

Copyright information

© Plenum Press, New York 1980

Authors and Affiliations

  • J. M. Berkson
    • 1
  • R. L. Dicus
    • 1
  • R. Field
    • 1
  • G. B. Morris
    • 1
  • R. S. Anderson
    • 1
  1. 1.Naval Ocean Research & Development ActivityUSA

Personalised recommendations