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The Determination of
Signal Coherence Length
Based on Signal Coherence
and Gain Measurements in
Deep and Shallow Water

William M. Carey
Massachusetts Institute of Technology
77 Massachusetts Avenue, Room 5-211
Cambridge, MA 02139
tel: (617) 253-7639
email: wcarey@mit.edu

Abstract Experimental measurements of signal coherence and array signal gain are reviewed for deep and shallow water sound channels. The signal gain is related to the horizontal coherence length through relationships from the statistical theory of antennas. Signal gain measurements in the transverse direction are proffered as a practical measure of coherence length for both broad-band and narrow-band signals. Using this technique, measurements at frequencies near 400 Hz are presented that show for the deep water cases lengths on the order of 100 wavelengths can be achieved while under downward refraction conditions in shallow water waveguides with sand silt bottoms lengths on the order of 30 wavelengths are realized. The measurement of broad-band and narrow-band coherence functions are discussed with emphasis on the role of partly-coherent noise backgrounds, multipath interference effects, and averaging constraints. These results are interpreted with coherence models consistent with sound scattering from the volume and boundaries of the waveguide.



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