FIFTH
ANNUAL
ASAP '97 WORKSHOP
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FIFTH
ANNUAL |
The Determination of |
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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