The determination of signal coherence length based on signal coherence and gain measurements in deep and shallow water

Abstract

Experimental measurements of signal coherence and array signal gain are reviewed for both 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 broadband and narrowband 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 in the downward refraction conditions of shallow water waveguides with sand-silt bottoms, lengths on the order of 30 wavelengths are realized. The measurement of broadband and narrowband coherence and correlation 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.