Array signal-gain measurements in shallow-water

Abstract

Shallow-water transverse coherence lengths can be estimated by the measurement of the narrow-band coherence function, broadband correlation function, or the signal gain either from a direct measurement with a filled aperture or the steered beam response of a sparsely filled aperture. Signal-gain measurements with an array of sensors have a larger number of degrees of freedom, spatial filtering of unwanted noise sources, and higher signal-to-noise ratios. Given a Gaussian coherence function, the coherence length may be estimated from these gain measurements. This paper discusses signal-gain measurements performed in five shallow-water sandy-bottomed areas with known environmental conditions using horizontal-bottomed arrays, omnidirectional explosives, and continuous sources between 100 and 600 Hz to ranges of 40 km. The coherence lengths determined by using a Gaussian coherence function yield a consistent representative value of approximately 30 wavelengths at a range of 40 km and a frequency of 400 Hz, which decreases with an increase in frequency. Comparisons show these lengths are consistent with previous measurements. Analytical considerations indicate these lengths are due to the combined effects of the water column variability and bottom. Implications for shallow-water array processing are discussed.