Effects of random bottom bathymetry on temporal and spatial coherence in shallow water propagation

Abstract

Fixed system measurements for three experimental shallow water sites reveal systematic dependence of temporal and spatial coherence of individual mode arrivals on frequency and mode number that cannot be fully explained with internal wave variability. Here random fluctuations in bottom bathymetry are introduced to propagation models. Mode structures are randomized and coherence times and lengths decrease with increasing amplitude of bathymetry fluctuations for the same internal wave variations. Distinct mode features blur as frequency of pulse signals increased. Mode coupling and eventually a smearing of continuous modes are observed with increasing frequency and magnitude of bathymetry variations. For low frequencies and bathymetry variations constrained a small fraction of a wavelength, the bottom appears flat and nearly perfect discrete modes are formed. Modes are reinforced by specular reflections all along the path of propagation. Coherence depends entirely on internal wave fluctuations. All mode arrivals have nearly the same coherence times. But modes break up as height of bathymetry fluctuations become comparable to acoustic wavelength. Then sensitivity to sound speed fluctuation increases and coherence is reduced. Two dimensional bathymetry fluctuations account for observation out to 10–20 km and three-dimensional effects become important at longer ranges.