Effect of Sound-Speed Inhomogeneities in Sea Bottom on the Acoustic Wave Propagation in Shallow Water

Abstract

The effect of volume inhomogeneities in the sea bottom upper layer (sediments) on the long-range sound propagation in shallow water is analyzed. The experimental basis for developing inhomogeneous bottom models are the data of 3D seismic survey on the spatial structure of sediments in the Kara Sea, where variations of ~10% in the average sound speed in the bottom are observed at ranges of ~1 km. The mode coupling due to bottom volume inhomogeneities is revealed within the normal mode approach. Coupled mode equations are derived not only for propagating modes but also for leaky modes and quasimodes; including these types of modes may be urgent in analysis of the waveguides where the sound speed in the bottom (c1) is less or approximately equal to the sound speed in water (c). Numerical simulations show that the coupling between energy-carrying modes is maximum at c1 ≈ c. Calculations of the attenuation of depth-averaged sound field intensity in a water layer, performed for the Kara Sea at ranges of up to 5 km, show that the mode coupling due to bottom volume inhomogeneities can be neglected, and the depth-averaged transmission loss can be calculated using the adiabatic approximation at frequencies below a few hundred hertz. When analyzing the depth-averaged intensity, the effects of mode coupling on bottom inhomogeneities can be observed only by implementing special conditions, for example, using mode arrays providing maximally pure (free of other modes) radiation of a specified mode.