Local Anomalous Sound Field Zones in Shallow Water: Experiment and Simulation

Abstract

The paper studies experimentally and theoretically the interference structure of the low-frequency spatial amplitude and phase responses of the scalar field and three projections of the vibrational velocity vector formed by tone signals from towed omnidirectional acoustic sources in zones near the interference maxima and minima of the sound pressure. The experimental dependences of these field characteristics on the distance obtained by four-component vector-scalar receivers when the sources are towed are compared with the calculated ones within the Pekeris model and waveguide model with a three-layer seafloor, the parameters of which were calculated based on acoustic calibration of the operations area. Satisfactory agreement was established between the amplitude and phase responses of the field, calculated by acoustic calibration and measured experimentally. It is shown that in the zones of maxima, a slow change in the angle of arrival is observed and the phase gradients are “smooth”, while in the zones of the minimum, sharp jumps in the amplitudes and phases form in the horizontal and vertical planes, leading for deep minima to the formation of circulations: local vortices around the poles. Numerical analysis of the fine structure of the sound pressure and vibrational velocity projections in the acoustic vortex zone is carried out, and hodographs of the vibrational velocity and phase gradients of the sound pressure are calculated, confirming the formation of vortices in the vertical plane in the zone of the poles.