Search for: All records

Abstract —An approach is proposed for estimating the dispersion characteristics of waveguide modes from analysis of ship noise recorded by two closely spaced and synchronized vertical arrays. This approach was used for an experimental study of the mode structure of a low-frequency sound field in a shallow-water waveguide with a gas-saturated bottom in a wide frequency band (from 20 to 250 Hz). The experiment was carried out in Lake Kinneret (Israel), known for its high methane bubble content in the sedimentary layer (~1%) and, consequently, for the low sound speed in this layer (~100 m/s). The maximum depth in the area of the experiment was 40.4 m. The receiving system consisted of two 27 m vertical arrays spaced 40 m from each other and covering part of the waveguide below the thermocline. The noise source, the R/V Hermona , moved along a straight line connecting the arrays at distances of up to 1 km from them. The approach made it possible to isolate the frequency dependences of the phase velocities for the first 12 modes; these dependences proved close to those for a waveguide with an perfectly soft bottom, except for the frequency region near the cutoff frequency. The limitations and possible development of the technique are discussed. 

Measurements by sensors mounted on compact platforms are affected by sound scattering from the platform. Assuming a spherical shape of the platform, this paper investigates the differences between the ambient and measured characteristics of low-frequency signals and noise for scalar and vector sensors. In the near field of the platform, low-frequency perturbations in oscillatory velocity are generally much larger than pressure perturbations. These perturbations prevent mounted vector sensors from correctly measuring the direction of the free-field oscillatory velocity. The feasibility of a compensation of the distortions in scalar and vector sensor measurements is discussed. 

Simple, analytically solvable models of normal mode propagation in the coastal ocean are developed and applied to study the effect of the seafloor bathymetry on modal travel times. Within the adiabatic approximation, horizontal inhomogeneity of the waveguide is found to change the modal dispersion curves in a way that helps separation of the modal components of the acoustic field using the time-warping transform. It is shown that moderate seafloor slopes can lead to surprisingly large errors in retrieved geoacoustic parameters and cause a positive bias in bottom sound speed estimates if horizontal refraction is ignored.