Retrieval of Dispersion Dependences of Waveguide Modes from Ship Noise Measurements Using Two Synchronized Arrays

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.