Infrasonic Interference Field of the Directed Sources in Shallow Sea

The numerical study of interferential structure of the low-frequency hydroacoustic fields in shallow sea excited by the concentrated directed sources of signals is made. The study is executed with the assumption that the amplitudephase distributions in the vertical and horizontal plane formed in a wave guide by extended volume real sources can be formed by system of local sources as superposition of a monopole and various image of the focused three dipoles and five quadrupoles concentrated in one point. The calculations are performed for the analysis of hydroacoustic fields in the near- and far-field, for different frequencies and depth of the source and receiver locations. It is found that the structure of the fields at distances of less or more than 1–2 depths of the waveguide is substantially different. It is shown that the intensity decrease of fields generated by a monopole, horizontal dipoles and horizontally oriented quadrupoles goes noticeably slower at the distance increase than the decrease of signals from vertical dipoles or vertically oriented quadrupoles. It is also shown that the signals from the sources or got by a receiver located at shallow depths or bottom area decrease significantly faster than the intensity of the fields in case if the receivers and sources are located close to the equivalent middle of waveguide. It is recommended to measure the characteristics of the fields of low-noise sources by vector-scalar receivers or antennas in the areas of signal interference maxima and in the middle of waveguide. When predicting the given noise values and processing the experimental data it is recommended to take into account the transfer functions of the waveguide, resulting from acoustic calibration of soil models in the study area.

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