Abnormal interference of very low frequency sound field in shallow water

Abstract

In a multi-channel seismic exploration experiment in the South Yellow Sea, a tuned air gun array with a shooting interval of 37.5m was used as broadband pulse source, and a horizontal line array with 336 elements is used to receive transmitted signals. The received single shot records in the range-time domain are transformed into an interference pattern in the range-frequency domain using Fourier transform. It is found that there is an abnormal interference pattern of the very low frequency (VLF) sound field in the stratified shallow water environment. It is a complex nonlinear structure, no longer regular striations in the range-frequency domain. The mechanism of this particular interference pattern is interpreted according to normal mode theory. In shallow water stratified leaky waveguide, VLF sound propagates as waterborne modes and bottom-trapped modes. These two kinds of normal mode interfere with each other and form the particular stable interference pattern. In the experimental environment, the group speeds for these two type normal modes change differently with frequency. And it causes slope of interference striations change from positive to negative with increase of frequency. The theoretical simulation results are in good agreement with the experimental data. It is shown that the structure of interference pattern is sensitive to bottom structure and geoacoustic parameters.