Experimental study on horizontal longitudinal correlation of the signal at the continental slope area

Due to the complex underwater propagation environment, such as environment disturbance, interference between normal modes, and ambient noise, the waveforms of received signals change over space and time and cause the attenuation of the spatial correlation between the received signals. The spatial correlation between the received signals affects the performance of the array signal processing directly. Thus, it is important to study the spatial correlation characteristics between received signals. In the paper, the horizontal longitudinal correlations between the received signals for 4 different locations at the continental slope area and the changes of the horizontal longitudinal correlations during the movement of the ships are studied by analyzing the experiment data. The experiment is carried out in the South China Sea. In the experiment, the receiving array, a horizontal linear array with 192 sensors, is dragged by a ship moving with 2 m/s, and the source with a depth of 95 m is dragged by another ship drifting at sea. The transmit signal is linear frequency modulation signal, with a frequency of 260 to 360 Hz. To reduce the effects of the ambient noise on the horizontal longitudinal correlations, the band-pass filter and the matched filter are applied first. Then the receiving array is divided into many partly overlapped short subarrays and the broadband beamforming is applied to the signals after filtering on each subarrays. Finally the beamforming outputs are used to calculate the horizontal longitudinal correlation coefficients between received signals. Results show that the horizontal longitudinal correlation length of the received signal is at the range of 13 to 60 m, which is generally less than the correlation length in the shallow water environment, and that the horizontal longitudinal correlations between the received signals change over space and time obviously and are dependent on the interference structure of the sound field and the environment disturbance.