The fluctuation of sound field due to internal waves on the thermocline in typical shallow water

In this paper, the results of experiments concerning the fluctuations of internal waves on the thermocline and the fluctuations of sound amplitudes at the different range in typical shallow water are presented. Recorded time is 48.96 hours. Thickness on the thermocline is about 2-4 m. The deviation of temperature recorded at the fixed depth has 10-15℃. The vertical displacements of isotherm curves are about 5-6 m, which are dominated by the lowest mode. The measured spectrums of vertical displacement have obvious spectral packet at the range of 0.05 cpm (20 min) and 0.143 cpm (7 min) besides diurnal and semidiural tide frequency bands. Spectrum attenuation coefficients are between -1.5 and -1.7. Numerical calculation shows that phase speed and group speed of internal waves are equal to 0.329 m/s at the range of lower frequency. And with the increasing of frequency, group speed drops more quickly. The measured fluctuation of acoustic amplitude is about 8-10 dB. With the increasing of ranget the fluctuation trends to be faster and acoustic signals are declining. Spectrum attenuation coefficients are between-1.45 and-2. At last, PE method is used to simulate the acoustic amplitude fluctuation,in which internal waves are the important element.