INTENSITY FLUCTUATIONS DUE TO MODE COUPLING IN THE PRESENCE OF MOVING INTERNAL WAVES IN SHALLOW WATER AND ESTIMATION OF BOTTOM PARAMETERS

Spectral properties of the sound intensity fluctuations in the presence of non-linear internal waves (NIWs) are used for estimation of bottom parameters. The subject of this analysis is the time dependence of the total intensity: summed over the entire depth of the waveguide, in other words, over all hydrophones of the vertical line array - VLA. Data of the measurement of sound fluctuations (frequency 224 Hz) on an acoustic track ~ 30 km in the presence of high-amplitude NVWs moving towards the coast, obtained in the ASIAEX 2001 experiment are considered. The peculiarity of this situation is that the acoustic track consists of two parts: "deep", with a smoothly changing depth (average value ~ 270 m) and "shallow" ~ 120 m. The transition between the two sections is quite sharp. This feature leads, firstly, to a change in the properties (evolution) of NIW propagating first along the “deep” and then along the “shallow” sections, and secondly, it gives reason to assume different properties of the waveguide (in particular, the bottom sound speed and attenuation) in these sub-tracks. The paper proposes a technique for estimating the attenuation coefficients independently in two sections based on the study of the field spectrograms at the VLA and the variability of the total intensity, as well as the corresponding agreement between the experimental data and theoretical modelling. The obtained values for the loss factor are quite realistic, they are compared with the data of other authors for the same area.