Research On Acoustic Field Fluctuation Caused By Shallow Water Internal Waves In 2005 Acoustics Experiment Of Yellow Sea Oceanic Front And Internal Waves

Oceanic internal wave is considered to be one of the main reasons for acoustic field fluctuation. Recently, it more and more becomes one of study focuses of ocean acoustics in shallow water.Three parts have been studied in this paper. Firstly, extracting internal wave modes and their power spectra using temperature data is discussed. Secondly, the rule of sound normal mode travel-time fluctuation caused by shallow water random internal waves and its corresponding relationship with the statistical rule of local temperature data are studied. Thirdly, the fluctuation of sound signal amplitude caused by one kind of non-linear internal waves is analyzed.The data acquired from moored sensors of AEYFI 05, Acoustics Experiment of Yellow Sea Oceanic Front and Internal Waves, are processed in first part A method of extracting internal waves' modes is given. In conventional ways, buoyancy frequency must be known and the internal waves' normal modes are gotten by numerical computing, then the modes' coefficients are determined by orthogonal decomposition. But in this part, internal waves' mode functions and mode coefficients are extracting from experimental data at the same time.Both of the temperature temporal-spatial and the sound travel-time statistical characteristics are analyzed in second part. Results indicate that the change of M2 tide and coherent-structure are clearly reflected in the frequency spectrum of the 1^st Normal Mode travel-time fluctuation. This spectrum is qualitatively similar to the spectrum of temperature time sequence near thermocline, two spectra present power lawω^-α(α(?)1.7-1.8) attenuation in the range over 30cpd. The relation equations between Normal Mode propagation travel-time and sound-speed fluctuation are used to explain the qualitatively relationship between two spectra.The fluctuation of sound signal amplitude caused by one kind of non-linear internal waves is analyzed in third part. Strong and "regular" sound signal amplitude fluctuations can be discovered in part of hydrophones. This phenomenon can be explained by the theory of sound field receiving sensitivity fluctuation caused by an "up-flange" weak non-linear internal waves passing receiving vertical array. Based on Normal Mode theory, the explanation of sound field fluctuation characteristic caused by internal waves is given.