Analysis of transmission loss, signal gain, and coherence in shallow water

Experiments in the Strait of Korea were performed to study sound propagation in an oceanographically complex shallow water environment. First a geoacoustic model is developed based on narrowband transmission loss measurements and using estimated profiles and measured bathymetry. The comparisons between measured and calculated transmission loss are made through an effective attenuation coefficient, which measures the rate of change of mean transmission loss with range. Environmental model parameters are selected to achieve the best agreement in the comparisons. Nonlinear frequency dependence in the sediment attenuation profiles permits good agreement between the calculations and measured data. The developed geoacoustic model is then used to obtain predictions of broadband transmission loss and signal energy spread. Very good agreement between these predictions and corresponding independent measurements validates the geoacoustic model. Next signal gain measurements taken during the experiment are examined. Using the previously developed environmental profiles the signal gain is computed. The calculations are in agreement with measurements for shorter ranges. For longer ranges and higher frequencies disagreement is found between the calculations and measurements. The cause is random fluctuations in the signal induced by the random medium. These effects can be included into the signal gain through the coherence function. Using a previously developed theory preliminary calculations of coherence are made. By choosing physically reasonable parameters of the random fluctuations in the medium, close agreement with measurements is achieved. Next this theory is extended to include scattering from inhomogeneities with arbitrary correlation functions. This allows a treatment of random fluctuations described by physically based spectra. The correlation functions corresponding to these spectra for mechanisms such as internal waves, turbulence, and wind driven sea surface variations are derived and included into expressions for the coherence. Finally the dependencies of the coherence function on various parameters are thoroughly examined using Gaussian correlation functions to describe environmental random fluctuations. The results using physically realistic mechanisms are also obtained. The signal gain is calculated using physically based correlation functions of fluctuations and found to be in very good agreement with experimental measurements.