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ocean environment. Improper optimization of the frequency or the depth of operation at a particular location and time of year can degrade sonar performance.

In shallow-water environments, the optimum frequency of propagation is often the result of competing propagation and attenuation mechanisms at either end of the frequency spectrum. Jensen and Kuperman (1979, 1983) investigated this problem and concluded that the optimum frequency is strongly dependent on water depth, is somewhat dependent on the particular sound-speed profile, and is only weakly dependent on the bottom type. Jensen and Kuperman (1983) also noted that shear waves in the bottom were important in determining the optimum frequency of propagation and the actual transmission loss levels at lower frequencies. A major loss mechanism for low-frequency acoustic propagation in shallow water is the attenuation in ocean sediments. Research results (e.g. Focke, 1984) indicate that variations in attenuation as a function of sediment depth have a significant impact on propagation.

An important analytical tool used by Jensen and Kuperman (1979, 1983) was a frequency-range representation of transmission loss, as illustrated in Figures 5.5 and 5.6. This type of representation has been used to characterize acoustic propagation in different waveguides (e.g. Milne, 1967). For the particular environment studied (Figure 5.5), a normal mode model was used to generate repeated TL runs versus frequency. In Figure 5.6(b), the

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