Abstract
Relatively simple approximations for transmission loss in shallow water based on ray theory were presented by Marsh and Schulkin (1962), Weston (1971), Smith (1974), and Rogers (1981). Formulas for averaged transmission loss versus range in stratified shallow water waveguides over sandy‐silty sediments are derived entirely from mode theory. Results are contrasted with other mode based expressions obtained by Brekhovskikh (1960) and Grachev (1983). Convenient expressions for loss in Pekeris waveguides are developed by approximating the mode number dependence of the attenuation coefficients as quadratic, which is valid in Westons’s mode stripping region. Similar loss expressions for environments with downward refracting linear sound speed profiles are constructed using modal attenuation coefficient approximations for the “limiting mode” of Denham (1969). This quantity illustrates how changes in environmental parameters, particularly thermocline strength and depth, produce corresponding changes in averaged transmission loss. The loss formulas have several terms, each having a natural physical interpretation, and each is compared with the well known results of Rogers from ray theory. The corresponding expressions have the same parameter dependence, thereby providing a modal foundation for Rogers’s formulas. [Work partially supported by the ONR.]
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