"Title from cover."

"29 March 1982."

Bibliography: p. 67

Nine shallow water areas of strategic interest were selected for a preliminary study of surface ducts in shallow water. The objective was to determine the relative percentage of surface ducts versus downward refractive conditions. Sound speed profiles from National Oceanographic Data Center (NODC) Nansen cast files and XBT profiles were classified by computer as either positive or negative (non-positive) gradient profiles. Ninety percent of the fall-winter and 31 percent of the spring-summer profiles were of positive gradient (or surface duct)type. Percentages are given for each of the nine cases. Representative profiles and sea floor data from six of the sites were used to compute propagation loss. These losses, computed by normal mode theory, were used to determine the frequency of optimum propagation at these sites for positive and non-positive gradient profiles. Positive gradient (winter) profiles generally resulted in at least 10 dB less loss at 50 km range than did non-positive gradient profiles. Optimum propagation for positive gradient surface ducts varied from 200 to 1000 Hz. The reflectivity of the bottom models varied greatly. Where propagation was relatively good by bottom reflected paths, optimum frequencies were around 500 Hz. The greatest differences between propagation losses calculated for the positive and non-positive profile cases are seen where such reflectivity is poor. (Author)

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