LEARNING OBJECTIVES Recognize characteristics of the SLD. Evaluate SLD product. Identify the graphic output products.

The SLD product displays the layer depth that can be used to locate areas of strong sound propagation in the near-surface layer. The sound field in a layer depends greatly upon the layer depth. The deeper the layer, the farther the sound can travel without having to reflect off the surface and the greater is the amount of energy initially trapped.

Figure 9-4.-A shaded bottom bounce probability display.

There is only one graphic output available with the SLD product. It is a shaded sonic layer depth display. The amount of shading indicates the range of depth in feet. See figure 9-5.

SURFACE DUCT CUTOFF FREQUENCY (SFD)

LEARNING OBJECTIVES: Describe the two conditions under which a surface duct may occur. Evaluate the SFD product. Identify the graphic output of the product. The SFD product displays the cutoff frequency values where a surface duct may occur in the mixed layer of the ocean if one of two conditions exist: (1) the temperature in the layer increases with depth or (2) an isothermal layer is near the surface. In condition 1, sound speed increases as the temperature increases. In condition 2, there is no temperature or salinity gradient and pressure causes sound speed to increase with depth.

In the mixed (or surface) layer the velocity of sound is susceptible to the daily and local changes of heating, cooling, and wind action. Under prolonged calm and sunny conditions the mixed layer disappears and is replaced by water where the temperature decreases with depth.

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