thermal gradient condition, the layer depth is thedepth of maximum temperature. Above layer depth,the temperature may be uniform, or a weak positive ornegative gradient may be present.Layer effect is the partial protection from echoranging and listening detection, which a submarinegains when it submerges below layer depth. Reportsfrom surface vessels indicate that effective ranges onsubmarines are greatly reduced when the submarinedives below a thermocline, and that the echoesreceived are often weak and sound “mushy.”Figure 4-4.-Isothermal conditions.(constant temperature). Refer to figure 4-4 as youread the following text. The surface layer of water inthe figure is isothermal, but beneath this layer thetemperature decreases with depth. This causes thesound beam to split and bend upward in theisothermal layer and downward below it.Remember, when no temperature differenceexists, the sound beam refracts upward due topressure. When the temperature changes with depth,the sound beam bends away from the warmer water.Under normal conditions the sea’s temperaturestructure is similar to that shown in figure 4-5. Thisstructure consists of three layers as follows:1. A surface layer of varying thickness withuniform temperature (isothermal) or a relatively slighttemperature gradient.2. The thermocline, which is a region ofrelatively rapid decrease in temperature.3. The rest of the ocean, with slowly decreasingtemperature down to the sea floor.If this arrangement changes, the path of the soundbeam through the water will change.Layer depth is the depth from the surface to thetop of a sharp negative gradient. Under positiveFigure 4-5.-Normal sea temperature structure.4-5
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