ADVANTAGES OF THE SURFACE DUCT

The potential for using these ducts in long-range detection was not fully realized in early sonar operation since the equipment was generally in the supersonic frequency range (24 kHz and above) and attenuation due to leakage and absorption was great. As a result of the continuous trend in sonar toward lower frequencies, the use of this duct is an aid for both active and passive detection.

At low frequencies, sound will not be trapped in the surface duct. This occurs when the frequency approaches the cutoff frequency; that is, the wavelength has become too large to "fit" in the duct. This does not represent a sharp cutoff. However, at frequencies much lower than the cutoff frequency, sound energy is strongly attenuated by scattering and leakage out of the duct.

Figure 9-5.-A shaded sonic layer depth display.

The quality of transmission in the surface duct varies greatly with the thickness of the duct, surface roughness, gradient below the layer, and frequency.

There is one graphic output available with the SFD product. It is a shaded surface duct cutoff frequency display. The amount of shading indicates the range of frequencies. See figure 9-6.

LEARNING OBJECTIVES: Understand the conditions under which DPRs are most likely to occur. Evaluate the DPR product. Identify the graphic output of the program.

The DPR displays the most probable ranges that can be expected for acoustic surveillance system modes that use direct path propagation. The direct path is the simplest propagation path. It occurs where there is approximately a straight-line path between sonar (source) and target (receiver), with no reflection and only one change of direction due to refraction. The maximum range obtained in the direct path propagation mode occurs out to the point at which the surface duct limiting ray comes back up and is reflected from the surface.

There is one graphic output available with the DPR product, a shaded direct path range display. The amount of shading indicates the range in nmi. See figure 9-7.

HALF-CHANNEL CONDITIONS (HAF)

LEARNING OBJECTIVES: Understand the situations that are most favorable for HAF. Evaluate the HAF product. Identify the graphic output of the program.

The HAF product displays areas where positive sound speed profile gradient (half-channel) conditions

Figure 9-6.-A shaded surface duct cutoff frequency display.

Figure 9-7.-A shaded direct path range display.

exist. Half-channel conditions exist where the water is essentially isothermal from the sea surface to the bottom, so that sound speed increases continuously with increasing depth. Under these conditions, the greatest sound speed is at the bottom of the ocean, and sound energy will be refracted upward, then reflected downward at the surface, and refracted upward again. The effect is similar to a strong surface duct, so long ranges are possible. Half- channel propagation is common during winter in the Mediterranean Sea and polar regions.

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