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.
100 350 ft
SURFACE DUCT CUTOFF
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
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.
ADVANTAGES OF THE SURFACE
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.