Surface Duct Propagation
In the mixed layer, we know that there is a slightly positive gradient in the sound speed due to increasing pressure, so the rays will bend upward to the surface. When a ray reaches the surface, the ray is reflected downward toward the bottom of the mixed layer (the SLD), and continuously refracted back toward the sea surface. This reflection and refraction results in trapping of sound rays and is known as surface duct (SD) propagation. The sound can propagate long distances as long as there is a positive gradient in the SSP.
An important aspect of surface ducting is the shadow zone that results below the SLD. You can see how this forms in the figure to the right. Sounds waves are trapped between the sea surface and the bottom of the mixed layer (or SLD). The thicker the mixed layer, the stronger the duct, and therefore, the longer the detection ranges that can be expected.
How does the sea state affect the surface duct? In general, with a higher sea state there is more mixing and a deeper mixed layer. However, very high seas will degrade the duct by causing too much scattering of sound rays when they reach the rough surface.
Frequency is also a factor in surface duct usability. When the wavelength of a sound wave becomes too large to "fit" in the duct (low frequency sound), it will not be bounded by the duct. The wave will experience spherical spreading loss and there will be significant leakage through the SLD. The frequency of sound which has a wavelength that just "fits" into the duct is termed the low frequency cutoff. All sound frequencies higher than this cut-off will be trapped in the duct; lower frequencies will penetrate the layer. The low frequency cutoff is directly proportional to the SLD. Operationally, this is determined by a nomogram such as the figure to the left. As an example, if the SLD is at 250 feet and the frequency of interest is 300 Hz, ducting is probable. If the SLD were any shallower, the rays would penetrate the layer.
Next: Half Channel propagation