Which term describes the layer of the ocean where the sound channel exists, enabling long-range acoustic propagation?

• A. Shadow Zone
• B. Surface Zone
• C. Sofar
• D. Sofar Layer

Answer: (D)

Sound speed in seawater changes with depth due to temperature, salinity, and especially pressure. This creates a refractive effect: sound waves bend toward regions where the speed is lower. Where there is a minimum in the speed of sound at mid-depth, sound rays tend to curve back toward that layer, effectively trapping the energy and forming a natural acoustic waveguide. That mid-depth layer where this trapping occurs is the Sofar Layer, the specific band where long-range propagation is possible.

So, the term that describes the layer is Sofar Layer. The Shadow Zone is a region outside the channel where direct paths don’t reach, and the Surface Zone isn’t where the channel forms. The shorthand Sofar can refer to the channel concept, but the exact layer designation is Sofar Layer.