Shallow water is classified as water less than 100 fathoms that is, water over a continental shelf. Deep water is classified as water 1,000 fathoms or deeper. Water between 100 and 1,000 fathoms deep is most common over continental slopes. It is not considered overly important in active sonar operations because it exists in such a small portion of the world s oceans. Shallow-water propagation paths are classified as direct path and surface duct.
Direct path is the simplest mode.
Direct path sound propagation occurs where there is an approximate straight-line path between the sound source and the receiver, with no reflection from any other source and only one change of direction due to refraction. A surface duct is simply a near-surface layer that traps sound energy. The temperature increases with depth. An isothermal layer is near the surface. In condition 1, sound velocity increases as the temperature increases.