Sonar (Sound Navigation and Ranging) was originally designed to assist surface ships to navigate in bad weather. Later, it was employed on submarines, and today it is our primary means of locating submarines. There are two types of sonar searches: active and passive. An active sonar employs a transmitter to send out sound pulses and a receiver to record returning echoes. A passive sonar listens for sounds generated by other ships and submarines.

Active-sonar search is classified into two modes: shallow-water transmissions and deep-water transmissions. Theoretically, the essential difference between shallow- and deep-water transmissions is the interference effects pro-duced by the multiple reflections of sound in shallow water.

Shallow water is classified as water less than 100 fathomsthat 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 worlds oceans.

SHALLOW-WATER TRANSMISSIONS. Shallow-water propagation paths are classified as direct path and surface duct.

Direct Path. 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.

Surface Duct. A surface duct is simply a near-surface layer that traps sound energy. Surface ducts exist in the ocean if the following conditions are met:

In condition 1, sound velocity increases as the temperature increases. In condition 2, there is no temperature or salinity gradient; however, the in-crease in pressure with depth causes the sound velocity to increase with depth.

The greater the depth of a duct, the greater the difference between the surface velocity and the velocity at depth. There are also a greater number of sound rays trapped in the duct. Of course, the efficiency of a surface duct in transporting sound is dependent also upon the smoothness of the sea surface.

ENVIRONMENTAL CONTROLS. The success of active sonar searches in shallow water depends a great deal on environmental factors. Temperature gradients, horizontal as well as vertical; water depth; and the physical characteristics of the sea surface and bottom all impact shallow-water transmissions. Of these controls, water depth is the most important. Water depth determines the range and angle at which sound rays strike the bottom (angle of incidence) and to some extent the types of transmission paths that occur. Variations in the vertical temperature gradient, which result in sound speed variations, are of ut-most importance where sound is propagated through a surface duct. A change in gradient of .2C per 30 meters can be the difference between an excellent duct with good ranges and no duct and poor ranges.

Horizontal velocity gradients in the ocean are not as great as those in the vertical; however, they can completely destroy a duct if they occur between the sound source and the target. Bottom composition and roughness control, to a large extent, the reflective and absorbent capabilities of the bottom. Shallow-water sediments are quite diverse. Areas of mud, sand, mud-sand, gravel, rock, and coral are not uncommon over shelf regions.

In shallow water, as in deep water, the sound velocity profile controls the degree of refraction of sound rays. For an example of how similar profiles effect shallow- and deep-water transmissions, consider the following:

In deep water, where a strong negative gradient exists, sound rays are refracted downward and result in shadow zones. On the other hand, in shallow water the downward refracted rays bounce off the bottom, travel up-ward, and bounce off the sea surface. This process continues until the shadow zone is completely insonified. Consequently, this results in better detection probability.

DEEP-WATER TRANSMISSIONS. In deep water, sound may travel from and to a sonar via surface duct, convergence zone, bottom bounce, and sound channel transmission paths. Figure 2-2-4 illustrates the deep-water sound-transmission paths.

Surface Ducts. Surface ducts occur in deep water just as they do in shallow water.

Sound Channels. A sound channel is formed when a negative-velocity gradient overlies a positive-velocity gradient. The thermal gradient necessary to produce a sound channel is negative over isothermal or negative over positive. The sound channel axis is found at the point of sound-velocity gradient change. The axis is the point of minimum sound speed. Sound channels trap sound rays and provide extremely long ranges. Such thermal conditions are found in shallow and deep water.

Shallow sound channels are found in the near-surface layer. They are rare and transitory (they move), and occur when thermal conditions are unstable (cold water over warm). In the Pacific Ocean, shallow sound channels are most common in the area north of 40N between Hawaii and the continental United States. In the Atlantic, they are most frequently observed in the vicinity of the Gulf Stream.

Deep sound channels are far more prevalent than their shallow counterpart. In the deep ocean, temperature generally decreases with depth (the main thermocline). This produces a negative-velocity gradient and sound rays that refract downward.

In the Atlantic, such gradients exist to a depth of approximately 700 fathoms. Below 700 fathoms, the gradient becomes isothermal. In the Pacific, the isothermal layer begins around 500 fathoms. Below these depths, the greater pressure combines with the isothermal-tempera-ture gradient to produce a positive-velocity gradient. The sound rays are refracted upward.

The depth of the gradient change is known as the deep-sound-channel axis. The deep-sound-channel axis varies from 1,225 meters in the mid-latitudes to near the surface in the polar regions. Extremely long sonar ranges (on the order of thousands of miles) are possible within a deep sound channel. Deep sound channels are also known as SOFAR channels.

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