Sound in the Ocean

The oceans are notoriously opaque, providing the submarine with a strong veil of stealth. The only thing that oceans don't effectively block is sound, which is transmitted more effectively through water than it is through air. As a result, sonar remains the main method of detecting submarines, either in active form, producing sound that bounces off the target and is reflected back, or in passive form, listening for noise from the target. But sound doesn't travel through the ocean in a straightforward way, and as sonar has grown more sophisticated, its operators have had to contend with effects that either attenuate the sound, drown it out or bend it in surprising ways.

The Deep Scattering Layer (green) on a sonar scan

Attenuation can come from a number of sources. Obviously, the intensity of a sound weakens as it spreads out from its source, and in an infinite ocean, this would follow the inverse square law. In practice, it's often lower than this, thanks to effects discussed later, but the ocean doesn't transmit sound perfectly. Some is absorbed and turned into heat, a process that occurs more strongly at higher frequency. Some is scattered when it runs into discontinuities, which can be the surface and bottom of the sea or solid objects in the ocean, usually marine life. Most notable is the deep scattering layer, composed of deep-sea fish and the plankton they feed on that tend to cluster a thousand feet or so below the surface. Scattering is a particular problem for active sonar, as the reverberations from the surface/bottom/creatures can make it hard to pick up the actual return.

Sonar also has to contend with background noise, of which there is a lot in the ocean, although unevenly distributed across frequencies. Low-frequency noise is poorly understood, but includes factors like turbulence in the ocean and seismic activity. At higher frequencies, noise is caused by distant shipping, wind and wave activity on the ocean surface, and even the movement of water molecules. Other sources of background noise can also complicate matters. Nearby shipping, drilling, or the use of explosives for oil exploration can drown out a submarine. Biological sources of noise, such as snapping shrimp, croaking fish, or cetaceans (whales and the like), are generally easy to identify, but also raise the background. And seismic activity is a problem in geologically active areas like the mid-Atlantic Ridge.

But the real fun comes from looking at the paths sound takes through the ocean. These are very complicated, but come from some quite basic physics. The speed of sound in water increases with each of increasing temperature, pressure and salinity, and per Snell's Law an increase in the speed of sound will change the angle of an ray of sound passing through it. The effect of Snell's Law is that rays bend towards areas where the speed of sound is low, and away from places where it is high, producing some quite intricate paths.

Half-channel propagation¹

The most basic case is one where salinity and temperature are constant throughout the water column, leaving only the effects of pressure, which obviously rises with depth. The result is increasing velocity with depth, which will bend the sound back towards the surface a ways away from where it started. When it reaches the surface, it might, depending on frequency and the state of the sea surface, bounce off it without too much scattering and continue on, a phenomenon known as the half-channel, common in the Mediterranean and the Arctic. This dramatically cuts down losses from signal spread, as the energy from the sound source is spread in two dimensions instead of three dimensions.

The surface duct and shadow zone

But temperature is rarely constant in the water column, producing more interesting effects. In the deep ocean, a cloudy, windy day will typically mix the water near the surface and produce an isothermal layer down to a few hundred feet, resulting in something quite similar to half channel propagation for objects near the surface, known as the surface duct. However, below the layer is a gradient of decreasing temperature which overwhelms the effects of pressure, bending sound downward. This produces a "shadow zone" below the layer where a sonar near the surface can't see, an effect that submarines like to exploit. Counters to this include variable-depth sonars, which have a towed body that can be lowered below the layer, and finding a path that bounces off the seabed. Exploiting bottom bounce is rather difficult, as the bottom has to be smooth enough to reflect well and not so hard that it absorbs a lot of the signal, as well as being at the right depth. It also suffers from constructive and destructive interference, which means that signals tend to peak and fade unpredictably.

The Deep Sound Channel

Temperature (and thus sound velocity) below the layer drops off fairly consistently until it reaches the 4°C of the deep ocean several thousand feet down, at which point pressure takes over and sound velocity begins to rise again. The result of this minimum in sound velocity is that rays on either side are bounced back towards it, producing the so-called deep sound channel. The US and its allies made extensive use of the Deep Sound Channel during the Cold War to track Soviet ships and submarines, sometimes across an entire ocean basin. Smaller sound channels sometimes form higher in the ocean, although far less predictably and over shorter distances.

A Convergence Zone

A related and more tactically useful phenomenon is the convergence zone, where the same process that produces the deep sound channel focuses noise near the surface at specific distances from the target. This only occurs in reasonably deep water, usually over 2000', and produces a set of "rings" that raise the volume of the target by 15-20 dB,² a major boon for ASW forces. Exactly how far out the convergence zones are varies depending on local conditions, but the innermost convergence zone usually is somewhere between 20 and 35 nm out, with subsequent CZs at multiples of that distance. Each covers 5-10% of the distance from the target, and they play an important part in long-range ASW, with helicopters often sent out to nail down and kill submarines picked up in the convergence zones.

Not mentioned so far is salinity, the third factor that can affect the speed of sound in the ocean. For most of the ocean, it can be ignored because it is stable throughout the water column, but it becomes a problem when two bodies of water of different salinity mix, either due to ocean currents or particularly in shallow waters where rivers run into the ocean. The result is a complicated and unpredictable set of sound paths, made even more convoluted by the topography of the bottom. The result is that hunting submarines in shallow coastal waters remains the most difficult challenge for anti-submarine forces.

Of course, this is only the broadest overview of underwater acoustics as applied to modern anti-submarine warfare. Books have been written on the subject, taking into account not only the things mentioned here but also issues like bottom topography and the differences between active and passive sonar. But at the very least, this should give a decent idea of how the primary underwater sensors have worked for the past century.