http://en.wikipedia.org/wiki/Sonar
Wikipedia
Sonar
Sonar (originally an acronym for SOund Navigation And Ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, communicate with or detect other vessels. Two types of technology share the name "sonar": passive sonar is essentially listening for the sound made by vessels; active sonar is emitting pulses of sounds and listening for echoes.
Sound propagation
Sonar operation is affected by variations in sound speed, particularly in the vertical plane. Sound travels more slowly in fresh water than in sea water, though the difference is small. The speed is determined by the water's bulk modulus and mass density. The bulk modulus is affected by temperature, dissolved impurities (usually salinity), and pressure. The density effect is small. The speed of sound (in feet per second) is approximately:
4388 + (11.25 × temperature (in °F)) + (0.0182 × depth (in feet)) + salinity (in parts-per-thousand ).
This empirically derived approximation equation is reasonably accurate for normal temperatures, concentrations of salinity and the range of most ocean depths. Ocean temperature varies with depth, but at between 30 and 100 meters there is often a marked change, called the thermocline, dividing the warmer surface water from the cold, still waters that make up the rest of the ocean. This can frustrate sonar, because a sound originating on one side of the thermocline tends to be bent, or refracted, through the thermocline. The thermocline may be present in shallower coastal waters. However, wave action will often mix the water column and eliminate the thermocline. Water pressure also affects sound propagation: higher pressure increases the sound speed, which causes the sound waves to refract away from the area of higher sound speed. The mathematical model of refraction is called Snell's law.
If the sound source is deep and the conditions are right, propagation may occur in the 'deep sound channel'. This provides extremely low propagation loss to a receiver in the channel. This is because of sound trapping in the channel with no losses at the boundaries.
http://www.e-reading.org.ua/bookreader.php/79701/Clancy_-_Red_Storm_Rising.txt
Clancy Tom, Red Storm Rising [ RACKETEER INFLUENCED AND CORRUPT ORGANIZATIONS US Title 18 ]
Tom Clancy
Red Storm Rising
USS PHARRIS
"I think this one's for-real, Captain," the ASW officer said quietly. The line of colored blocks on the passive sonar display had lasted for seven minutes. Bearing was changing slowly aft, as though the contact were heading for the convoy, but not Pharris.
The frigate was steaming at twelve knots, and her Prairie/Masker systems were operating. Sonar conditions were better today. A hard thermocline layer at two hundred feet severely impeded the utility of a surface sonar. Pharris was able to deploy her towed-array sonar below it, however, and the lower water temperature there made for an excellent sound channel. Better still, the layer worked in both directions. A submarine's sonar had as much trouble penetrating the thermocline as a surface sonar. Pharris would be virtually undetectable to a submarine below the layer.
"Red Storm Rising"
"How's the plot look?" the tactical action officer asked.
"Firming up," ASW answered. "Still the distance question. Given the water conditions and our known sonar performance, our sonar figure of merit gives us a contact distance of anything from five to fourteen miles on direct path, or into the first convergence zone. That predicts out from nineteen to twenty-three miles . . ." A convergence zone is a trick of physics. Sound traveling in water radiates in all directions. Noise that traveled down was gradually turned by water temperature and pressure into a series of curves, rising to the surface, then bending again downward. While the frigate could hear noise out from herself for a distance of about fourteen nautical miles, the convergence zone was in the shape of an annulus-the area between two concentric circles-a donut-shaped piece of water that began nineteen miles and ended twenty-three miles away. The distance to the submarine was unknown, but was probably less than twenty-three miles. That was already too close. The submarine could attack them or the convoy they guarded with torpedoes, or with surface-to-surface missiles, a technology pioneered by the Soviets.
"Recommendation, gentlemen?" Morris asked. The TAO spoke first.
"Let's put the helicopter up for the near solution, and get an Orion working the far one."
"Sounds good," ASW agreed.
Within five minutes, the frigate's helo was five miles out, dropping Lofar-type sonobuoys. On striking the water, these miniature passive sonar sets deployed a non-directional sonar transducer at a preselected depth. In this case all dipped above the thermocline layer to determine if the target was close. The data was relayed back to Pharris's combat information center: nothing. The passive sonar track, however, still showed a submarine or something that sounded like a submarine. The helo began moving outward, dropping sonobuoys as it went.
Then the Orion arrived. The four-engine aircraft swooped low along the frigate's reported bearing-to-target. The Orion carried over fifty sonobuoys, and was soon dropping them in sets both above and below the layer.
"I got a weak signal on number six and a medium on number five," a sonar operator reported. Excitement crept into his voice.
"Roger, confirm that," the tactical coordinator on Bluebird-Three agreed. He'd been in the ASW game for six years, but he was getting excited, too. "We're going to start making MAD runs."
"You want our helo to back you up?"
"Roger that, yes, but tell him to keep low."
Seconds later the frigate's SH-2F Sea Sprite helicopter sped off north, her magnetic anomaly detector trailing out by cable from a shroud on the right side of the aircraft. Essentially a highly sensitive magnetometer, it could detect the disturbance in the earth's magnetic field made by a large chunk of ferrous metal-like the steel hull of a submarine.
"Signal on number six is now medium-strength. Signal on seven remains medium." The plotting team took this to mean that the submarine was heading south.
"I can give you a working range figure," ASW said to the TAO. Forty-two to forty-five thousand yards, bearing three-four-zero to three-three-six." The frigate relayed this at once to the Orion.
As they watched on radar, the P-3C quartered the area, flying very precise tracks across the box of ocean defined by Pharris's sonar data as the probable location of the submarine. A computer system plotted the lines as they extended to the south.
"Pharris, this is Bluebird. Our data indicates no friendly subs in the area. Please confirm, over."
"Roger that, Bluebird. We confirm no reports of friendlies in the area." Morris had checked that himself half an hour before.
