Titanic sub: The sonobuoys being used to scour the Atlantic

A desperate search is underway for the missing Titanic submarine containing five people who are rapidly running out of oxygen.

Every 30 minutes, popping noises have been detected by a Canadian aircraft that used underwater sonar equipment called ‘sonobuoys’ to search.

This has led to new hopes that the crew of the ‘Titan’ ship is still alive and is deliberately making noises against the hull.

But what is sonar, how does it work and what does it mean for the rescue mission? MailOnline takes a closer look at the technology.

What are sonobuoys?

As the Canadian plane flew over the North Atlantic search area, it dropped devices called sonobuoys — an important tool for underwater surveys.

When dropped into the water from the aircraft, they descend to the surface of the water, carried by parachutes.

Only when they reach the water can they properly deploy to the required depth and communicate with the aircraft above.

Once in the water, the sonobuoy separates into two, with a radio frequency transmitter at one end that floats back to the surface.

On the other side, an array of underwater microphones called “hydrophones” is deployed, pointing to the depths of the ocean.

Both ends are connected by a cable and any sound picked up by the hydrophones is sent down the cable to the radio frequency transmitter floating on the surface.

The transmitter then relays the signals to the plane that dropped it, potentially giving rescue teams a crucial indication of sounds happening thousands of feet below.

How does sonar work?

The way sonobuoys work is based on the principle of sonar, short for Sound Navigation and Ranging, which is the use of sound waves to detect underwater objects.

Sonar works in two different ways: active and passive detection.

A Canadian plane (file photo) heard ‘thumping’ at 30-minute intervals in the area where the submarine disappeared, a leaked memo suggests

A glimmer of hope lit up the bleak search yesterday when the Coast Guard announced that “thumping” noises had been detected underwater. It remains unclear whether the thump came from the submarine, but it has now become the ‘focus’ of the mission. Pictured is the Deep Energy rescue vessel – the latest hope in the ongoing hunt for the ship – in the middle of the Atlantic Ocean

Active detection involves bouncing a ‘ping’ into the environment and listening for the return echo.

READ MORE: How CAN the Titan submarine be rescued at 12,500 feet?

When rescue crews manage to locate the missing submarine (file photo of the Titan) thousands of feet below sea level, they face a series of barriers and an incredibly complex mission to retrieve the ship unlike any they’ve ever seen before.

Passive sensing involves listening for sounds produced by propellers and machinery.

Which one is used here?

Search teams will usually use passive sensing – picking up sounds that the crew is hoping for banging against the submarine’s hull.

Active detection is much more difficult around the Titanic wreckage, as it would be incredibly difficult to distinguish between the submarine and the surrounding debris.

Another third category of sonobuoys is also called ‘special purpose sonobuoys’ because they provide additional information about the environment, such as water temperature or wave height.

When were sonobuoys developed?

Sonobuoys were initially developed to detect German U-boats during World War II.

Any underwater acoustic signals detected by the hydrophone, for example caused by a nearby U-boat, are then transmitted to the aircraft via the radio transmitter.

But sonobuoys are now used for a variety of purposes, including search and rescue.

They can map the location of a plane crash, a sunken ship, or survivors at sea.

The 21ft submarine (pictured) has an oxygen supply of up to 96 hours, but the crew of five is thought to have less than 24 hours of breathing air left, as of Wednesday

The search site is located about 900 miles off the coast of Cape Cod, 400 miles southeast of Newfoundland, Canada. It’s hard enough to get there without finding the missing submarine below the ocean’s surface

Can the submarine be located now?

Unfortunately, it would take three buoys to pick up the sound so experts could “triangulate” the submarine’s position.

Triangulation – often used by geologists to find the locations of earthquakes – makes it possible to pinpoint a more precise location by using angles.

How hopeful can we be about these sounds?

Dr. Jamie Pringle, Lecturer in Forensic Geosciences at Keele University, said: ‘The fact that they are 30 minutes apart is a good sign.

‘It is unlikely to have come from another submarine, which typically only goes down to 900 metres, or from a surface ship’s propeller which would make a continuous noise – so this noise is probably man-made.

‘Acoustic sound travels far into the water – whale sounds for example – so that’s both good and bad news.

Timeline (British Summer Time) of the search for the Ocean Gate submarine, if Titan has lost power the crew will be in complete darkness with temperatures of 3C

The shipwreck lies 12,500 feet underwater. It is feared that the Titan is now trapped inside

“You would still need three static buoys to triangulate the noise to get a position fix.”

‘I should also add a caveat that the sound could of course be coming from something other than the sub, let’s not give people false hope here.

“The lack of oxygen is key now — even if they find it, they still have to surface and unlock it.”