What is the link between the wreck of the Titanic and the solving of missing persons cold cases in the USA?
Well, both were found with sonar, a technology that uses sound to locate objects and measure distances in water.
We currently know more about the surface of Mars and the moon than we do about the seabed here on Earth. If flight MH370 were lost on Mars it would be easier to find than just 4km beneath the Indian Ocean somewhere off the western coast of Australia.
We have mapped all the land from space and have visited or used almost all of it in some way or another but as far as the sea is concerned only around 23% has been mapped and there are many mysteries down there that we would really like the solve like what happened to flight MH370.
70% of the earth is covered by water with the best visibility being about 80m in the Eastern Weddell Sea, Antarctica. But in most of the seas and lakes around the world, visibility is much less than that, if that were in the air it would be considered like trying to see through thick fog.
Diving down in the deep sea with subs or specialist ROVs is also fraught with problems, huge costs, and a general lack of interest because of the difficulties that make working in space seem easy by comparison.
So how can we see what is literally less than a 13-minute drive in a car distance away even in the deepest part of the ocean without being crushed like a coke can under a car wheel?
To “see” in deep water with high enough resolution to pick out objects we need sonar which is short for Sound Navigation and Ranging which is similar to the technique used by bats, namely a short burst of sound to be sent out and the reflections or echo’s are picked up.
The further an object is away the longer it will take for the reflected sound to return and as such you can accurately measure the distance to an object like a moth if you’re a bat, or from the surface of the sea to the sea bed if you are a ship.
This is made easier in the sea because water is 830 times denser than air and because of this water is effectively noncompressible and the sound waves don’t lose energy like they would in air and can travel much further.
Whales can communicate over thousands of kilometres in the right conditions because of this, if they were to do the same in air it would measured in just kilometres.
The first use of echo-location underwater was thought to have been prompted by the Titanic disaster of 1912, with the first patent for an underwater echo-ranging device just one month after the sinking of Titanic.
These systems were very basic but in WW1 to counter the threat of submarines, The British Navy developed hydrophones, basically underwater microphones to listen for the sounds made by the German U-boats and by 1915 active sound devices had been developed but the technology of the transducers for making the sound and picking it up was still in its infancy and nothing much changed with the basic technology from 1915 to 1940.
To keep the work secret the British admiralty made up a name “Allied Submarine Detection Investigation Committee” or ASDIC which the underwater active sound detection system became known as to throw people of the fact that they were using quartz piezoelectric crystals using Rochelle salt and Magneto restrictive transducers.
In September 1940 as part of the Tizard mission, which I have spoken about before, the British ASDIC technology was given to the US. Because of the large losses of US merchant ships to the German U-boats in the North Atlantic, submarine detection was given a top priority and a new development of Ammonium dihydrogen phosphate (ADP) which was superior to Rochelle salt the transducers were now much more sensitive.
To this end, there are two types of sonar, Active which sends out a signal and listens for the return, and Passive which just listens.
For the navy and in warfare situations, active is used mainly by surface ships to try and find submarines. Submarines also use it but because it acts as a massive beacon that can be heard for hundreds or thousands of kilometers, its not the sort of thing you want when your trying to be as undetectable as possible.
So passive is used by submarines most of the time to listen for ships, other submarines and things like torpedoes heading their way. They do this by having arrays of hydrophones around the hull of the ship. One example is the Sonar 2076 which is a submarine sonar detection system designed by Thales for the Royal Navy for three Trafalgar Class and three Astute Class submarines .
Although the details of the system are secret, its thought that it uses over 13,000 hydrophones made up of bow, flank and towed array sensors which as of 2010 was more than any other submarine in the world or attack ship.
But is not just the huge number of hydrophones that matter, having all this data needs to have a huge amount of processing power to make sense of it. When it was announced in 2010 it was said to have the equivalent computing power to that of 60,000 home PC’s.
With the latest stage 5, it uses an open architecture system utilising the current state-of-the-art commercial off-the-shelf or COTS processors. This allows the hardware of the hydrophone arrays to stay but the processing and software can all be easily upgraded as time goes by. To give an idea of how sensitive this system is, if it were on land it could track a double decker bus going round Trafalgar Square from a distance of 100km or 60 miles away.
In the 13 years since there have been great advances in digital processing and now neural network engines that you can find in graphics cards and smartphones which allow AI to be used in ways that wasn’t even thought of back in 2010 which will make data processing even more important.
Passive systems don’t have to be fitted to sea-going vessels, they can be in the form of sonobuoys, aircraft-dropped disposable hydrophones that once they enter the water split into two parts. One is the hydrophone which is below the surface and a radio transmitter which is above and sends the signals picked up to aircraft or surface ships to go after any submarines it detects.
As every vessel that travels in the sea makes some form of noise once their unique signature sound is known they can be recognised anywhere in the world they go.
But detecting other submarines with an active system that could be up to 100km away requires a lot sound energy. This is normally done by surface ships because they can’t hide and knowing where they are is easy to find out from the air.
This sound energy is measured in decibels a bit like for sound in air but here is difference between underwater db and db in air.
An Underwater db when measured in water is 1 μPascal @ 1 meter, whereas in air is 20 μPascal @ 1 meter.
To convert from water to air just subtract 62 db from the water figure. Or if it’s the other way around add 62db to go from air to water.
So when people say that the sound of a jet taking off is 140db in air, if that were in water it would be 202db or when an active sonar signal is 240db @ 1 meter, in air that would be 178db @ 1 meter. So although sonar systems are incredibly loud in water and would do serious damage to anyone close enough, that doesn’t translate into the same figures that we would hear in air.
However, mapping the ocean floor from the surface requires a lot less acoustic energy. That’s because a signal would need to travel 11km in the deepest part of the ocean, at the Challenger deep, and at the Titanic wreck site it would be only 3.8km depth.
To map the ocean bed in high resolution, multi-band sonar is used, this sends out multiple beams of sound at different frequencies from the bottom of a ship in the shape of a fan usually up to 8km wide.
The time taken for the sounds to return gives the distance to the seabed and can show the shape of any geological formations but the strength of the beam, known as backscatter can also tell the operators what type of material it is made of. A strong signal would indicate rock, whereas a weak signal would be soft mud. It can also show features in the water column like bubbles from undersea thermal vents or leaking pipelines.
However, at great depths, scanning from the surface will reduce the resolution so they are often towed on remotely operated vehicles (ROVs) or autonomous underwater vehicles (AUVs) close to the seabed.
But the get the best images of smaller objects like shipwrecks or planes, side scan active sonar is used. This uses side-mounted transducer arrays that scan at frequencies from 50khz to 1Mhz depending on the size of the scan and the resolution required.
The images that are made from the sonar data show as dark and light areas. Hard objects send a strong echo and create a dark image. Shadows and soft areas, send weaker echoes and create light areas. These dark and light images can look like photographs but they are only visual representations of the acoustic data.
These are the type of systems that have been used to try and find MH370 but they can only scan a small area in comparison to the size of the ocean and the price of doing that often becomes too much for governments and the search stalls.
Side scan sonar doesn’t measure depth so when surveying it is often used with multiband sonar but this very high-resolution side scan sonar is very expensive but there are now low-cost versions and sonars made for finding fish which can be well under $20,000.
Some of these have been used by enthusiasts to look for interesting objects in lakes and rivers and it was only a matter of time before they were finding cars that had either been dumped or had crashed some times a decade or more ago with unlucky occupants still in them that had been unable to escape.
These had become missing person cold cases that the police couldn’t solve and yet sometimes they might have been in a river or lake next to the road they left but the dark muddy waters obscured them for often years until someone with a sonar rediscovered them.
High-resolution sonar will continue to become better especially when coupled with neural networks and machine learning to process the data and it will become just a matter of time and money before many of the secrets now hidden are revealed.
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