The sound of water
Does the climate crisis have a soundtrack? It does - when you listen underwater with the right tools. At the University of Bath, scientists led by physicist Professor Philippe Blondel are pioneering research into acoustics (sound waves) to listen to the ocean’s melodies.
With their game-changing technology, they can decode the meaning of hidden underwater sounds, and tell us how rising temperatures and human activities are impacting the world beneath the waves.
Philippe Blondel
Philippe Blondel
So what exactly are scientists listening to?
Melting icebergs, for starters. When an ice cube melts in a glass, it does so quietly, mostly unnoticed, but when an iceberg thaws, a symphony of sound can be heard for several kilometres.
By deciphering this concert – the acoustic signature of each change in the iceberg as it melts – we can monitor the impact of climate change on our warming seas.
Much of the technology that allows us to make sense of underwater sounds was developed in France over a century ago, but in recent years it has been radically advanced, thanks largely to research carried out at the University of Bath.
The techniques developed at Bath are now widely used by international teams to capture and interpret every conceivable sound made by Arctic ice, from the loud boom of icebergs capsizing and giant ice blocks falling into the water before bobbing to the surface, to the susurration of drifting islands of ice, and the delicate sizzle, pop and creak of tiny air bubbles being released from the ice as it steadily melts beneath the surface of the sea.
This mysterious maritime soundscape is picked up using hydrophones – waterproof microphones that can operate even kilometres beneath the sea surface. Underwater probes register the sound, and researchers decipher it to get an accurate reading of how warming waters and increasing human activities are impacting the fragile polar ecosystem – and by extension, the entire planet.
"What the data shows clearly is that with climate change, the underwater noise from melting icebergs is intensifying, and human-made sounds can be heard over large regions of the Arctic."
Listening to the deep blue
Sonar was first used to record underwater sounds in 1917, and the science was refined into a practical tool during World War One. Spurred in part by the sinking of the Titanic, scientists learned to detect objects in the ocean – including icebergs and submarines – by analysing the echoes of sound waves.
Modern sonar techniques, however, do far more than simply help researchers find objects. Sounds are converted by hydrophones to electrical energy. These signals are then processed by a computer and displayed on a screen, where they are studied and interpreted by a skilled operator.
By deploying more than one hydrophone, the operator can hear how noises are located in space relative to one other.
“What’s important is to know where to listen and what to listen for,” said Professor Blondel. “This is where Bath’s research has provided new approaches, which have been validated in environments around the world.
“For instance, we have helped teams uncouple ambient noise from sound coming from icebergs by determining how close to the source they should record – from one metre or five kilometres from the source? – and whether or not they should use one hydrophone or multiple.
“But it’s not just a matter of listening and recording – we need to divide the noise into frequency bands, which is like using a mixing table on a symphonic recording, where the bass has to be isolated from the violins, drums and brass instruments.”
"When we listen to a glacier, we’re looking for signs that the environment is changing. And if the sounds are increasing or becoming more frequent, we can determine if it’s a sign of trouble to come or just the normal cracking you’d expect in the summer months.”
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Diving deeper with artificial intelligence
In a project involving University of Bath mathematician Professor Chris Budd OBE, Professor Blondel and his team have set out to improve the accuracy of their data by identifying the best ways to position underwater sonar equipment and the best times of day and year to take measurements.
Professor Budd said: “The ocean is a noisy place, with sounds coming from ships, weather, icebergs and animals such as whales.
“By listening carefully to the ocean and using advanced mathematical and AI tools, we can take this jumble of sound and turn it into useful data. This allows us to do several things – for instance, to detect where whales are and how their behaviour is changing, and also to look for signs of climate change.”
Professor Budd describes his work with the Bath sonar team as “a wonderful, and joyful, collaboration between physics, mathematics and statistics, with us all bringing different strengths to the team.”
He said: “The potential impact, as a way of understanding what is going on in the ocean and its wildlife as the climate changes, is huge.”
What’s driving the rapid melting of our glaciers?
The world’s seas absorb over 90% of the heat from the greenhouse gases that pour into the atmosphere through human activity.
As a result of rising emissions, ocean surface temperatures are at a record high, and icebergs and sheets throughout the Arctic are melting.
"This trend is captured by the hydrophones we deploy from ships or located on underwater monitoring buoys."
Philippe Blondel checks signals on an oscilloscope.
Philippe Blondel checks signals on an oscilloscope.
Why does the rapid Arctic melting matter?
- Trapped atmospheric heat - Arctic ice helps regulate global temperatures by reflecting the sun’s heat back into space.
- Hotter seas - Warmer seas fuel extreme weather, with heat accumulating faster and reaching deeper – measurements show warming down to 700 metres below the sea surface.
- Locked carbon, now released - As the climate warms, melting permafrost releases greenhouse gases that would otherwise remain trapped on the Arctic seabed.
- Expanding saltwater - As seawater expands when heated, rising ocean temperatures are pushing up sea levels across the globe. This threatens coastal communities, with serious flooding already forcing some to migrate to higher ground.
- Habitat loss - Warming seas are pushing marine species northwards in search of cooler habitats. But what happens when no water is cool enough?
- Mini-tsunamis - Mini-tsunamis, triggered by collapsing glaciers, can damage infrastructure in regions such as Alaska and Greenland.
- Freshwater spillage - Freshwater from melting glaciers disrupts fragile ecosystems, killing plankton and impacting the wider food chain in an ecosystem.
"We monitor the melting both from icebergs and glaciers collapsing above the water and from glaciers collapsing beneath water. Underwater sound warns us about these collapses before they happen. They can also tell us how melting ice is affecting marine animals."
Migrating sea life
Captivating as it may be to eavesdrop on glaciers, Professor Blondel’s research interests extend beyond melting icecaps: his mission is to use physics and acoustics in as many ways as possible to help us better understand our oceans – and ultimately, to create positive change in our interactions with the natural world.
Marine animals are actively monitored through the sound signatures they create. Using techniques developed at Bath involving deep learning, researchers keep tabs on the adaptations sea creatures are being forced to make as their habitats warm up.
Professor Blondel said: “Suddenly you have salmon living in areas that were previously home to cod, and you pick up the signatures of whale species that a decade ago would have been found much further south.
“And it’s not just the animals that are affected – we have also looked at marine vegetation. Kelp protects the shore from waves, and without it you get more shore erosion. It forms underwater forests that are highly sensitive to changes in ocean conditions. Everything is linked.”
Britain’s warming waters
Like other coastal nations, Britain is experiencing warming seas. Today, the UK is home to the pistol shrimp. Until a couple of decades ago, this warm water-loving species was unknown in this part of the world.
“These creatures once lived exclusively in the tropics but now we can hear the pistol-like snap of their claws through hydrophones right here in our waters.”
Pistol shrimp have made themselves at home in Britain's warmer seas
Pistol shrimp have made themselves at home in Britain's warmer seas
Disrupted life on the coast
Researchers also use Professor Blondel’s tech to monitor how human activity at sea is being impacted by warming waters. As polar ice thins and disappears, it’s possible to navigate new parts of the ocean. More ships mean more underwater noise.
The thump of propellers, sonar from vessels and construction noise as new infrastructure is built close to the shore all emit sounds that penetrate deep into the ocean, increasing underwater noise pollution.
Many water-dwelling creatures use natural sounds in their essential activities, producing clicks, whistles and songs to communicate with their young, search for food, find mates, navigate and avoid dangers. The boom of an airgun can have a devastating effect on their hearing and on their ability to stay alive and healthy.
"If ice around a bay is melting and there are risks to a community, people need to know about it. And if ships are so noisy, fish are being frightened away and nothing is being caught for food, communities need to know this too."
Professor Blondel and his colleagues gather data on these changes – often invisible to observers on land – and feed this back to coastal communities that are left struggling after their once stable lives have been turned on their heads by climate change.
All these underwater sounds are linked and it’s important for us to understand where the effects are and what are the causes. This is what we have set out to do, for example, in a big European project led by Norway, which started in 2023.”
Coastal communities have had their once-stable lives upended by climate change.
Coastal communities have had their once-stable lives upended by climate change.
Philippe Blondel holds his favourite transducer, key to the development of multistatic sonars
Philippe Blondel holds his favourite transducer, key to the development of multistatic sonars
Mapping the ocean floor
The human desire to explore and discover can seem insatiable, yet at the bottom of the ocean – which covers more than three-quarters of the Earth, often to a depth of 6 km – the world remains largely uncharted. Professor Blondel says filling in the seabed gaps with data and evidence, “so we can marvel at the diversity on Earth and so others can take action” is a major preoccupation for the Bath team.
“We know more about the surface of Venus than the surface of the seabed,” he said. “If you were to take the distance on the ocean floor between Bristol and London, for example, you’d find just one data point linking the two. Bath, Reading and just about everywhere else between the two cities would be missing.”
Underwater maps are created using an active sonar system, where instruments known as transducers emits an acoustic signal into the water – the famous ‘ping’ of sonar – and this bounces off the target object and returns to the transducer as an ‘echo’. The echo reveals the object’s distance from the transducer. (By contrast passive sonar, traditionally used by submarines that want to detect objects but not be detectable themselves, does not emit its own signal.)
Some sound waves become scattered as they bounce between the transducer and target, and to date, much of Professor Blondel’s research has involved finding ways to harness this diffused energy to extract new information about the seabed or objects in the ocean.
Philippe Blondel analyses signals from the ocean with an oscilloscope
Philippe Blondel analyses signals from the ocean with an oscilloscope
“We’ve basically designed new sonar as well as new ways to extract information from the sonars we already have,” he said.
Professor Blondel’s 2009 book Handbook of Sidescan Sonar, which describes these new techniques, remains the undisputed textbook across the world on sidescan sonar and the interpretation of sonar images. It is used both by universities to teach marine geologists and surveyors, and by industry players working in this field.
In addition to sidescan, the Bath Physics team has developed a surveying technique they have named multi-aspect sonar. “Adopting this, we can use sound waves in different ways to give us ‘surround sound’. It works by separating the device that makes the ‘ping’ from the hydrophones that pick up the sound. This gives us incredibly detailed information about targets on the sea-floor targets, such as toxic waste,” said Professor Blondel.
Operating this underwater imaging and mapping technology from ships and underwater vehicles, Professor Blondel and his global collaborators are creating maps detailing slopes, trenches, ridges, cliffs, seamounts, coral reefs, plains, submerged volcanoes and oceanic plates. These maps are important for laying underwater cables and pipelines, and for installing wind farms. They are also vital for identifying biodiversity hotspots, such as seamounts, where wildlife tends to congregate.
So far, around 20% of the ocean floor has been mapped. The aim is for all sections to be pieced together within the next few years.
Flourishing partnerships
In their quest to improve humankind's ability to study the oceans, Professor Blondel and his colleagues collaborate closely with universities and other institutions across Europe and North America.
Most recently, they joined a five-year European project looking at acoustics in the high Arctic. Partners in this project are all members of the High Arctic Ocean Observation System, and their shared goal is to design new technologies and approaches to monitor changes to Arctic environments.
The Bath sonar team also assists with the development of commercial products, working closely with industry through Innovate UK’s ‘Knowledge Transfer Partnerships’ – collaborations that smooth the way for academic expertise and innovation to be transferred from campus labs into the world beyond.
“Our work is about finding better ways to monitor sound to protect marine life, but the main idea is for our technology to become part of companies’ everyday practice."
Through their collaborative work with Seiche Ltd and other partners, the team has developed systems to protect marine animals from underwater industrial noise.
Devon-based Seiche supplies sound monitoring systems to offshore oil, gas and renewable energy companies. These firms face growing pressure to monitor the marine environment around their vessels and halt operations when needed – for example, if whales, dolphins, or endangered animals are detected nearby.
Seismic exploration ships often use airguns to survey the seabed. Blasts of compressed air travel through the water and into the seabed, sometimes for kilometres, before reflecting data on buried mineral deposits. These pulses are as loud as a blue whale’s call – the loudest natural sound in the ocean – and can be fatal or cause lasting harm, including deafness.
Underwater sound technology aims to reduce this risk.
Philippe Blondel and his team are uncovering secrets from the oceans so we can marvel at the diversity of life on Earth – and so others can take informed action to protect it.
Philippe Blondel and his team are uncovering secrets from the oceans so we can marvel at the diversity of life on Earth – and so others can take informed action to protect it.
As Professor Blondel explained: “Once you know how much power is coming from an airgun, you know how wide an area you need to monitor for endangered marine life. You can have people looking for animals over this distance with their eyes, which is challenging, especially when there’s a rough sea, or you can use sonar to detect life far more accurately.
“You can think of acoustic tomography as being like a surgeon searching the whole body for a possible tumour and not only finding the tumour but also producing a 3D image of it. Using sonar, we’re mapping the environment looking for relatively small objects – like a 30-metre whale – roaming freely in a huge body of water.
“Once we detect the whale, we can tell how far away it is, what direction it’s moving in and how fast. Hydrophones pick up this information and radio it through to a satellite which then sends it to the company’s head office. The company then radios back to the ship and says, ‘stop everything now, there’s a whale heading your way’. It all happens very fast, 24/7.”
The tomography system that makes it possible for commercial operators to respond to an ever-shifting marine environment in real-time was developed in partnership with the Department of Electronic & Electrical Engineering at Bath.
Together, Bath and Seiche have created the Sound Source Verification’ (SSV) – a method that maps underwater sound fields using computer modelling and field measurements. It is used globally by offshore industries and commercial shipping to reduce their impact on marine life and meet regulatory requirements.
Mind the turbine
When, in 2012, Professor Blondel designed the FLOWBEC project – along with partners that included the University of Aberdeen, and with leadership from the National Oceanography Centre (NOC, Liverpool) – he knew the work would be challenging.
The project set out to explore the feasibility of using underwater turbines and wave energy converters off the UK shoreline to convert marine energy into usable electricity. Tidal energy is produced by the movement of ocean waters as tides rise and fall. Would sea animals, riding the tides and currents, get sucked into the turbines, which can be metres wide, along with the surging water?
“There was real fear of harmful collisions between marine animals and turbines,” he said. “Environmental groups were understandably concerned by their installation.”
His team combined two state-of-the-art sonar systems on a seabed frame placed within 25 metres of test sites operated by the European Marine Energy Centre, off Orkney, north Scotland.
Sonar located below the water monitored fish and diving seabirds that passed through or fed within the location, to see how they interacted with the devices. Marine radar (operated by NOC-Liverpool) also tracked the behaviour of birds and marine mammals in the immediate area, with the instruments pinging away and collecting data seven times per second during multiple two-week study periods.
“To our relief, the data shows that fish, marine mammals and birds move freely around the devices, not straight into them: there were actually more fish around than elsewhere because people can’t fish in the area,” said Professor Blondel. “So we can say with increasing confidence that marine renewable energy is both sustainable and great for biodiversity.”
From planets to plate tectonics
In the course of his career, Professor Blondel has collaborated with chemists, biologists, oceanographers, engineers, sailors and naval personnel – all people in need of help from physics to solve complex problems involving the sea. His projects have seen him apply his expertise to waters from 50 centimetres to 4,000 metres deep.
He is the product of the French education system, having completed his undergraduate degree in Physics at the University of Rouen and then a PhD in Physics with Remote Sensing at the University of Paris VII. For his PhD studies, he used radar (which employs electromagnetic waves – radio- or micro-waves rather than sound to detect objects) to create high-resolution maps of the planet Venus.
“Working with NASA, we wanted to see if Venus, like Earth, had plate tectonics – spoiler: it doesn’t.”
After a stint at a lab in Seattle, USA – where he continued his working with sonar on plate tectonics – and another at the Institute of Oceanographic Sciences in Surrey, which later became part of the National Oceanographic Centre in Southampton, he joined the University of Bath in 1999 to work more closely with sonar technology.
He has not, however, closed the portal entirely on space: for the past 10 years, he has continued to teach planetary Physics and to research space debris, as well as editing the textbook Sonar System Updates.
What motivates him in life, beyond his natural curiosity about our Earth and other planets, is a desire to understand the link between humans and their environments, present and future.
“Scientists can’t work in a vacuum or an ivory tower,” he said. “We need to understand what our research means and how to link it to the world around us with all its challenges and all its magic.
"As the French philosopher and moralist François Rabelais (born in the 15th Century) once said: ‘Science without conscience is but ruin of the soul.’ ”
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