Sounds of the planet

Acoustic remote sensing and its uses in underwater environments

Tagged: acoustics

Acoustics - From the deep sea to outer space

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📥  From the lab

Asteroids near Earth

"In space, no one can hear you scream" ,,, This was the motto from the first Alien movie and it has been used and abused in the decades since. I lost track of the number of times I have worked on ships and seen T-shirts or posters with "In the deep sea, no one can hear you scream ..." or similar. Although sound travels very well in water (and better than in air) ...

So how would it work in space? Sound does not travel in vacuum, despite what countless movies seem to imply (and yes, I love the sound effects of Star Wars or Farscape, even if I need to leave my scientific mind aside for a time). But space is made of much more than vacuum.

There are planets, first, and some of them are getting increasingly closer as plans for their exploration are firming up. Elon Musk gave a much talked about presentation at the International Astronautical Congress in Mexico last week. I was interviewed on Al Jazeera to talk about the actual feasibility of the scheme, and this still seems rather far off. The exploration of Mars, which needs to take place before any attempt at colonisation, will need to include assessing potential resources in the ground. We know there is water below the surface, from remote sensing by satellites in orbit around Mars. And there are very strong indications that some of this water is still freely flowing at the surface in specific areas. But what about other deposits, for example oxygen? How easy would it be to drill and get it? How stable would exploration platforms, or habitats, be as resources get extracted from below them? Like on Earth, these questions can be answered directly with acoustics. Sending sound waves of different frequencies through the surface, we can listen to their returns and find out how deep they are, and what they are made of. Just like seismics or non-destructive imaging (think about ultrasound scans of human bodies ...)

Planets are great (I love planets) but they are far away, expensive to get to, and with the current state of space exploration programmes, it will be a while before we can get more than passing glances at their marvels and really explore them. Closer to home, though, we have asteroids and comets. They are big pieces of rock or gravel (comets are often compared to "dirty snowballs"). They contain all sorts of minerals and oxygen or frozen water. And they often pass in the neighbourhood. So what about exploring asteroids?

Asteroids near Earth

Asteroids often come to the neighbourhood of Earth. Artist's view from the European Space Agency (Copyright ESA - P.Carril).

This seems much more feasible, and there are actually several companies aiming to mine asteroids for their riches. These companies have been in existence for several years, so they are no "seven-day wonders" but real companies, with serious investors. The government of Luxembourg recently announced it had its own plans to support space exploration and space utilisation. And they organised a great workshop two weeks ago, inviting space industries to meet space scientists. It was a packed programme over two days, with world-famous scientists describing the main discoveries of recent missions to asteroids and comets, and their plans to learn more over the coming years. The ASIME-2016 workshop is now leading to a White Paper, which will lead to recommendations on space mining, the science behind it but also the implications for protection of any possible astro-biology (none found yet, but one can hope ...). I was there to present seismo-acoustics to a different audience, and it was very encouraging to see possible avenues for acoustic exploration of still mostly unknown planetary bodies ...

Once we get to them ... (but at least we have the tools ready ...)

 

 

Finding Nessie … or close enough …

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📥  From the field

The Loch Ness monster is one of the most famous mythical beasts, supposedly lurking below the cold waters of Loch Ness in Scotland. But there has been no conclusive proof of its existence yet, despite many people searching for it over the years … Until …

Looking for animals, even very large, in a long and deep fjord like Loch Ness is like looking for the proverbial needle in a haystack. During a visit to the Loch Ness Monster Museum, long ago, I had seen old sonar records purportedly from “something” lurking in the deep, although my sceptical eye interpreted this as long-range noise in the records … Later, more accurate sonar maps of the bottom of the Loch Ness had revealed structures, more or less circular, build of rocks and rubble and placed at regular intervals. Some journalists of course used the occasion to talk of “monster nests”, but the truth was much more prosaic, if as interesting. As I wrote in my 2009 Handbook of Sidescan Sonar, these structures were associated with the building of the road along the shore. Debris from the construction were loaded onto barges, which dumped everything unceremoniously in the deeper parts of the loch. And, like any collection of objects falling in deep water, these rocks arranged themselves in rough circles.

But this time, even the BBC reported it. So it looked much more serious, and I started reading … This Nessie was a lost prop from a movie several decades ago, not a real, live animal. It was found by my colleagues at Kongsberg Maritime, using a combination of the latest technologies now available: high-resolution sonars, capable of mapping both the topography of the loch’s bottom and its acoustic reflectivity, and the Autonomous Underwater Vehicle MUNIN, their latest model. Their website has a very nice (and short) movie of how they found it. It really is a needle in a haystack!

 

28 July 2014 - The bright side of life

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📥  From the field

Polish TV recently showed the farewell concert of Monty Python and "The bright side of life" (my Polish is still extremely limited, but I could understand that much ...) (especially the English part ...) For the last 24 hours, the sun has been shining and I can see its bright rays illuminating the glaciers and moraines on the other side of the bay of Isbjornhamna. Today, we are going to do experiments in the deepest parts of the fjord, and sail there for a large part of the day. We launch our boat shortly after another party of scientists has left for the fjord of  Hans Glacier.  Amongst them is a friend and former Bath postdoc, Aleksandra Kruss. A multibeam sonar expert with an international reputation, she is out there to test the latest generation of high-resolution sonar in this challenging environment. She also kindly agreed to take measurements of the front of the glacier, to help us in our interpretations. She has extensive Arctic field experience so we know she won't get too close to the glacier for safety, but the sonar should do an excellent job at mapping what it looks like underwater.

The bright side of life: this side of Isbjornhamna is graced with a few rays of sunshine ...

The bright side of life: this side of Isbjornhamna is graced with a few rays of sunshine ...

In the meantime, we move to a slightly different type of landscape: very eroded mountains, and much less green. No moss, no lichen visible from the boat: no animals either. The Polar Station has disappeared at the horizon: even finding the peak below which it stands, we cannot see it. The ride to the other side of the bay took us 40 minutes, bumping into the waves on our small Bombard C5. After 20 minutes of measurements, we realise the site is not suitable, and move to a different location, on the original side of the bay (another 40 minutes, bumping into the waves, making sure we fall back into the boat and clinging tightly to the ropes ...). There, we can make the right kind of measurements. We also understand why no one had ever made this before: it's hard, it's cold, and we have to do several trials and use our field knowledge of underwater acoustics before getting it right ...   After many hours drifting in the cold wind and short waves, we have all the results we want and head home. Cold to the bones (it takes me until this evening to warm up, despite the many layers of clothes), but delighted with the results. Which we start analysing immediately after cleaning the equipment, untangling several hundred metres of cables and of course manhandling the boats back on shore (Aleksandra and her colleague are back too: but their boat is fortunately lighter, or maybe it's the fact we are now 5 people hauling it ...)

 

27 July 2014 - Ships passing in the night ...

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📥  From the field

The small icebergs brought to the shore have fallen silent with the evening. The sun is currently hidden behind the 500-metre mountain just behind the base, and the beach is in shadows. What suddenly made these icebergs silent? Curious, of course, we came to investigate after some colleagues told us there was no noise ... (motivated by some aspects of our research, they had combed the beach looking for icebergs with the most bubbles to add to their end-of-work drinks ...) We take measurements in air and in water, and conclude it is a conjunction of the type of icebergs, the contrasts in temperature (or rather the absence: air and water are both close to 1 degree Celsius), and the very calm seas ... We also take some samples to measure in the laboratory ...

Traffic in the Bay of White Bears has increased tonight: there is a large cruise ship at anchor in the deepest part (around 200 m deep), and we can hear the noise of its engines over the several kilometres of water. Another ship (further left in the picture), much smaller, decided to moor very close to where we had deployed an acoustic-recording buoy ... What about the noise it will make, covering what we want to measure over the coming months?

They are Norwegian hydrographers, though, so we do not really begrudge them: they must be doing exciting work too. And we all "comrades-in-arms": we all want to understand more about the polar regions and their climate. Thinking a bit more about it, our first buoy has been there for several months already. The second buoy, 25 metres away, will not start recording until November. So a few hours of engine noise will not really affect our different measurements ...

Ships passing in the night? If only ... One of them has been above our buoy for more than 24 hours now ...

Ships passing in the night? If only ... One of them has been above our buoy for more than 24 hours now ...

 

26 July 2014 – No birds, please …

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📥  From the field

After the last days, we have plenty of field data to analyse and more experiments to run and test different theories. Is the noise coming from the bubbles? From the cracking? What influences how loud it is? Is it the temperature of the air? Of the water? Of the ice itself? Does it depend on how much salt there is in the water? And how does the noise from one iceberg combine with the noise of the others to give what we measure in the field?

This acoustic experiment fits nicely on a table top, and measures the noise of individual growlers as they melt ...

This acoustic experiment fits nicely on a table top, and measures the noise of individual growlers as they melt ... The box on the left contains hydrophones for use in the field, listening to icebergs (or anything underwater) in stereo.

This makes a lot of parameters to investigate. The high-speed photography rig is used fully, day and late into the night, and we have another small tank to run tests in.  The second part of our laboratory is actually in our dormitory: I annexed the desk and put a small aquarium on top. The hydrophone measuring the sound close to melting ice is connected to different bits of kit, and everything is recorded on my computer for later analyses. Melting full growlers in different conditions requires as little background noise as possible. This is not always possible in the local conditions: the station was built to be a polar base, not an underwater acoustics lab, and it is mounted on stilts (to separate from the snow and cold permafrost, in winter). This means that people walking in the corridor outside make the floor move. Doors closing too fast because of the wind do not help. But the main culprits are outside: squabbling geese or cheerful snow buntings settling just below the window.

Snow buntings are the only songbird in Svalbard.  The same size as sparrows, they are a delight to see and hear. But what do they need to sing loudest when the experiments are running :-) ?

Snow buntings are the only songbird in Svalbard. The same size as sparrows, they are a delight to see and hear. But why do they need to sing loudest when the experiments are running?

 

 

22 July 2014 – Pulled under by a mini-tsunami

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📥  From the field

7 am: The weather has cleared up and we can start field work again. According to Internet, we are in the sunshine. According to my own eyes, the clouds are still very low and all grey from one end of the horizon to the next. This morning, the beach was littered with small and big icebergs. Their clinking noise could be heard even from the base, 200 m away. After piling all the equipment in the boat,  we are going back to Hans glacier, first to set up the stereo cameras on the hill overlooking it, second to get more underwater measurements. After the rain of the last days, the glacier has melted a bit more, and there is a lot of ice, including blue ice, everywhere around (blue ice is more compressed ice, coming from the base of the glacier).

9 am: Just as we approach the beach, a loud bang can be heard, and we turn back in time to see parts of the glacier falling off. We have seen that before, and it is always spectacular. As blocks of ice fall into the water, they create long waves, which do not seem that bad from our place. We use the first ones to help haul the boat onto the beach. And then the big one comes in … It looms large very quickly, bringing a 5-tonne iceberg too close for comfort … And then the gravel below my feet is pulled down, and I feel sucked into the water deeper than I expected … Luckily, the Arctic immersion suit is once again very useful …

I should have known: mini-tsunamis are a common risk near glaciers, and like other tsunamis, their height increases as they reach shallower water. Here in this fjord, the seabed is very steep, so the waves increase only very close to the shore. In other places in the Arctic, they can crash big icebergs onto structures or small harbours (when the places are settled).

Not very fetching, but it does the job: the Arctic immersion suit is ideal for very cold water.

Not very fetching, but it does the job: the Arctic immersion suit is ideal for very cold water.

10 am: The stereo cameras were easy to set up, with practice, and we will gather pictures every 5 seconds for the several hours to come. During this time, we’ll be drifting with the freshwater outflow from the glacier, measuring the noise underwater and finding ourselves in the middle of a very large and noisy ice flow.

Ice, ice everywhere ... and we are in the thick of it, drifting for several hours in cold wind ...

Ice, ice everywhere ... and we are in the thick of it, drifting for several hours in cold wind ...

Icebergs, icebergs everywhere … We have to push the pointiest ones away from the rubber tubes of our boat. Icebergs make clunking noises as they move below the aluminium hull, or big thumps when colliding with us (more or less gently).

11am : we get close to a very nice iceberg, made of blue ice and roughly 15 m large. Sea gulls use it as a vantage point, and it has already started to melt in a variety of interesting shapes. The seabirds turn around us with interest, before flying back to their resting place. We take the paddles out to move away (without too much noise: this would be bad for the measurements), and a few minutes later, this iceberg capsizes several times … Lucky escape …

A big blue iceberg: it will capsize violently (and noisily) ten minutes later. The watching seagull at the top will be unfazed, and use the occasion to dive for more fish.

A big blue iceberg: it will capsize violently (and noisily) ten minutes later. The watching seagull at the top will be unfazed, and use the occasion to dive for more fish.

1 pm: The rest of the measurements get by without excitement, as we get colder and colder despite our warm clothing. The walk up the hill to recover the cameras is a good way to warm up (I am on polar-bear watch and carry the rifle and emergency radio, fortunately not too heavy).

3 pm: Back to the shore. Someone is waiting for us to point at a relatively less dense group of icebergs. With Grant, we jump down in the water for the last few metres, moving the icebergs by hand away from the boat so that we can beach it safely. We are back in time for a good lunch, and can relax later to the sound of Mozart on an old vynil from the station’s collection, in the communal room. Then it will be back to work, downloading the data, checking the dozens of emails that accumulated at work back in our respective institutions, and preparing for tomorrow’s deployment. The network is slow again (kilobytes/second) and most connections do not work well … Typing letters. I see them on the screen 5-10 seconds later …

Midnight:: The autonomous recorder is now ready for tomorrow, and we cleaned our hands from the lubricant used to waterproof all the joints and connections. Time for more work: the rain outside looks suspiciously like sleet but I am not going to investigate. It’s warm and cosy inside.

 

 

16 July 2014 - More about the experiments

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📥  From the field

Last time, I talked at length about the science, and why we are here. But it's not all work in the field. Arctic weather changes fast, and even the best forecasts (using data from the weather station 100 m away) are not as accurate as local scientists. By looking at the wind and the sea, they can predict much more accurately when it is worth going out, when we should think about going back home in a hurry, or when we can carry on ... Sometimes, in a bright blue sky, Jarek will say: "OK, in 30 minutes, we should start back". And once we have cut short the experiment, and started going home, we already see fast winds and increasing waves moving in ... That's field experience worth listening to.

When the weather is not as good, or just when we have plenty of iceberg samples to study in detail, we do experiments using two setups. Mine is simple, with a small tank and a high-precision hydrophone to record the noise from an iceberg as it melts, from start to finish. We keep our samples in the station's deep freezer, at -23 degrees (Celsius). The cook nicely put the frozen fish to one side, and we have the other half for all sorts of icebergs, cleanly packed and well separated.

Grant Deane, our American colleague (but originally from New Zealand), has designed and built a separate rig to take high-speed photographs of small samples of ice as they melt, and to record the very short bursts of noise they make each time a bubble escapes, or something else noisy happens.  To hold the tiny samples, Jarek Tegowski built a special holder, which adapts to the melting as the samples become smaller and smaller and smaller.

Combined visual/acoustic studies

Grant Deane (Scripps Institution of Oceanography), calibrating the visual/acoustic high-speed rig he built.

 

14 July 2014 – Starting the science

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📥  From the field

The last two days have been very busy, starting with the first experiments and really testing our equipment for good.

Ice flow

Small and large icebergs flowing out from Hans glacier

Being in the field is always very nice, although life back home carries on as usual. The deadline for submitting papers to an international conference was today, and I finalised a paper with colleagues from Aberdeen and Liverpool on acoustic monitoring of impacts from marine renewable energies. Through several deployments in the challenging waters of Orkney, we tested devices and control areas at the European Marine Energy Centre and used different types of sonar to detect and track marine life, including seabirds (also seen on radar and confirmed by very experienced visual observers).

But what are we going to do here? Yes, we are going to listen to underwater noise from ice, and from glaciers. But why? And who cares? What are the reasons for this work, and how will it be applied later?

Noise underwater spans a large range of frequencies. The lower ones (a few tens of Hertz) can propagate for hundreds or even thousands of kilometres. They correspond to earthquakes or similar natural processes. The higher ones (up to several hundreds of kiloHertz) correspond to animal vocalisations (e.g dolphins) and man-made sonars. In the middle, there are all sorts of sources of noise: natural ones, like the weather (rain falling on the sea surface, waves crashing in the wind) or animal life (whales, fish, even shrimps), and artificial ones like sonars, industrial activities (seismic surveys, offshore building) or divers using acoustic modems. How much noise is there in the ocean already? How much can we make before becoming “noisy neighbours”?

We have answers to some of these questions, with emerging international standards and work done by different technical committees (e.g. the British Standards Institution in the UK: I am there as member of the Centre for Space, Atmospheric and Oceanic Science, University of Bath). But we do not know that much about what is happening in polar regions, because of their remoteness, and because of the very difficult conditions in which the measurements have to be made. Going to Svalbard in 2007, we showed how environmental processes and weather could be unravelled from noise from icebergs. People knew icebergs were very noisy, but not at frequencies as high as the ones we listened to (well into the ultrasound). So we decided to investigate this a bit more. We created artificial icebergs in the lab (that was fun), using different techniques. But nothing beats the real stuff: so more data was collected in the Arctic in 2009, in a fjord with lots of ice and in a fjord with no ice at all. Both sets of recordings were done in very flat seas, so we would have expected to have very little noise. In fact, the recording in the place full of ice was as noisy as if the sea state was 4 Beaufort (rather rough). So we knew the ice was really noisy, and we did some more measurements with the real stuff: tank experiments with small ice blocks carried back from Svalbard in 2012, field measurements in summer 2013 and this summer.

Bubbles in icebergs

Seen from up close, glacier ice contains a lot of small bubbles. When exposed to the air, they create loud noises that we can detect underwater.

Understanding the sources of noise, and putting numbers on how loud ice can be, at what frequencies, will help many other people. If glacier environments are very loud already, this might help assess how certain types of human activities will impact the environment. With the noise coming from the ice, we can measure how many small ice blocks there are (they are usually small enough to be very hard to detect with ship radar when sailing in icy waters, but large enough to create damage to the ships). With the noise coming from the glaciers, we can detect when they are melting (even if no one is nearby), how they are melting, and how much fresh water they contribute to the salty oceans (too much fresh water will kill zooplankton, near the base of the local food chain).

This is why we will listen to these different types of noise, using acoustics underwater, assessing where the noise comes from, and how we can explain specific processes (e.g. the noise created by air bubbles trapped in the ice), but also how the noise from glaciers is working.

 

11/12 July 2014 – The Polish Polar Station

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📥  From the field

We arrived late last night … or was it already the morning? At this latitude (77°00.0’N), the full polar day lasts from 24 April to 18 August, and we cannot see any difference between night and day. This means it is very easy to work as much as we want, or as much as we can. The first day is spent unpacking all the boxes of equipment, settling in the half-workshop we share with an oceanographer from the University of Wroclaw, and settling in the station itself.

Polish Polar Station

Entrance to the Polish Polar Station, Hornsund Fjord. In our honour, our flags have been added to the Polish and Norwegian flags. A very nice attention ...

The Polish Polar Station is named in honour of Stanisław Siedlecki, famous polar explorer and geologist, and one of its founders in 1957. It is inhabited by a permanent crew, spending 13 months on site and overlapping for 1 month with the next year’s team. With the summer complement of scientists, the station can quickly become very crowded. We are around 30 people, although it is difficult to count as some people spend the day in the field, go out for several days or weeks, or come back briefly for a day or two. The geographers are for example leaving in a few days for a hut 15 km away along the fjord, whereas some ornithologists are currently off studying birds in the mountains but will come back in a few days. On Saturday, the official transition between the outgoing and the incoming permanent teams is marked by a nice and well organised ceremony. All speeches are in Polish, of course, but the good humour and sense of welcome easily cross language barriers.

Breakfasts are at 8 am and lunches at 2 pm, both marked by the sounding of a bell which can be heard throughout the station. They are communal affairs, and everybody takes turns to help the cooks with the service (and washing up). Dinners are more informal, with no fixed times, and people come and go as they please. The communal area, next to the mess and the kitchens, is well stocked with books, music and a satellite TV. It is also the focus of other social activities, including guitar-playing and singing, sometimes interrupted by radio calls (the radio is always on: Channel 16 for coastguard and emergencies).

The science starts, at last, and after checking equipment we begin looking at the local icebergs. The closest glacier, Hansbreen or Hans glacier, is two kilometres away as the crow (or the Arctic goose) flies. Results from its melting are visible in the bay as they cross in front of our window: small and big icebergs, ranging in colour from blue to white, sometimes smudged with gravel or even small rocks (from the bottom of the glacier), and with sizes from less than a metre (growler) to larger (bergy bits: that's the official designation). The ones washed up on the closest shores are picked up for later laboratory analyses (we'll start that in earnest tomorrow).

First icebergs

Checking ice blocks washed on shore by the currents. The rifle is compulsory (because of polar bears).