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Mapping Oceans With Sound And Mapping the Sound In The Oceans

Daniel:

Hey, Chris, welcome to the podcast. Today we’re going to be talking about mapping the acoustic marine environment and using acoustics to map the marine environment. I wonder if for the sake of context, if you could take the time to just introduce yourself to the audience, please?

Chris:

Thank you very much for having me. Happy to be here. My name’s Chris Verlinden, I’m am oceanographer, and acoustician. I’m originally from Portland, Oregon in the United States. I spent about 14 years in the coast guard as an officer, primarily serving on icebreakers in the Arctic. I spent the last few years of my career on loan to the Navy doing ocean acoustics research, everything from finding submarines to whales. I got out of the military about four or five years ago and helped found a small company called Applied Ocean Sciences, where I currently serve as the CTO. We do research in ocean acoustics and marine environmental protection.

Differncen between a Soundscape and a Soundshed

D:

I noticed during the introduction there, you refer to it as ocean acoustics. A lot of the time I hear people calling acoustic environments, soundscapes. For me it would make much more sense to call them soundsheds- the same way we talk about a watershed or a viewshed. I imagine that this acoustic environment has geographic limits. Could you explain to me why we don’t often refer to this as being a soundshed?

C:

I actually love that question because I’m secretly starting the movement to start the term soundshed. I’m referring to the study of ocean acoustics, which is really just the study of sound in seawater and how sound propagates, its impact on the environment, and how you can use sound to sense the environment. When I use the word soundscape, I think of it as a way to describe everything that you’d hear in the ocean.

It’s sort of the cacophony of man-made sounds from ships in industry, oil exploration, the sound of biologics like marine mammals, fish, snapping shrimp, flocking fish, things like that. As well as natural sounds, everything from breaking ice, breaking waves, and wind and seismic activity like earthquakes and volcanoes. 

I think of soundscape the same way I think of a landscape. In a landscape, if you’re standing in an environment, is everything that you can see. Well, the soundscape is everything that you can hear.

I think of the term like viewshed or watershed. A watershed is an area where all the water in the area flows to a single point or, a viewshed is all the places that I can see from a point. I think an appropriate use of the term soundshed might be to refer to the sound footprint from a single source, or maybe the sound footprint of a single receiver. Practically, if I have a microphone or an underwater microphone called a hydrophone in this location, what are all of the things I can hear?

If I look at the footprint of everything I could hear, that would be the soundshed of that microphone.

Similarly, if there was a whale, let’s say a blue whale calling at its usual noise level and frequency, there would be a certain area over which you could hear that whale. That would be the whale’s soundshed. The soundscape would be the full abundance of sound in the ocean. It’s the cumulation of all of the various sounds that you’re hearing.

A soundscape might not have a clear definition because, depending on the time or what is happening or what the dominant source of noise is, the variability of the soundscape will change. For example, if a hurricane is moving through the ocean or within a few 100 miles, all you’re going to be able to hear is breaking waves and wind and the noise from the hurricane. Whereas similarly, if you’re in an area near a shipping port, all you’re going to be able to hear is these ships, but that could change.

Maybe the port closes at night, and over the course of the day that soundscape changes. So to me soundscape describes all of the sounds that you can hear in a given area and those geographic boundaries will change and shift over time.

Mapping using Active and Passive Acoustics

D:

When you were describing soundscapes, you were talking about all of the different noises you could hear. You also mentioned this idea of using sound to sense the environment. Oftentimes when we think about remote sensing from satellites we’ll think about active and passive sensors. When we think about active sensors and sensing the marine environment, people may be familiar with using acoustic measurements to measure bathymetry. Could you give us a few examples of passive sources of acoustics that we can use to measure things, and what we can measure with them?

C:

This is a question I’ve devoted my life to, so I get really excited, I think it’s a pretty interesting topic. First of all, just like with remote sensing from space where you have active sensors like synthetic-aperture radar (SAR) and passive sensors just collecting imagery, you have that same paradigm in the ocean. 

Fundamentally, when you’re studying the ocean, you have to use sound because sound is how we see underwater.

What I mean by that is all the electromagnetic radiation, all the visual band optics. The light just doesn’t propagate well underwater, and neither do most frequencies of radio that we can use for communications. What that means is that if we want to image the bottom of the ocean, whereas on land, we could use radar or LiDAR- all the DARS. DARS don’t work underwater. We pretty much have to use sound to image the environment. Similarly, marine mammals use sound to navigate, find mates, communicate.

If you want to communicate, if you want to see and sense your environment, if you want to navigate, sound really is your only option. Just like in space and air where you have passive and active sensors to do all of those things, similarly underwater, you have active and passive acoustics. 

Active acoustics would be where you transmit a sound and use the way that that sound interacts with the environment to learn something about your environment. Just like you pointed out, the real quintessential example of that is bottom mapping sonars or multi-beam sonars, bottom-penetrating sonars. Where you put out a pulse of sound, you wait for the reflection off of the bottom of the ocean and sub bottom layers. You can also image what’s underneath the sea floor. You use the travel time of that reflection back to your sensor to determine the distance to those things. If you’re determining the distance to all things in all directions, you start to construct an image of your environment. 

Active acoustics are a very effective way to measure certain things in the environment, but there are still downsides. One is that some of these active acoustic techniques can be damaging to the environment. It can be harmful to marine mammals and fish that you sound to navigate. In this new paradigm of swarms of autonomous vehicles exploring the ocean, every watt of power matters. If you have to put out active of pulses of sound, you’re going to kill your battery a lot quicker. 

Passive sensing techniques have the advantage of being far less energetically expensive, and the equipment is far less expensive too. The easiest example is passive sonar compared to active sonar. If I’m looking for a submarine with active sonar, I put out a ping. I wait for that ping to reflect off of objects such as submarines. If I want to define that submarine passively, I just have to listen very, very carefully for the sound that submarine makes and then use triangulation, with my array of hydrophones to determine the location of the source. Passive acoustic sensing isn’t limited to just hearing whale song or finding submarines. You can do a lot of really cool things passively. 

There was a really wonderful paper written in the nineties by a scientist at Scripps, Mike Buckingham. He was actually on my committee and he’s a brilliant and wonderful guy. He wrote a paper called Acoustic Daylight. What he did was he drew a comparison to the way that we use our eyes to see and sense our environment in air to what we might be able to do with acoustics underwater.

Think about the way that we interact with and that we sense our environment around us above water. We use our eyes. Our eyes aren’t active sensors, we don’t have laser beams shooting out of our eyes like LiDAR measuring reflections off of everything. We just take the ambient electromagnetic radiation or light that’s reflecting off of objects and scattering all around us. We receive that light on arrays (our eyeballs) and do some very clever signal processing in our brain. We are able to reconstruct an image of what the world around us looks like from nothing but back-scattered ambient light.

Michael Buckingham wrote this paper of how we might be able to use acoustics to do the same thing. He used noise from things like breaking waves on a pier to image objects underwater. It’s a non-trivial problem, it’s very challenging to do. He opened the door for a lot of really interesting work in using passive acoustics to sense the environment. 

We have used noise from ships to measure the temperature and salinity, and possibly even the pH of the ocean using the way that the temperature, salinity, and pH of the ocean affects how sound propagates through seawater. This can help with all kinds of things, from climate and weather forecasting to military operations, to understanding ecosystem dynamics.

Another really interesting example, Dr. Aaron Thode wrote a paper a while back where he used blue whale calls to measure  seawater and bottom properties of the ocean. 

Creating Underwater sensor networks

D:

You mentioned this idea of using an array of sensors to gather this information. You talked about it being a non-trivial problem. When you were talking about using the acoustics from a ship to measure seawater temperature, what kind of sensor array do we need to have in place in order for that to make sense?

C:

I’ll let you know when I figure that out, it’s a non-trivial problem and there are a lot of really bright people working on it. In fact, I have a current ongoing research program with the Office of Naval Research studying exactly that problem, which is what sensors and what combination, what configuration do we need in order to make use of those types of sources to measure the ocean? 

What I want to do is I want to create algorithms that can take an arbitrary system of hydrophones or sensors anywhere in the ocean, and quantify how well we can use those sensors to measure the ocean.

You’re not going to pursue a new remote sensing technique unless you really understand how well it’s going to perform. Some of the mathematics that go into inverting ocean profiles and seawater properties using noise from ships are not non-trivial because you don’t know exactly where that ship is. You need to localize the ship. You need to account for any type of timing or position offsets in both your sensors and the receiver, and you need to measure the properties of the ocean all in one big mathematical calculation.

There’s a whole field of mathematics that really center on how you can cheat in that problem by using prior information. Things like AIS ship tracking data and the fact that your array is only so long, so your hydrophones can only vary in certain positions. I’ve really made a career out of finding clever ways to cheat in order to do more with less information on measuring the ocean with sound. I’m currently working on trying to find ways to determine the information content of sound on arrays of hydrophones so that we can use that to answer your exact question of what exactly we need to measure the environment. The answer is as many hydrophones as possible, as perfectly surveyed as possible, with as quality of synchronization as possible. How good is good enough and how many hydrophones do you really need? That is a really an open question and something that we’re hoping to answer.

Getting Good GIS Data from Aquatic Sensors

D:

I heard you mention AIS data. AIS data is this broadcast that comes off ships and it gives you speed, direction and a little bit more information about the actual ship and its position. Can we use that and say, “Oh, we know that ships travel along these lanes here,” and put an array of hydrophones along those shipping lanes and start doing something there? We know where the ship was, we know the size of the ship. We can make some assumptions about how much noise it was producing with an array of hydrophones that hopefully weren’t moving. Is that a feasible way forward when we think about using these passive sources to map the marine environment?

C:

That’s something that I’ve been wanting to do for ages. We’ve got colleagues at University of Hawaii who’ve been thinking about that same thing for a long time as well. Instrumenting underwater cables and things that are already on the bottom of the ocean in areas where you have access to sources of sound like ships in order to do exactly that.

The advantage of fixed sensors is you only need to survey their location once and they don’t move. There’s a lot of advantages to those types of fixed sensors. So yes, I think 100% the more hydrophones that are out there in the ocean the better. You shouldn’t put them out randomly. There will be places where we have more data, like next to a shipping lane. Then you also have to think about places where we really need those measurements. There are a lot of places where we just kind of know what the ocean looks like, either because we have a lot of sensors already in place or because the variability of the ocean in those areas isn’t so significant. 

I’ve done a lot of acoustic experiments off the Pacific Northwest. And I could probably draw for you what the temperature and salinity profiles look like 500 miles off of The Olympic Peninsula in Washington state right now, and be within a degree or two. The variability is very predictable and very well understood there. So with a couple sea surface temperature measurements, a few floats, a good oceanographer can tell you really what going on with the dynamics in that area fairly accurately. 

If you look at a place like the north Atlantic Ocean off of the UK, for example, that is a complicated ocean. If you take a measurement in one place and you take a measurement 10 kilometers away, I don’t know what they’re going to look like, but I guarantee they’re going to look different. There are a lot of really complicated oceanographic processes, deep convection sites, where water sinks in from the ocean bottom, and deep water masses spread throughout the ocean at ease. Currents meander hundreds of kilometers on a time scale of a few days. Oceans like that are very important to get more measurements from. 

The real holy grail of ocean measurements right now is the Arctic. The Arctic is for much of the year covered with ice, and that makes it very difficult to get research vessels and sensing platforms that we have access to under the ice. The Arctic is one of the most critically important places to measure because it’s one of the places that’s changing the fastest due to climate change. This means acoustic measurements for measuring the ocean properties under the Arctic are incredibly important and challenging, because there are not a lot of ships in the Arctic. If you need to know as much as you can about the sensors and the environment in order to use those sensors to make meaningful measurements of the environment, well, the Arctic where there are a lot of unknowns is a challenging place to work. 

Your question was a really good one. “If we just put these fixed sensors where we know exactly where they are all over the ocean, can we just use all the sound sources that are already in the ocean to map and measure ocean properties?”

Yes, but there will be places where that’s not practical. That’s why I think we need to do a lot more work on using moving platforms and sensors that might not be perfect. There was an interesting study done by DARPA here in the United States recently called Ocean of Things, where they wanted to put tens of thousands of biodegradable floats out in the ocean and instrument them with sensors, including potentially hydrophones, where these things are just drifting around and moving all over the place. That creates challenges because you need to really get the most accurate position of those sensors, you need to get timing synchronized perfectly. If you can do that, you could potentially use all of those sensors like the cube sats, but in the ocean. 

There is a lot of really exciting work going on in this area. There’s a company I love to work with called Subsea 7 that’s making this autonomous sailing vessel that’s a fraction of the cost of anything else that’s out there. You could put hundreds, if not thousands of them out all over the ocean and position them optimally in order to get the best sample measurements of the ocean everywhere. It is a silent sensor because it’s a sailing vessel and its semi-submersible nature makes it a really quiet platform. You could also instrument platforms that are already out there, like whales.

I used to teach oceanography at the Coast Guard Academy and I’d always start off with the description of why it’s so important to get these measurements. I always use El Niño as the example. El Niño is a phenomena where warm water from the Western Pacific slashes across the ocean into the Eastern Pacific. It creates a big pool of warm water, which causes air to rise, and precipitation which disrupts global weather patterns. It can cause floods, famine, disruption to crop cycles, and disruption in ecosystems all over the world.

It can put thousands of people out of work, it can cause starvation, billions of dollars in economic disruption, thousands of lives impacted, people dying from the impact of El Niño. It is dynamics that we understand, we get geophysical fluid dynamics, we get how water moves and yet we cannot predict El Niño. This is a solvable problem in our time. If we understand the ocean better, if we make better measurements, if we make advances to ocean modeling techniques and ocean sensing techniques, including the acoustics ones we’re discussing today, it’s achievable to do things like predict El Niño.

We can save lives. We can preserve the economy. These are solvable problems in our time if we can just make better measurements of the ocean. 

Using Sea Creatures as Spatial Sensors

D:

You’re so enthusiastic about the problems that you’re trying to solve. You said a couple things that you need to go back and clarify for me. You talked about the ocean of things, and it sounded like you were talking about using marine animals to crowdsource this mapping work that needs to be done. Maybe putting sensors on whale. Can you tell me more about that please? Earlier in the conversation, you talked about using whale’s acoustics to passively sense the environment. Are we then talking about putting a sensor on it and using a whale like a cube sat?

C:

First of all, DARPA’s Ocean of Things- it was a program that they created in order to see if you could make tons of measurements of the ocean in order to resolve all the things that we just talked about. If you could instrument the ocean to that degree, you could do things like predict El Niño. You could track where whales are in order to more accurately predict and mitigate human impacts on ecosystems. Having more sensors in the ocean is critically important. 

Sensors, electronics, data exfiltration, these things have all gotten cheaper and more available over the years. It used to be that if you wanted a temperature sensor in the ocean, you better have 70 grand. Now there’s an Open CTD project. It is a conductivity, temperature, and depth sensor. You can make it for a couple hundred dollars worth of parts and a PVC pipe. There’s access to technology that we never had before. There are hacker websites, there’s Arduinos, Raspberry Pis, Odroids, Teensys. Similarly, there is the Iridium satellite constellation so you can get data off of any of these instruments.

There is Starlink of course, and Kepler Space, which is a really exciting company out of Canada that’s making a really nice communications constellation. The ability to interact with low-powered, low-cost sensors all over the world has never existed before, and it does now. I think in oceanography, we’re entering into a new paradigm. It’s kind of a revolution right now. 

The space industry I would say is still about 10 or 20 years ahead of us. In space, it used to be that there were are a few dozen satellites. They cost billions of dollars, they were the size of school buses, and they had really nice cameras or sensors, but you only had a few of them. Now we have tens of thousands of sensors in communications hubs. Each of those cameras might not be as good, but when you combine the information from tens of thousands of lower end, lower cost, lower size platforms, you get a lot more information. We just got that memo in oceanography, and we just realized that having more sensors is good and having more sensors needn’t be expensive. 

I think it’s established that having lots of sensors, even if they’re poor quality sensors that don’t report as often, or in the case of whales, have complete control over where they go, are still a really good thing. So now let’s talk a little bit about what would happen if you instrumented a whale or an elephant seal with a sensor. I say elephant seals, because they’re one of the deepest diving marine mammals. They have a higher blood oxygen content and a higher concentration of hemoglobin in their blood than any other animal I’m aware of. They can stay down forever. 

Put a temperature sensor, a CTD, like we just talked about on elephant seal, you’ll get really incredible profiles of the ocean. You can listen to what the animal is doing. It is really important to understand the way that animals interact with their environment, like how whales use acoustics. Just the biological information we could gain from that would be incredibly valuable. If we can localize those animals using the acoustics around us and other technologies, perhaps we can analyze across groups of animals just like an array of hydrophones. 

I think there are two really critical things to understand here and that’s that you can use ambient noise to measure the environment. You could use the noise of ships like a sonar to localize things underwater or to measure temperature, salinity, things like that. You can also use the differences in ambient noise, from things like ships and waves recorded on groups of hydrophones to measure how much time it took for the sound to travel between those two instruments. Temperature and salinity affect the speed of sound, and that can give you estimates of the temperature and salinity of the ocean. You can also use passive acoustics of sources like whales and ships to measure the ocean bottom’s location, shape, and material. 

A good example of that is a paper by Martin Siderius at Portland State who created something called the passive fathometer. Essentially, he takes an array of hydrophones, no sources of sound anywhere, and he does something called beamforming, which is a signal processing technique that allows you to listen in just one direction. He beamforms up and he beamforms down. “All the energy coming from the sea floor is coming from the surface, just reflected off the sea floor, because all the things that make sound are at the surface.” “The time difference between the up beam and the down beam has to be the travel time between the hydrophone array in the bottom and back up.” So just like an active sonar, he’s able to use the ambient noise of breaking waves and things at the surface in order to image the bottom. It’s not inconceivable to think you could do passive multi-beams, or passive sub-bottom profilers. Martin and I have actually written papers about measuring bottom loss, which can tell you what the bottom’s made out of.

If you have passive hydrophones anywhere and everywhere you can get them, on floats, on animals, or fixed platforms all over the ocean, you can make very meaningful measurements of both the soundscape and the environment, including temperature, salinity, pH and bottom properties. This matters for ecosystem dynamics, economics, oil and gas exploration, national defense, etc. This stuff matters, and I do think having hydrophones everywhere, including potentially on biological cube sats would be really beneficial.

Comparing Earth Observation From Space To Remotely Sensing The Deep Ocean

D:

This is a real eyeopener for me, this idea of using biological cubesats as you call them. You were referring to the idea of using groups of these in a mesh network to image and sense our environment. You started making those comparisons to what’s happening in the space industry at the moment. Why do you think we are pouring money into space and earth observation platforms as opposed to earth observation platforms in the water? Is it more difficult to put things in the water than in space?

C:

I apologize if I offend any of my space colleagues now, but yeah, I think that’s exactly why. Don’t get me wrong, the assets we’re putting in space are very valuable. This cube sat revolution has changed the world in a positive way. That being said, those space guys just need to stop complaining. When they put something into space, they act like it’s this engineering marvel. They’ve got one atmosphere of pressure to deal with between the inside of a platform and the outside of the platform. Try putting something on the bottom of the ocean, where you have hundreds of atmospheres of pressure. Everything needs to be perfect to millimeters, otherwise it’ll literally explode or implode. We’re putting our sensors in acidic baths of some of the most corrosive stuff known to being- salt water. Yes, space is sexy.  Space is important. Space-based sensors and space-based communication assets can really improve our lives, but understanding the ocean is critically important as well. It is very challenging and it’s very expensive. 

You hear people say things like, “We know more about the surface of Mars than we do the floor of the ocean on our own planet.” I once wanted to examine how true that statement was. I was hosting a panel for the American Geographical Society between Don Walsh, Bob Ballard, Sylvia Earle, and Kathryn Sullivan, four pillars in the ocean exploration community, and we had this exact discussion. I asked them, “To what degree is it true that we know more about the surface of Mars than we know about the sea floor on our own planet?” And it really depends on what you mean. 

We have a bathymetry map of the whole world. There aren’t places where we know nothing because of the space people. Satellite-derived gravity measurements, gravitational anomalies, the Shuttle Radar Topography Mission, because of things like that we’ve been able to map out with at least very poor resolution everywhere on the surface of the earth. If you want to talk about resolution that’s good enough for things like seismic exploration, or understanding ecosystem parameters, or where mineral deposits are or what the bottom’s made out of well enough to be able to model all the acoustics in order to do things like find submarines or whales better, then you’re talking about the need for maps of the seafloor with resolution better than 10 meters, and we don’t have that. Depending on who you ask, I think it’s between 5 and 15% of the ocean is mapped well enough to do that, whereas the entire surface of Mars is.

By at least some very meaningful metrics, we do know more about the surface of Mars than we do about the seafloor on our own planet, and I think that’s problematic. There’s a lot of really exciting efforts going on and I think Seabed 2030 is one where there’s a lot of academic institutions, government programs, research labs, nonprofits, and companies that have combined together to try to create a comprehensive seafloor map of the entire world by 2030. It also represents this level of international cooperation, which I think is rare and incredibly powerful. We should use that as an example of ways that we can share data more effectively on a lot of different levels, acoustic data, temperature, salinity measurements, things like that. 

Kathryn Sullivan, the former director of NOAA, the first person to have walked in space and gone to the bottom of the Mariana’s Trench, a low key hero of mine. She once said, “We need to API the ocean.” What she meant is we need to work on data availability, data sharing, just like those folks who are working on the seafloor map for 2030, and we need to do that for all of our ocean measurements. 

I think that people are seeing that sensors are becoming more affordable. The problems that we can solve by understanding the oceans better are coming into our daily lives more. Things like understanding if storms are going to get worse or better with climate change, things like knowing which fisheries are healthy, or knowing which coral reefs are going to die and which sites might be viable for coral reefs in the future. People are starting to really understand how these things map to their daily lives. As a result, we’re going to see a real increase in how much people care about and think about ocean exploration and ocean sensing in the future.

Understanding Marine Ecosystems

D:

I just want to try and make one more point here about this before we move on and perhaps talk about noise pollution and what that means for the ocean. You talked about the bathymetry model that we have for the deep oceans, and you were saying it’s not good enough to accurately model the acoustic environment. Early on in the conversation, we were talking about how acoustics is the way we see underwater. Is that actually a limiting factor when we think about the bathymetry model that we have today? Is that the limiting factor when we think about seeing underwater, about sensing our marine environment?

C:

Yeah it is, think it this way, all over the world people are talking about 5G communications now. 5G communications could be very wonderful, very high bandwidth comms everywhere, but it’s a challenging media to work with because 5G doesn’t travel through objects. Heaven forbid that there are leaves in the way, so having really incredibly accurate maps of the landscape is really important for understanding this 5G communication infrastructure.

There’s been a really sharp increase in the commercial, as well as government need for very accurate maps of the surface of the planet. In order to understand what you can communicate with, what you can see with acoustics underwater, you need to be able to understand what the bathymetry looks like.

Similarly, radio propagation depends on atmospheric properties. For certain things like long-range radio communications and over-the-horizon radar, you need to understand those things intimately. 

However, for knowing whether or not you’re going to get Wi-Fi somewhere, you don’t necessarily need to pull out your temperature sensor and measure the humidity, but you do need to do that underwater. If you want to know where you can hear a humpback whale, you put out a hydrophone say, near a shipping lane because you want to be able to warn mariners if there are a lot of whales in the area so they can slow down. Nobody wants to hit a whale. If you want to know where your hydrophone can hear whales, you need to know the bathymetry perfectly. As well as you possibly can, within a few meters. You need to know that the depth accuracy everywhere, you need to know what the bottom is made of. You need to know if it’s sand, is it rock? Is there sand with a little bit of rock underneath it?

You need to know the temperature and salinity of the ocean everywhere, because temperature and salinity affect the speed of sound, and the speed of sound can cause it to refract differently and bend back up to the surface or possibly down into the bottom. Finally, you even need to know the pH because the attenuation of sound is dependent on the pH. If you really want to understand your soundshed, going back to what we talked about earlier today, you need to know as much as possible about the ocean.

There’s hope that if you have hydrophones out in the ocean, you can learn more about the ocean, and that improves your models. In order to do ocean sensing, we need to know the ocean as well as we possibly can. For example, whales when they interact with their environment, they’re doing it acoustically. They can’t see very far underwater, so if a beluga whale wants to know where it’s food is, it’s finding its food with acoustics. It knows its food likes ocean environments of certain types, maybe they hang out on ocean fronts, boundaries between warm and cold water, or maybe the food likes to live above ocean bottoms made of certain properties. Whales know what the sea floor is made out of. They know where ocean fronts are. They know how to navigate. They know how to find mates. They know how to find prey. They know how to find dangers in their environment.

Whales know a lot about their environment from what they’re learning acoustically. We’ve really only scratched the surface of what we can do with acoustics. Yeah, we can find a submarine, a giant hunk of steel underwater, but we don’t know where every single ocean front is everywhere all the time. We don’t know what the bottom is made out of everywhere. We spend a lot more money than beluga whales do. Beluga whales keep me up at night and they should keep you up at night too.

I helped with a study by some researchers at Woods Hole, including a gentleman named Aaron Mooney, just a really brilliant scientist who was looking at how well beluga whales could direction find. One of the things they’ve found is that they can determine the direction sound is coming from, something like an order of magnitude, 10 times more precisely than they should be able to given the size of their sensing organ. They can defy modern mathematics in ways that we don’t understand. If we can understand that a little bit better, then we can figure out how to use ocean acoustics to sense our environment a little bit better. 

We can understand the environment better for its own sake. Understanding ecosystem dynamics, marine environmental protection- If you know where coral reefs are dying, you know where to focus. If you know the viability of the ocean for certain species, you know where to concentrate your efforts. There is just an abundance of discoveries to be made that have world-changing applications. To me, that’s why studying sound in the ocean is so exciting right now. 

Managing Underwater Noise Pollution

D:

I’d like to move on now and talk about mapping and measuring the sound in the marine environment. When we think about sound in the marine environment, a lot of us will think about pollution, noise pollution. My question is, how do we define noise pollution? Is it simply all anthropogenic noise that is put into that environment? Is it a certain frequency over a certain time period? Could you give us a working definition of noise pollution when we think about the marine environment?

C:

If you’re in the European Union, you define noise pollution as ambient noise created by humans that can be harmful to the environment defined as marine organisms or anything else. We don’t define underwater sound as a pollutant in the United States as of yet. The Noise Control Act of 1972 in the United States really only defines noise pollution as it affects human beings. Noise pollution for us is defined as creating sounds that are harmful to either creating stress or physical damage to human beings. Underwater sound where there aren’t a lot of human beings is not currently classified as a pollutant in the United States. And I think that’s going to change very soon. And I think we’ve already seen the beginnings of it.

There’s been a lot of development lately in offshore wind, which I think is a really wonderful here in the United States. It’s existed for quite a while in much of Europe, but we’re really starting to hit it hard in the United States, which I think is a really good thing. These offshore wind developers have come from Europe, where they’ve already had to go through the process of determining what the environmental impact of these platforms is going to be. They’ve already got the procedures in place.  They already have done studies to see how the sound from these offshore wind platforms could impact the marine environment. When they’re coming to the United States, the Bureau of Ocean Energy Management is requiring them to do that same work. Even though noise isn’t technically a pollutant here in the United States, they’re really treating it as one in environmental impact assessments. 

If you want to put in a new pier and you’re going to do pile driving, pile driving’s loud. You have to be able to determine what your impact is going to be on the environment and take necessary mitigation actions. I think that’s a really good thing, and it’s been forward-leaning to require that of the wind companies that are doing the work, and who are already prepared to do that work. 

There’s a wonderful company called Jasco based out of Canada, they’ve got a U.S. office as well. We used to compete with them, but we got tired of losing, so now we work with them as often as possible.

They do just a great job at understanding the environmental impact of industrial activity on the habitat on marine mammals. So if we take a step back again, you said, “How do you define noise pollution?” There are legal definitions, and that’s what we just talked about, but there’s just working definitions as well. I think underwater noise pollution will be sound made by human beings that impacts the environment. That can be more nuanced and more complicated than it sounds on the surface.

Considering the original example I gave of sound in air, it’s pretty easy to determine if that’s having a negative impact on human beings. Human beings are difficult animals to work with. That’s why I became a physicist, but it is possible to ask a person, “How are you feeling?”. It’s possible to ask them to come in for regular tests so you can evaluate their health. You can determine if regular loud noises are causing stress hormones like cortisol and aldosterone to increase in their systems. It’s possible to stick a camera in their ear or whatever doctors do to determine what sort of impact you’ve had on their hearing. 

Whales are a little more difficult to get them to come in for regular appointments where you can ask them how they’re feeling. Moreover, the way they use acoustics is so much more complicated and more nuanced than the way human beings use acoustics. It’s hard to know how the sound that we produce impacts them. 

We use sound to hear and it’s about it. We use our ears to communicate with each other. Maybe find things generally, but the whales use sound for everything. Think about this, if I put you in a room with nails on a chalkboard, just constantly going for a month, the sound of nails on a chalkboard, you would lose your mind. Your stress hormones would be elevated, you’d lose sleep, and there would be significant health impacts on you. There is no physical reason why that should be true. The elevated levels of that sound, the frequencies, there is nothing physically about that sound and how it interacts with your ears drums that should mean that that’s really painful for you. The only way that I know that nails on a chalkboard are painful for some people is because we can talk to people. We can’t talk to whales about that. We don’t know if there’s an equivalent of nails on a chalkboard for a whale, because the way that they perceive sound is so much more nuanced and so much more complex than human beings.

That being said, we can do our best. I’ve recently been very lucky to participate in a study in collaboration with the World Wildlife Fund, and NOAA, here in the United States and Penn, a working group on the Arctic Council to predict future noise pollution levels in the Arctic from changes in shipping traffic. We ran some really incredible acoustic models that we’re very proud of. We modeled every ship everywhere in the world all the time. Every minute, every day, every month, every year in order to create statistics on what the sound has looked like over the last 10 years and what it might look like over the next 10 years. We showed our results to our partners at the World Wildlife Fund and they were like, “Cool, nerds. What does that mean?” They didn’t say that, they’re very nice. What we realized is we had all these statistics and decibels and charts, but it didn’t mean anything because it didn’t tell us how that sound might actually impact the environment. 

We asked one of our lead marine biologists, Dr. Kerri Seger, if she could put together some sort of a map by species. What she did is she took all the information that we had on the spatial extents for all of these critical species, so we had maps of where they were. Then she took all the information we had about the audible range of these species as best we could tell, and integrated the sound energy on their audible bands in the area where that animal lives. We were able to start coming up with risk scores by species and region that told us how much sound energy there would be. 

Of course, there are differences. A gunshot or an explosion is a really intense sound that lasts for a second, but it can irreparably damage your ears and your eardrums. Similarly, if I played really loud, really bad music at 130 decibels for a month straight and never gave you any reprieve, that would have different physiological impacts on you. You would be stressed, you wouldn’t be able to communicate, you wouldn’t be able to talk. If you’re a whale, you wouldn’t be able to navigate, find food, find mates, etc. How you quantify how bad sound is for an organism has to be classified in different senses. There’s a whole set of guidelines created by NOAA here in the United States, and there are similar guidelines in Europe and all over the world that help us understand what the impact of sound is on marine mammals.

One of my favorite studies was by a group called JAMA in Northern Europe, where they wanted to understand the impact of shipping noise on marine mammals in the North Sea. One of the statistics they reported on was the percentage of the time that sound was above certain thresholds that would be either damaging or uncomfortable to marine mammals. That is really important from a stress perspective, how often can this whale find mates, etc. Maybe if a ferry crosses through that whale’s habitat once every few hours, it’s not a big deal because they can still find mates, food, and communicate in between trips. If it’s all the time though, that really matters a lot. So, to your original question, how do we define sound pollution? Well, it’s man-made sound that affects the environment. But how do we decide what the impact of that pollution is on the environment? I think there’s a lot of research that still needs to be done.

Bubbles as Sound Barriers

D:

I used to work for a company that was building offshore wind packs, and they were experimenting with this idea of using a bubble net, blowing bubbles up from the seafloor to protect the area that they were working in from letting too much noise escape from the point source. When you think about the future, is there a world where we noise proof our shipping lanes, noise proof our other activities? Can we set up barriers in the ocean? Can we use bubble nets? Is there any way to protect the marine environment from the noise that we are creating?

C:

We just submitted a grant this week to see about the viability of doing bubble curtains around ships. It’s a pretty far-out idea. I would say a lot of experts in the field have said it would be very challenging. First, why might it work? Well, if you’re doing pile-driving and let’s say, you’ve determined that it’s going to be really loud. You’ve determined that there’s a lot of marine mammals in the area. You’ve done your homework and your due diligence and you don’t want to impact those marine mammals. One of your options is to essentially lay hosing down on the bottom of the ocean. It’s more complicated than this, but basically, it’s a tube with a bunch of holes poked in it that you pump air through. It emits these big strings and curtains of bubbles around the pile-driving thing, and that blocks the sound. You might think to yourself, “Well, that seems silly. Why do bubbles block sound?”. 

If you’ve ever been in a swimming pool, could you hear the people talking above the swimming pool? Probably not. Similarly, if you were in a swimming pool and made a bunch of noise, the people outside the pool couldn’t really hear you. That’s because the pressure difference, density difference, and speed of sound difference between water and air is so great that sound waves see that boundary between air and water as almost like a perfect reflection surface. Very little energy can travel from water to air and vice versa. If you start putting all these little bubbles in the ocean, you create a wall where the sound reflects off that wall and it can’t propagate through it. So in theory, bubble curtains can be a really effective mitigation strategy. 

There’s a researcher at The National Oceanography Center (NOC) in Southampton named Tim Leighton. We call him the bubble king. He wrote an 800-page book called The Acoustic Bubble, about the acoustics of bubbles. They are indescribably important. Bubble acoustics can cause sound to be trapped on the surface. It affects underwater communications, it affects everything. Bubble acoustics is an entire discipline of study that is really important for reasons like this.

To your question, can we just soundproof entire shipping lanes? Most people would just flat out say, “No,” because it’s going to be too expensive, shipping lanes are massive. But maybe just in places where it really matters. Maybe just in places where you have a shipping lane going through a critical habitat. Look at the Santa Barbara channel in the United States. LA Long Beach is the highest volume port in the United States, and for a lot of vessels, the most efficient route is to go through the Santa Barbara channel. It is a habitat for a ton of highly endangered marine mammals and other species that are sensitive to sound. If you’re in a ship that operates often in areas where species might be really sensitive to sound, could you inject a bubble curtain around the whole of your ship? It would be very challenging, there are a lot of significant engineering challenges to be able to do something like that.

Icebreakers actually inject bubbles along the whole of their ship to reduce the friction with the ice. So there are systems, albeit a lot less complex, that do things that are similar. I don’t think it’s impossible, and there might be other ways to reduce the sound from ships. There are really good engineers out there who specialize in making things quiet. You would never accept a car that was so loud that you could never hear anybody while you were inside of it. That’s because really smart engineers tried very hard to make those things quieter. I think there’s probably a lot of gains to be had just purely in engineering of ships.

I forget the name of the company, but some folks have designed these sort of… They’re not springs, but basically shock absorbers that you mount to things like generators and engines. That decouples the vibrations from the machinery with the whole of your ship, which prevents some of the acoustic emissions. There’s a lot more that can be done. The Navy’s been trying to make submarines quieter and quieter for going on 80 years now. Taking some of that engineering and applying it to commercial shipping, it’d be expensive, but I think there’s a lot more work to be done there. As more and more people start to think of sound as a pollutant, there are going to be requirements that come out that make us take some of those engineering steps.

D:

We have come a long way in this conversation and I’m really enjoying it. This has been absolutely great. You’re working on a ton of different things, you’re exposed to a ton of different ideas. What’s the thing out there that’s got you most excited for this year and maybe for the next five years?

C:

Oh, man, that’s a tough one. You might be able to tell, I get very excited about my work. I love oceanography and I love ocean acoustics for all the reasons we talked about. There are so many unsolved problems that are really, really important.

D:

Let me just stop you there. You talk about unsolved problems that are really important, and it sounds like you’re excited by this. You’re not overwhelmed by the challenges, you’re excited by these unsolved problems?

C:

Yeah, of course I’m excited. I don’t know about you, but if I do something that I think is impactful, it makes me happy, it makes me think I made a difference. If I can make tiny little advances in our understanding of underwater sound and of the ocean, I think that there are huge benefits to society and to the world. There are so many problems that I could contribute in a small way to solving that can make the world a better place, and that can help us understand how to protect whales better, help national defense, that can help us address climate change. The amount of unsolved problems that are solvable in our time and the impact those solutions would have to humanity is what gets me up in the morning. So to your question, there are two projects that have me most excited right now, one of them, because it’s just so darn cool, and the other one, because it’s so darn hard.

The one that is cool is the project I told you about already with the World Wildlife Fund and the Artic Council and NOAA. First of all, our government program manager is wonderful. She is a solving hard problem, she’s pushing us hard to really do the best job possible to understand how shipping noise in the Arctic is likely to affect Arctic ecosystems in the future. We have just gone all out. We’ve taken the most sophisticated acoustic propagation models we’ve ever made and analyzed and optimized them for all sorts of computing architectures, to be able to simulate every ship, everywhere, all the time.

We’ve brought in marine biologists, psychologists, physicists, and atmospheric scientists to work with the World Wildlife Fund. The work we’re doing is good. We’ve gotten to work with world experts in everything from economics who will drive how shipping will change, to some of the best ecologists and marine biologists there are, and department of transportation people who just get shipping traffic. The work is good, and it will be delivered by the World Wildlife Fund and these Arctic Council people to decision makers who will get to look at these maps of, “Oh, there’s a shipping lane here and there’s a critical habitat for this whale there.” Maybe if we just moved the shipping lane, or maybe if we made this regulation or that regulation, it would have this huge impact on the future of conservation in that really critical environment. I think the combination of how good the work is, the fun people we get to work with, the potential impact the study has, makes it all really exciting to me.

The second project that has me really excited is one we’ve alluded to, it’s a basic research program funded by the Office of Naval Research here in the United States. It’s about the information content of ocean noise. It was inspired by work done by my PhD advisor, Dr. Bill Cooperman at Scripps a few years ago where we’re trying very hard to understand everything you can learn about the ocean, from sound that’s already in the ocean. Be it from ships, be it from whales, regardless of the source. How can we better understand what we can learn about the ocean from underwater sound?

I got a chance to collaborate with brilliant scientists at Scripps, at Georgia Tech and Woods Hole and Portland State, University of Washington, and in my own company, at Applied Ocean Sciences and it’s so darn hard, the math, the work you need to do to try to synchronize all your clocks and position all your ships and your sensors and parameterize the ocean in really clever ways to reduce that mathematical search space. It’s so darn hard and therefore very rewarding when we make progress. So yeah, those are the two most exciting things I’m working on, the ocean conservation, noise pollution stuff in the Arctic, because of the impact and the information content of ocean noise stuff for the Navy, because it’s so hard and so interesting, and I also do think critically important.

D:

Chris, I really want to thank you for your time. I have thoroughly enjoyed talking with you. You’re brilliant, you’re enthusiastic, and you’re human. This has been a really inspirational conversation for me. So I really appreciate it. Let’s say somebody else is listening to this and they think, “Well, who is this guy? How do I get in touch with him? How can I reach out to him?” Where would they go to do that?

C:

If anybody after this still to hear more, so the Applied Ocean Sciences website has contact info. It’s appliedoceansciences.com. My email is chris.verlinden@appliedoceansciences.com. I’m always happy to talk acoustics with anyone and everyone. Feel free to cold call me or email me, I love talking about this stuff.