50 Wonders! Sorry I'm late. No! Pub Death again! Oh, it looks like we got a theme for this chapter.
Maybe Pub Death is the theme. Let's see how many shirts I have. Keep this thing going.
If you just remember, a few chapters ago we did a Black Sabbath. Now we're going to do napalm death theme. All right.
Here we go. So we were, 251ers, eyes over here. We were talking about bathymetry and why it was important. And now we are going to continue. and talk about measuring bathymetry.
There's several different ways to do it, a little bit the history of it, and kind of what the hotness is nowadays. So back in the day, a long time ago, the way that people used to measure bathymetry is by using soundings. Poseidon made the first soundings in 85 BC and he did so A sounding is basically, you have a rope with a weight on the end of it so it'll sink when you throw it over into the water. And then you have, you know, usually markings on it. I think, you know, here you have kind of these red markings.
You might use tape or something to indicate, uh, how much rope you've let out so that when it you know a rope with weight as it lowers in the sea floor you're gonna be able to you know you can kind of feel it through the tension and once it hits the sea floor uh then you there's a release of tension and so you could tell tell when it's at the seafloor. And then you say, okay, well, I let out 85 meters of rope here, so it must be 85 meters deep. And that is how this works. And this is how we did it for, by the way, Poseidon, right? Remember?
That's like, you hear, Poseidon, that's what the, I forget if, the Roman or the Greek, you know, god of the deep, back in the day. This was, should say BCE, this is, you know, a long, long time ago, and then for the next 2,000 years, this is basically how we. we measured the depth of the ocean.
So not 2,000 years ago, that's pretty impressive. 200 years ago, not that impressive. But that's what we had.
You can see here a more recent version, right, where they're sailing around and they actually have like a kind of winch that they lowered up. So one of the complications with this, as you can imagine, is if there are strong currents and you lower a rope into it, it's going to get pulled out with those currents. And so more rope will be let out before that thing actually hits the sea floor somewhere further.
away from the ship than right below it, which gives you kind of a false indication of how much rope was let out. There's probably ways, as we got better at math, that you could figure out from the angle that it's going out, you know, how much you need to correct for that, I would imagine, but I don't know if they did that. So we've talked about it before, but the first...
Like, big effort to measure bathymetry globally, right, by one kind of project was the HMS Challenger, the Challenger Expedition. They made the first systematic measurements in 1872 where they essentially, you know... used the same method and went all over the world making these measurements. Remember, do you remember that Captain Cook made systematic soundings in the late 1700s? But the Challenger did a lot more and specifically for science.
Remember again, Captain Cook and his crew, they were doing military work. I mean, they weren't fighting wars, but they were out working for the Navy, right? Whereas this is the first time specifically for the sake of science that a group went out to measure the depth of the ocean. They did a lot of other things.
The Challenger, as I maybe mentioned before, started with 216 people on the ship. Seven died, 26 left. in hospitals or some others deserted and there were 144 remaining at the end of the voyage. Here's a picture of just the scientists. This is an old picture from the 1870s.
They're British, so it's all white men allowed. But that's who it was back then and we're still the people that did the work. So quite impressive. Here's a picture of the Challenger. the ship that they were making these measurements of.
All of those soundings, when you put it together, it revealed, you know, for the first time in some amount of detail, that the seafloor has relief or variations in seafloor depth, right? So there were a lot of theories about how uniform the seafloor might be, you know, before the Challenger went out there, and also about, you know, how, what things might... if anything lives at the depth and a variety of things that the Challenger showed, hey, that's not actually the case.
So from all the soundings made on the HMS Challenger, we find out, hey, there's a shape to the C4 and it's variable. And that's pretty cool. All right, so jumping a little bit forward, the Echo Sounder was created in the early 1900s. This is something that I use. Acoustic pings is another way to say sound.
It shoots out sound. To determine the depth of the seafloor, it shoots sound at the seafloor, and then thinking about how fast sound can travel in water, and when that ping comes back or is recorded by the ship again, you can figure out how far away the seafloor must be. was used widely in the early 1900s.
There are some issues. Integration of a wider signal means that the sound coming back is a little bit diffuse. This is actually something that we'll talk about when we talk about biology.
This is how we discovered the deep scattering layer. Which, right, because when sound bounces off of something saying it scatters, right, it hits something of a different density and it bounces back. And so this layer called the deep scattering layer, they thought this was a seafloor.
This is actually a map, echo sound record of the US East Coast offshore. Right, you can see that there's shape to the seafloor, going down in meters, 700 meters down here. But you can see around 400 meters, a little bit shallower, there's this, it looks like a seafloor. And it turns out that's actually a bunch of animals, right, that go down during the day and then come up at night. And so the way they figured out that this was biology and not geology is that it started moving and the seafloor shouldn't move up towards the sea surface, at least not on a regular basis like once a day.
We'll talk about more of that later. So here's a more recent example of data from an echo sound where you could see essentially there's some interpretation here. So the raw data is kind of the grainy, dotty stuff in the background, but you can see that the geologists in this case put their interpretation of what the seafloor looks like.
You can also tell... Scale, right, it's giving you, it's a much, the scale, the width, the horizontal scale is not the same as the vertical scale. So in this case, this is 273 meters here, but this is 4 meters here.
So 4 meters is, it's kind of, yeah, the vertical scale is a lot more, I guess, a lot less compressed than the horizontal scale. Just some examples. I like showing you data, not just concepts, so you can see what it actually looks like. When this was first created, echo sounding, it was using essentially one signal, and then that would come back, or one wavelength of sound. Nowadays we have something called multi-beam echo sounding, where you essentially send out a bunch of different wavelengths and different angles.
come back to the ship. And you can see this is essentially what that would look like. This is a picture or image, figure from your book. And so if you're on a ship, you shoot out sound to the seafloor, it bounces off the seafloor, and it comes back to the ship.
It's multi-beam because you're shooting in a variety of different directions and then collecting that information. And doing this, you can make a pretty nice map of the seafloor. And as a matter of fact, a lot of the, until recently, a lot of the better resolution data we have about the seafloor is from this. This is, generally, you're closer to the seafloor, so it will give you better resolution than like a satellite, which you're going to learn about in a few slides.
An even better option... but slower, takes more time to collect the data so you can get higher detail but cover less area. It's side scan sonar where you tow something off the back of the ship, it gets closer to the seafloor, it shoots data out, you know, to the side, the two sides of the vehicle.
Okay, and you know, it can give you really detailed images of what the seafloor looks like, right? It gives you a swath of a few hundred meters there. An example... of what this data can look like is this. So this is real data from one of my cruises where I was the chief scientist, basically the lead science captain, sorry, let's think about it, but the kind of lead scientist organizing this particular expedition.
And what you see, so we use this robot called, it's an autonomous underwater vehicle or AUV. That just means robot that's not tied to the ship. You throw it in the water, it does its thing and it comes back.
So you throw this robot out, it makes maps of the seafloor using side-scan sonar. It can make really nice maps because it flies at a height of about 40 meters above the seafloor, so it's pretty close to the seafloor. And what you see here is Mid-Ocean Ridge.
This is... one plate, the Pacific plate, and this is a different tectonic plate, right? And this is where those two plates are spreading apart from each other. Typically we look for hydrothermal vents, but what we found here was this feature over here that we call Lucky's Mound. It's named after my grandma, by the way.
Which is an inactive hydrothermal sulfide mount, right? You can see there's this big kind of feature with a few individual chinties around it, right? So if you zoom in, you get really nice detail of this feature, which is probably about, you know, 20 by 30 meters. this part of it. So this is the type of thing you can get from modern side-scan sonar.
Really, really, really nice detail maps. But again, the thing to realize, right, is this this whole area here, this took days. Well, this maybe took all night. I can collect, but right? This is, this region is maybe, you know, a few miles wide, right?
A few miles wide. This is, you know, this is meters wide over here. And so it's slow, right? If, you know, this is faster.
because you don't have to be right at the seafloor, but that you're restrained by the speed that the ship is moving, right? This is even slower, but you get more detail. This is faster than that, but with a little less detail.
Okay, so in thinking about those two things, side-scan sonar versus multi-beam, and comparing them, we could say this. Multi-beam backscatter is the reflectivity measurement, whereas the side-scan sonar imagery is the actual intensity of the return signal. No?
I know, it sounds science-y. SideScan's sonar towing configuration provides greater maneuverability, which is great, so you can get around things more easily, as the depth of the tow fish above the seafloor can be adjusted in view of the swath width. For example, the side-scan imagery is less prone to be affected by the slope of the seafloor as it can be positioned, whereas multi-beam can only receive the backscatter intensity as it reaches the survey vessel.
So essentially this is telling you some of the advantages of side-scan sonar versus multi-beam bathymetry. All right. Why use ships? Why even be on this planet to measure this, right?
You can measure bathymetry from space. So you can use satellites. Math is complicated, definitely beyond the scope of this class, but you can cover much more area than ship measurements, right?
If you have satellites, it takes you a few days to circle the globe. You can get a map of global seafloor epithymetry within a few days, which would take you, you still haven't done it for ships, kind of globally, right? And also, right, the advantage of satellites is... the Southern Ocean is really rough, right? But satellites don't give crap.
They're not seasick. They're up there in space, right? So you can get maps of regions that would, for various reasons, be too difficult to gather information from using a side scan, sonar, or multi-beam with imagery. So how does this work?
Right? So you have a satellite in space. It's measuring something and then telling you the depth of the seafloor.
How does that work? Right? How the hell does a satellite...
the seafloor. It turns out it doesn't. It doesn't have to. So seafloor features influence Earth's gravitational field. And that influences the height of the surface of the ocean, because the bigger the feature, the more gravity it changes the height of the surface of the ocean.
So for example, a 2,000 meter seamount rising off the seafloor, like you know, maybe... something like this, creates a two meter bulge in the sea surface height. So basically, I said it makes more gravity, but essentially what happens is if you have a mountain on the seafloor, the sea surface height is gonna be a little bit higher over that mountain.
And you can measure that distance from space and actually with satellites you can tell differences of nowadays of like centimeters at the sea surface. They're exquisitely delicate. So you can tell some pretty cool stuff. This is still, it's, it's, may not, it lets you cover the entire planet. The resolution is a little bit coarse but it's still you can get global maps right.
This is an example of one satellite that I forget what it's called at this point. Right? And this is the result of that kind of work. So this is a map that was made of sequoia bathymetry from space using a satellite. This is Sequoia Topography Version 4.0.
I forget what, I'm sorry, I forget the name of the map that they used to do this. I think it was Topex Poseidon, that's what it's called. This is a...
picture of TOPEX, T-O-P-E-X, the satellite that's measuring the sea surface height and also where, using GPS, it can tell where it is measuring that and put it all together. And this is what that looks like. Let me point out a few really cool features of this map that you can see from space. So you can see the reds are the shallowest, the blues are the deepest, you can see the distance of the continental shelf globally, right?
You can see that there's... There's a big continental shelf over here, right, and over here. You can see the mid-ocean ridges, right? This is the southwest Indian Ridge right there. This is the East Pacific Rise, the Mid-Atlantic Ridge.
See all of that. You can see seamounts, right? So here we have Hawaii and the islands of Hawaii.
black because they're land, but then you can see that it's a hotspot island chain, right? And so you can see that that seamount chain goes all the way, and then there's a shift in the way the Earth moved at this point in history back then about... 60, 70 million years ago, and that this seamount chain actually goes all the way up to Korea and then subducts underneath Korea, which is amazing, right? You can see one of the things that I studied as a postdoc here, the Louisville seamount chain, which also kind of has this little bit of a kink in its origin. So all this really cool stuff that you can see from space.
But again, this is way less detailed than this. But this, I don't know, it would take... hundreds of lifetimes to gather a global map of, whereas this, we can get this global map in a few days.
Hopefully that makes sense. All right, so in all of these things that we've been discussing, Soundings, Echo Sounder, Multi-Beam, Side-Scan Soda, and Satellite Altimetry. These are all measuring the shape of the sea floor, right? But sometimes we want to know what it looks like below the sea floor. And so to do that, we would use something called seismic imaging to generate data and images of the subsea floor environment.
This is a standard tool for oil exploration as well, and so this is essentially how we know what's going on. where the oil is. And in this case it's a similar thing where you shoot sound at the seafloor. Sound will penetrate and bounce off of different things of different density.
And so the theory here is that essentially different layers of the seafloor, the most simple might be like you have the seafloor which is probably depending on where you are, but generally sediments unless you're at a mid-ocean ridge or over a seamount. So sediments are different density than water so you get a rich. return signal from that, but some of the other wavelengths will penetrate more deeply into the seafloor and they'll bounce off of the volcanic rock that makes up the seafloor underneath the sediments, or what I call basement, or what scientists call basement.
So that's just two different layers, but then there's different layers of sediment depending on what they're constructed of and when they were laid down. So essentially this is how we know what the seafloor looks like below the sea surface, so the sub-seafloor. And this is, and again... So in this case you have a ship, it's towing behind it an air gun that makes an explosion.
It says an air gun explosion emits low frequency sounds that can penetrate bottom sediments and rock layers. That sound goes down into the seafloor. It reflects off these different boundaries between layers and it comes back up. And a receiver picks up that reflected sound which are analyzed.
You might be like, how is this different than echo sounding? Like it sounds a lot like this. this, you're sending out signals coming back. The concept is the same but the source is a lot different. So this is like an explosion, more or less, really, really loud and makes the, so therefore the waves can go more deeply into the subsea floor and come back compared to multibeams.
This is what data would look like uninterpreted, so this is like the raw data, which just gives you the shape of the... the sea floor you can see some lines but you might not know what they are and this is this black line up here is the sea surface this is showing you the two-way travel time how many seconds between when I sent the sound down and when it came back And then this is an interpretation of that reflection profile which shows you faults. These faults are resulted from tectonic activity, so it shows some faults and there's, you know, reasons you might be interested.
All right, 251ers! You know, just because I like you, I'm going to give you a little more APOM Death. There you go. Throw it out at you.
And we will continue in this video series, this lecture, and we're going to talk... about the next few lectures about the different marine provinces. Remember, this is about marine provinces.
We've been talking about epitometry a lot. That tells us where we can find different marine provinces. But now I want to talk about what those different provinces are, what you can find in...
those different three provinces and so that is what we'll talk about in the next lectures until then as always please be good people treat people well with respect be nice to other people and i will see you in the next class