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Andres Perez Arana - Archaeology under the Radar. Remote Sensing and Geophysics for the Layman - Ep 50
594 Plays8 months ago
In our 50th episode, our Geophysics Technician Andres Perez Arana entertains a packed Red Deer to describe what archaeologists are able to observe without getting their hands dirty. He includes many examples of site she has worked at whilst in commercial and academic archaeology.
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Transcript
Introduction to Archaeology and Ale
00:00:00
Speaker
You're listening to the Archaeology Podcast Network.
00:00:29
Speaker
Hello everyone, and welcome to the latest episode of Archaeology and Ale, a free monthly archaeology talk brought to you by Archaeology in the City, which is the community outreach programme from the University of Sheffield's Department of Archaeology.
Guest Speaker: Andres Perefarana
00:00:42
Speaker
This month our guest speaker is Andres Perefarana, and his talk is entitled Archaeology under the Radar, Remote Sensing and Geophysics for the Laman.
00:01:03
Speaker
Right, good evening. So for those of you who don't know me, my name is Steve Hollings, and on behalf of Archaeology in the City, I'd like to welcome you to our first Archaeology in the Air top of the new semester. Yay! For the purpose of the podcast, we are at our spiritual home above the red deer in the heart of Sheffield, South Yorkshire. What we've got tonight is a newbie.
00:01:27
Speaker
So I'm just is going to, I was going to say Virgin, but I couldn't do that really. We're going to have a wonderful presentation, which will be put on to a podcast. So just in case I'm just get so carried away, and so fast, we can't understand him, I can either slow down and make the podcast or you can play to half speed.
00:01:52
Speaker
We've got about an hour's presentation with opportunity for questions after. So please be kind to him. Look after him. She's our own Andres. Thank you very much.
00:02:09
Speaker
First of all, I want to say thank you to Steve for organizing this event. This kind of thing is what keeps archaeology going. I want to thank the Redbeard as well for letting us stay here.
00:02:24
Speaker
I guess I need to introduce myself.
Remote Sensing in Archaeology
00:02:28
Speaker
As you can read there, I am Andres Perezerera. I'm the teaching technician in Filford and Vermont Medicine at the Department of Faculty at the University of Sheffield. So yeah, I deal with all the equipment and everything that makes BEEP, basically. So yeah, today I'm going to talk about
00:02:45
Speaker
You know, basically remote sensing and geophysics are the things I do the most. I hope you don't fall asleep too much. And I hope my thick accent is not too hard to understand. What did you say? You got it.
00:03:03
Speaker
So, first thing, when people, you know, when a layman hears words like remote sensing, geophysics, radars, what kind of things comes to mind? Something to say about the word layman?
00:03:21
Speaker
So what people think about, when you say like remote sensing, geophysics, radars. Well, you think you think it's time team doing actual geophysics, remote. Yeah, like things you do on your TV. Radars is what the Americans used to chase the U-Boots in the Second World War. But actually what we mean when we say remote sensing is, as you can read there, is the acquisition of information about an object or phenomenon without making physical contact.
00:03:50
Speaker
non-destructive technique. This includes a very wide range of techniques and approaches. Mainly it's aerial, satellite, and terrestrial sensors, which we use, depending on where we are. And then we also can include in this category geophysics, even if it's an Americanism, to include geophysics into remote sensing. But it's becoming a trend now, so I guess we have to adapt
00:04:20
Speaker
Types of remote sensing, we can distinguish them by how do they collect the data. It can be an active sensing, is when the actual technique, the actual device, sends a pulse of energy or a pulse of something. It sends the energy that then is going to rip later. Or a passive sensor, when
00:04:42
Speaker
It reads the data that comes from an external source. Very often it's the sunlight or it can be also the magnetic fields of the Earth. Another way to categorize remote sensing can be on how close to the object of study we are. So normally we divide them into three categories. Spaceborne, satellites, rockets, airborne, airplanes, balloons, kites, if you like. Drones is the most recent one.
00:05:09
Speaker
And then is the ground-based ones, which is close to the surface, whether it's the terrestrial surface or it's a water surface. We also do kind of like a bathymetric studies. So a lot of what we work about in remote sensing has to do with the electromagnetic radiation.
00:05:26
Speaker
The human eye can only see within the visible light spectrum, which is the rainbow color, basically, as you can see. This is what we can see with the eyes. But there are things we cannot see just on the naked eye, as we say. And that goes from infrareds, as everybody has heard about the infrareds, microwaves, radio waves.
00:05:49
Speaker
or on the other side, ultraviolet, X-rays, or gamma rays. We don't normally use X-rays and gamma rays in remote sensing. Other colleagues use it for other topics, like bone people. I don't know if there's any bone people here. They use them quite a lot.
00:06:06
Speaker
How is this defined is by the wavelength. Gamma rays, x-rays and ultraviolet at some extent are high energy, short frequency, while infrared, microwaves or radio waves are long wavelengths.
00:06:24
Speaker
One could argue it's less energy. So that's why it has less precision. So that's why you have the FM, AM, long distance radio. Any person who has a radio at home and speaks with people from the other side of the Atlantic, that's kind of radio waves that they control very far. While gamma rays, they are very short distance.
00:06:49
Speaker
And everything within that range, well, we try to use it for our own purposes, in these cases to identify and study archaeology. This is very important for what we do. It is how crop marks and how the underground affects the topsoil. A lot of archaeology is covered under soil. These days it's crop marks or grassland,
00:07:17
Speaker
pasture this what is underground affects quite a lot the crops so the most typical one will be a ditch which has higher content of moisture with an infill of soils which tend to be newer and maybe richer and for that reason the crops there grow better and higher
00:07:41
Speaker
On the other hand, we have a wall or rubble or stones that will hinder the growth of the crops. So we play with this a lot. It's giving us so much information without putting a spade in the floor, in the ground.
Aerial Photography and Drones
00:08:02
Speaker
So the typical examples, this is the staple of British archaeology.
00:08:05
Speaker
ditch, pit, there's no post hole, there's no post hole, but basically a post hole is a small pit and walls. So let's go into different techniques. Well, aerial photography is the first technique used in history. It's not new, as you can see the photos, has been used since the 19th century. All kind of techniques, like from balloons to kites, pigeons.
00:08:33
Speaker
Yeah, I think it's in Germany, it's from Dresden. I think the streeties doing crazy experiments with rockets and putting cameras in rockets and having nice pictures of the streetiest countryside. And then all these booms since the Federal War and the development of modern aviation, putting cameras, you know, you can put a machine gun or a grenade, why not put a camera and shoot? I'm not kidding anyone. So, yes.
00:09:01
Speaker
So, yeah, the message is aerial photography is nothing new, but it has developed quite a lot in the last decade. I'm going to put some examples. Some of you may know this place. This is from historic England, aerial archaeology mapping explorer. And you can see aerial photography is from Castleton in the big district.
00:09:24
Speaker
You can see some photos, so we can use photography to identify how things have changed since then, because it's been almost 100 years now, so we can see changes in the last 100 years. We can also see things that don't appear in more and more than... How did the video go? Was that when Colin started excavating it? I think so. I think so.
00:09:56
Speaker
He can tell us. So yeah, this tool is very useful. It's the IRL archaeology mapping explorer. So you can see people already have been working on it. I have been identifying medieval and post-medieval rich and thorough, iron age activity, medieval and post-medieval tracks. That is very fun because in this photo from 2005, you can see the tracks that have been digitized here. You can appreciate it.
00:10:24
Speaker
Right, carry on. Drones! Well, this is the last addition to... This is the last addition and a very useful tool.
00:10:36
Speaker
The magic of drones is that they have democratized aerial photography. Something that before was very expensive and very time-consuming because you have to plan very well and it was very expensive to put a plane in the sky. Now everyone for 100 pounds can get a second-hand drone and start flying as long as you are within the law, of course. So drones have definitely a game changer.
00:11:02
Speaker
With drones you can stock your colleagues. This is from Wales. But also you can do useful stuff like recording old buildings or landscapes or whatever you like. It gives you definitely a different perspective from ground level.
00:11:22
Speaker
Structural promotion is a very useful thing we can do with drones. It's basically a type of photogrammetry. We take several photos from different points of view and then with different algorithms in certain softwares, we can create 3D models out of several images of the same object from different points of view. I don't want to spend too much time because this is...
00:11:46
Speaker
for a different talk, and you can do things as this one. This is the same building I showed you here. It's Middlesbrough Old Town Town Hall, basically. Now it's in ruins. You can see that the whole town center of Middlesbrough has been completely destroyed. We went last a few months ago and we flew the drone. We collected some photos also from ground level. We made a nice 3D model of this one of the most
00:12:13
Speaker
One of the oldest buildings in Middlesbrough.
00:12:18
Speaker
Also, with photogrammetry, we can create terrain models. If your drone has RTK, which is a high precision accuracy GPS, you reference your photos and the photogrammetry will calculate how far away the drone is from the ground. And when you put all these photos together, you can create an actual elevation model.
00:12:49
Speaker
with actual giving you values of the syllable. So these photos are from Sheldon, some of you may know. This is Sheldon, the hench we did last summer. And this is new hole in Castleton. You can see the excavation and the spoiled hips. So to create these terrain models, we use
00:13:11
Speaker
First, we have to create a point cloud. A point cloud and a 3D model is what you can see in the bottom. So you can see each blue square is one single photo, one individual photo. So we put them all together. We need a heavy processing computers for this because it's very heavy data. It's quite resource-commanding. But yeah, so once we put all these photos together, you can create these three models and these terrain models. And you can see here,
00:13:38
Speaker
excavations, I don't know, it's like, can you see the trench number two, trench number one in Sheldon, the excavation at Newhall there? Right, again, this is Newhall excavation, you can also do with photogrammetry, you can also do plenty orthomosaic, orthophotos, which is like a correct photo of the site, so you can check.
00:13:57
Speaker
if someone is moving your stones around or bringing them home or something like that. These two photos are like five days apart from each other and you can see the advance of excavation. You can see these little legs that's coming up here. You can see how the spoiled hip is evolving. You can see how they went deeper in this, what was this, Colin? The cellar, yeah. How they went deeper in the cellar
00:14:26
Speaker
And they expanded a bit in these two, what were they like, flus? Or hearts? Yeah, I wasn't paying attention, sorry. Anyway, other techniques. Let's move into other techniques. Multi and hyperspectral.
Advanced Imaging Techniques
00:14:46
Speaker
This is when things get interesting, because we are going beyond the visual wavelengths. Multi-spectrals are usually composed of between 5 to 20 bands, and it can't... I'll go to the next...
00:15:02
Speaker
It covers the visible spectrum but it also goes into the infrared, not the far infrared, only the near infrared and it gets into the ultraviolet as well. It doesn't go the whole spectrum because that would be too much. This is the typical multispectral camera with different channels.
00:15:23
Speaker
One is just a normal camera, the other ones are like different colors, so like the blues, the greens, the reds. And then what's the difference between multi and hyperspectral? Well, the difference is the number of channels. I'm going to go back and forth from these two slides. So as you can see, the limits between both cameras is the same.
00:15:49
Speaker
The hyperspectral can't go beyond what the multispectral can go. The difference is the number of channels. So while in the multispectral you get different levels, like stacked, in the hyperspectral you can make gradients.
00:16:06
Speaker
It's like a continuum. So you can see here on the photo, like this is normal color, true color. Multispectral is more, how to say, the colors are starker, let's say. And hyperspectral is more, it's smoother. So depending on what you want to do, one technique order is more useful.
00:16:23
Speaker
So what do we do with multispectral? We collect all these fancy photos. What do we do with them? Well, as you may know, in archaeology, we basically steal techniques from other disciplines all the time. So in this case, we are stealing the technique from farmers. This is what farmers use to identify
00:16:45
Speaker
the crops health so they use multispectral cameras they see areas where the crops are not growing very well and they say okay we need to put more fertilizer there or insecticide or whatever they need to make it better so we take that from them
00:17:00
Speaker
And we say, OK, well, probably, if there's a Roman wall there, the crypts are not going to like it. So they are probably going to go very, very brown, very red. Or on the opposite, if there's a Bronze Age pit full of organic material, well, the plants are really going to like that because plants like organic material. So it's basically using these techniques on our own advantage.
00:17:26
Speaker
There are many indexes that can be used. LDVI is the most common one. It stands for Normalized Difference Vegetation Index. How it's calculated, I don't want to bore you with a lot of formulas and stuff like that. It's very simple. You take the near-infrared, you subtract the red, and then you make a division of the near-infrared, plus the reds.
00:17:50
Speaker
And then it gives you an index and based on this index, then it tells you the health of the plant. This very briefly, maybe someone here can tell you better, maybe a plants person here, I don't know.
00:18:06
Speaker
But yes, for example, if the index is from minus one, it goes from minus one to plus one. So it has that range. So we have an index of minus one to zero. It basically means like a dead plant or an animate object. There's no life there.
00:18:24
Speaker
If we are looking at index from 0 to 0.33, it's alive, but it's not very healthy. It goes from 0.33 to 0.66. It's healthy, but it starts to show some signs of something's not right. And then from 0.66 up to 1 is healthy plant. An example of multispectral imagery. This is from Green-Hamerton.
00:18:51
Speaker
Apparently with this technique they found an Iron Age settlement. These white lines here.
00:18:59
Speaker
and a Roman road. Going there. Other examples, I think this is from Spain. They use different techniques. There's the NIR infrared, one of the indexes, the ENVDI. So just say this one is different to this one. I don't remember exactly the difference now, but there are different indexes I was explaining. There's a whole plethora of different indexes that can be applied to the imagery.
00:19:29
Speaker
And here they found some structures as well, as you can see. I think these lines are modern, like agricultural activity. But the red lines are actual buildings. Other examples from far away.
00:19:48
Speaker
They use different techniques like thermal, I will go to thermal soon, NDVI to identify structures in the countryside and it looks very accurate. You can see the layout much better than in the normal photography. More examples as you like, I can give you as much as you like. Thermal cameras. This is another technique we use on drones, it's quite useful.
00:20:18
Speaker
it plays with the thermal difference moisture generates on different types of material. So it's normally the difference between the natural soil and the archaeological elements. So here you have a little diagram where you can see, for example, you have a concentration of artifacts on the surface that normally will gather
00:20:44
Speaker
will gather, they will heat more than the rest, so in a thermal photography they will appear as hotter than the rest of the soil. You have a ditch or a pit, it will concentrate more moisture, similar to having with the crop marks, more moisture means more water, more water means cooler, so the thermal photo will identify that area as cooler than the rest, so we can pinpoint it.
00:21:10
Speaker
A wall is the opposite, it will gather heat, it will be drier, so when the water evaporates, that area will be warmer than the rest, etc. So the trick of thermal photography is you have to choose when you are doing your survey, because there are optimums through the day. As you can see,
00:21:34
Speaker
the dry soil during the day hits much more than the moist. So you need to, what people normally do, they look for the coolest part of the day, so they normally fly the drones in the night just before or by dawn. So they use this curve here.
00:21:53
Speaker
to pick the optimum of the difference. Because one say, oh yeah, why don't you use the hottest part of the day? Well, what will happen at that time in many places, I don't say this always what happens, everything will be too hot. Yeah, true. The dry soil will be hotter, but everything will be too hot for the camera to distinguish. So people tend to look for the coolest part of the day. Yeah, some examples. Roman villas. Who doesn't like a Roman villa? A Roman villa.
00:22:24
Speaker
very, very clearly identified with thermal photography. This is the same field. If you didn't tell me there was a Roman Villa here, I will never guess it. I mean, now I can see the lines vaguely, but otherwise I will have never guessed it.
00:22:41
Speaker
There are other factors that can be used for thermal photography, but I don't want to bore you too much. LiDAR. Well, LiDAR. LiDAR is fantastic. It's a super, has revolutionized the life of archaeologists in the last, you know, five years, ten years. Fantastic. LiDAR stands for light detection and ranging. It's a remote sensing method, as it says, and it consists on, it's basically a laser. It's basically a laser like this one.
00:23:11
Speaker
stronger. So it sends a laser down the ground, normally it's on a plane. Now people are starting to do it on drones as well. And it sends pulses down the ground and the laser bounces back.
00:23:27
Speaker
Then the receiver calculates how long it takes to bounce back and it can tell you the distance. Very briefly speaking. What is the advantage of LiDAR? It can penetrate vegetation. So a normal aerial photography
00:23:43
Speaker
on forests or on long grass, you cannot see what's below that, or bushes, you cannot see what's there. With LiDAR, you can see the actual model. And it's so versatile, because if you're interested in the actual
00:23:58
Speaker
forest you can you can create a digital surface model because it's also pinging that down so you have because it has like several returns first return second return third return so you have several returns so you can choose which layers do you want to keep you can filter the data so for those interested in buildings or objects
00:24:16
Speaker
poking out from the ground. You have the digital surface model, when you can see the data sticks to the actual surface. But if you don't want them, you can filter them down and create a digital terrain model. And then you stick to the bare surface of the soil. And that's what we archaeologists normally like, because we can find archaeology where previously we couldn't find it.
LiDAR's Impact on Archaeology
00:24:40
Speaker
Like, as I said, normally forest, overgrown areas, you name it.
00:24:45
Speaker
Some good examples of Laidar. Well, the first one is the impressive one. It is Tikal in Guatemala. It's like a Mayan city. It's fantastic. I've been there. It is impressive. You go up the pyramids, you just see a sea of green. Sorry, I'm diverging. Sorry.
00:25:04
Speaker
In these kind of jungles, Laidar has been a revolution since in the last 10-15 years. It's been fantastic. You can get models of the actual city getting out of all the vegetation. It's wonderful.
00:25:23
Speaker
A bit closer to home, we have other examples when later has been useful. Some of you may recognize these places. This is Cairlehuis in Wales. It's a Roman fort we are working at. And in the later you can see the bank, perfectly going there.
00:25:40
Speaker
But then wait for later, because there's more about this. And in the bottom, there is Sheldon. There's the Henschwilmerdinger as well last summer. So you can barely see it on the satellite imagery, but on the LiDAR, it's so clear. The bank and the beach.
00:26:00
Speaker
Okay, well, enough of being up in the sky. Let's safely touch ground now. These are our photos from when I used to work in commercial archaeology. Good memories when we got the quad bike stuck in the mud. So, yeah, let's go back to the ground, but not that badly. Maybe it's a good time to... I don't know, like, should I...
00:26:24
Speaker
Has someone questions about what we've been talking so far? Because now we're going to move into the terrestrial techniques. So we are moving away from the aerial ones. No? Keeping them for later? Very well. That give me time to look some daughter. He's brewing it. He's brewing it. All right.
Magnetic Surveys and Technology
00:26:45
Speaker
Well, now we're going to move into the terrestrial techniques. Most part of the large geophysics.
00:26:52
Speaker
The key of these techniques so far is the magnetic surveys. Magnetic surveys, we use them, especially in Britain, are used everywhere all the time. Their versatile is quick and gives you quite good results.
00:27:08
Speaker
Here we got some examples, we have some weirdo here using a dual magnetometer, we have some people here using a single one, cards and tout cards. There are so many ways of collecting data. The principle is very simple, it uses magnetic fields to identify what's laying underground.
00:27:33
Speaker
As you can see in the picture, in this kind of Neanderthal here, we're in a magnetometer. So, yeah. Magnetometer is the generic word for the instrument. Radiometer refers to a certain kind of configuration of magnetometers. So, as a rule of thumb, that's how you say rule of thumb.
00:27:57
Speaker
All radiometers are magnetometers, but not all magnetometers are radiometers. How the radiometer works, basically it's composed of two magnetometers. You can see in the machine, one on the top, one on the bottom. They both take changes in the magnetic fields. I will go later on how these magnetic fields work. And then they average the readings to remove the background noise, because
00:28:25
Speaker
the earth has its own magnetic fields and there's magnetic noise all over the place. So we need to filter that. This helps to filter it down and then we can focus on the actual magnetic properties of the soil where it's standing.
00:28:40
Speaker
As I said, this is a very useful technique. Here is an example. They did a magnetic survey. They identified these strong anomalies here, and they decided to excavate one of them. Bingo! They had a pit with organic material inside, I think it is. So, yep, it's a previous effectivity. How it works? Well...
00:29:03
Speaker
As I said, it's based on the earth's magnetic fields. The magnetic fields are constantly going in the earth. I don't want to get too much into this question. We are archaeologists. We are not physicists. The magnetic fields is a constant flux of energy. This passive technique can measure that flow
00:29:32
Speaker
Sounds very, sounds very new age isn't it? Sorry, I go back. Other, other principles, it's the remnant magnetization, or, and this one is at the same time is related with the thermal reminiscence. So when, for example, a volcano erupts,
00:29:49
Speaker
All the particles, all the materials that are in it, aligned when it's in a lava state, aligns with the current magnetic field. Because as you know, the magnetic field changes over the long geological era. So there's been times when the North Pole has been able to, we understand to know as the South Pole, you all know that already, don't you?
00:30:13
Speaker
And it shifts as well, it oscillates as well, it oscillates. So it's also used as a adaptation technique, if you want to know. Anyway, so these deposits align with the current magnetic field of the Earth. And when the lava solidifies, they solidify as well, they stay in that position.
00:30:39
Speaker
After, when they become dust or debris, and they fall into a water body, for example, at the bottom of the sea or a lake, they will get the magnetic position when they fall down, when they touch ground as well. So there are different moments in these particles when they align.
00:31:02
Speaker
Another thing is the magnetic susceptibility is how some materials gain or lose magnetic properties. This French guy decided to put his name on it. It's Le Bourne mechanism.
00:31:21
Speaker
And for example, hematite reduces and becomes magnetite. In that process, it gains more magnetic enhancement. And then it oxidizes and it becomes magnetite. And that's the most magnetic element. It happens for several reasons, like fermentation, biochemical. It's like...
00:31:44
Speaker
This is just to give you some ideas. I don't want to get into the bug of hard science now, because it's not the scope of this talk. But if you have questions, you can ask me later. So typical results from a magnetic survey. Well.
00:32:05
Speaker
For example, one of the typical results for magnetometer, what magnetometer is good for, is for fire or fire-related activities, such as identified kilns or fireclay, brick walls, they appear as magnetic. So you can see here in this graphic, this wall made of bricks.
00:32:29
Speaker
has like a strong positive and then drops down in a slightly negative dipole. Things like wool, some stones are not magnetic, like sandstone, the sandstone is not magnetic, but if the soil where it stands has some enhancement,
00:32:47
Speaker
then it will appear as a negative because it's less magnetic than the rest. So it can go both ways. You can have a low magnetic background with a high magnetic feature or you can have a high magnetic background with a low magnetic feature. It goes both ways.
00:33:03
Speaker
And then you can have, well, of course, the strength of the anomalies will depend on how close to the surface they are, so the closer they are and how concentrated they are. Because, for example, you have a small, high magnetic feature there. It will be very, very small. If it's broad, it won't peak that much, but it will be wider in its imprint, in its signature.
00:33:30
Speaker
For example here, you see, depth is important if your feature is near the surface, enough to be picked by the magnetometer, it will appear, and depending on the magnetic properties, it will be stronger or more strong or less strong. But if it's deep enough, you won't detect it. So it doesn't mean... So people need to understand this, like in geophysics, just because it doesn't appear in the data doesn't mean there's nothing there.
00:33:58
Speaker
Other examples are the state of the actual feature. For example, this is different. This is the same material, bricks. But this is a wall that hasn't been disturbed. So it's like one solid block of data, one peak. Well, here is a rubble. And the rubble is created like different peaks in the data. Okay, sorry. Sound.
00:34:23
Speaker
Other typical anomalies, well, we can have in-situ burning, there's like hearts or kings. They are very characteristic for this double peak. It goes up, then down, and then up again. So it makes like a cut face. This is typical of in-situ burning.
00:34:42
Speaker
Then you have ferrous stuff like when the farmer breaks the plow or a horseshoe, they will be like scatter ferrous debris and they make like very small but very strong positive and negative by picks, picks, picks, up down, up down. Then you can have concentrations of ferrous.
00:35:05
Speaker
Because when someone spreads rubbish from a demolition, for example, and they spread it, it will look like this, like speckle. And then we have geological features as well, because different geology has different magnetic properties. For example, we have pockets of sand and gravel, and they appear like this. But they are much smoother.
00:35:32
Speaker
And they don't have peaks. It's just positive. They don't have a counter-negative part, like in the ferrous spikes.
00:35:41
Speaker
What causes? Well, all kinds of things like, well, mainly the usual suspects are various objects, burning of firing, as I said before, fermentation. This is strong for organic material. Sometimes you will find pits with decades and centuries of organic material deposition, you know,
00:36:03
Speaker
plants, your dead cattle, you throw it all into that pit, and the composition, the bacteria, enhance the magnetic properties of the pit. And you can see, for example, in this graphic here, you can see a pit. This is a typical... Excuse me, my pulse. This is a typical only positive organic pit.
00:36:28
Speaker
They are magnetotactic bacteria, but that's not very common in Britain, it's more common in northern France and Germany. It's associated with the lowest landscapes and the composition of wood in that kind of landscape. And then we have like modern magnetic debris, like farmers like to throw green waste these days, and green waste enhance the magnetic field and very often masks the archaeology below.
Electrical Resistivity and GPR
00:36:56
Speaker
Right, this is an example. This is from my adventures in Spain. This is a possible marching Roman fort. You can see it vaguely on the satellite image. You can see the line there, making a right angle. Some people claim it's a Roman fort. It's not definitely clear yet, but it could be.
00:37:18
Speaker
So I did a magnetometer survey, a magnetic survey. It's very funny because the content of iron in this soil is quite high, so it's very speckle. So you can see the line going up and then turning there. It's very hard to see, but it's there. I've been looking at this data for too many hours. I promise you. But why the magnetic survey was useful? Well, it was useful, especially for this little corner in the bottom.
00:37:48
Speaker
Because on the satellite, imagine the landowner changes, and this person here uses the land in a different way, so we cannot see the crop marks. Thanks. I have this crazy idea of doing a little corner in the bottom just to see what happens. And I think we got the corner of the fort there. You can see it very briefly. If you don't see, don't worry. It's hard to see.
00:38:18
Speaker
Electrical resistivity is another technique we use. There are two types of electrical resistivity. There's ERT. ERT is electrical resistivity tomography. This is a very cool article I was reading the other day. They are using high-density tomography. So they're basically doing how it works. You put a lot of probes, a lot of probes in a line, and then you send an electrical current, as you can see here. This is how it should be, but it's not like that. It's like this.
00:38:49
Speaker
So you put probes and you send an electrical current and this current creates arches between probes and depending on how the material resists the flow of the electricity will tell you the properties and then you can identify this obstacles or not.
00:39:06
Speaker
So they make, in this experiment, they make these lines, I think every 25 centimeters is very, very time consuming. But the results are very good, because later they excavated and then they got the walls bang on. And the same data on profile looks like this. So you have the walls with high resistivity.
00:39:29
Speaker
You have the wool, this is on a profile, this is in front, so that's why you cannot see that well. And then you have the moisture, more moisture area under the wool. This is the most common application of resistivity. Everyone who has been involved in some fieldwork probably has been doing some of this.
00:39:51
Speaker
is the mobile probes. It's basically how this works. You put two remote probes in a fixed place that will give you a constant value. And then you move the remote probes in a certain interval that you set up. And that will give you different readings each time. So depending on what is under the ground,
00:40:12
Speaker
the electricity will go faster or slower. So for example, again, as we were saying, if you have a wall, the electricity will struggle to pass through the wall, because it's drier. So the resistivity will be higher. On the opposite, if you are under a ditch or a pit, which has more moisture, then the electricity will go faster, because electricity goes faster when there's more moisture, and hence you will have a low resistivity value.
00:40:41
Speaker
And many commercial units say, well, resistivity is not useful anymore, because it's very slow, and it's true, it's lower compared to magnetometer or radar or other techniques. But it still has its value, still has its value. There's an example here. This is the same place. They did a magnetometer survey, and they picked very nice ditches. These are ditches with the infill, yeah, that's fine. Boundaries, plowing marks.
00:41:07
Speaker
But, there's a big lump here, it's not very clear what it is. And then they did a resistivity, sorbet, and boom, Roman Villa.
00:41:18
Speaker
Why this happens? Because Roman Villa probably was made with brick. The brick, being fire clay, creates too much noise for the magnetometer. Because as I explained before, it's in hands. So you cannot see, you cannot define the edges of the walls. On the other side, the resistivity is not subject to the magnetic properties. It doesn't care about the magnetic, it cares about the electricity. Then,
00:41:46
Speaker
they got the walls very well defined. They didn't get the ditches so well, but that's why the more techniques you use on your short base, the better. Other good bandages of resistivity is normally like a low cost compared to a radar, for example. It's very good for community projects or low budget research projects.
00:42:12
Speaker
So here we have some examples. This is from Wales. We were having some volunteers to help us. It's also very collaborative because you need two or three people to work around. So it's very nice to involve members of the public into the surveys. So we have the volunteers helping. This is Luke. This is from Spain. It's also quite sturdy. So it's easy to travel with it as well if you can pack it well. We have someone here, someone in the room who looks like this.
00:42:44
Speaker
This is from Spain in September. And this is something I found on the internet. It's the Scarborough Archaeological Society or something like that. It's like a local society that is interested in archaeology. And they were very happy because recently they bought this resistivity meter and they were trying it out. So it's like something very good for community projects and local groups.
00:43:08
Speaker
These results are from last spring in Wales. This is the Roman fort I showed you all later before. This is the resistivity data. And you can see here a nice building. Probably the Principia, which is the headquarters. Roads. A ditch. The ditch. Some more streets there.
00:43:28
Speaker
The road leaving to the north as well, probably this is like one of the bank and ditch here, the defenses. So yeah, oh yeah, of course the granary, the horia. Here you can see it's very typical Roman granary. Oh yeah, sorry, interpretation is in the bottom, sorry. And well, the star, the star of the show. And I'm not talking about Alleluia. I'm talking about the GPR.
00:43:58
Speaker
Ground Penetrating Radar is a very versatile tool because it can be used in different contexts that other techniques won't be useful. For example, indoors or on build surfaces like stones, tarmac, concrete.
00:44:15
Speaker
but it also can be used in the countryside as any other technique. So we have some photos here from previous surveys we've done. These two are from Spain. These two are from the Sheffield General Cemetery. This is me when I was in commercial in Sweden, looking for Viking burials and houses.
00:44:40
Speaker
So, yeah, so how does the GPR work? Well, the GPR sends pulses, it's composed of a transmitter and a receiver. It has a transmitter and a receiver, and what it does, it sends a pulse down the ground, and then when it hits something, it bounces back. It's quite similar to LiDAR, as I was explaining before. It hits something, bounces back, and then the receiver measures the time it takes to get the signal back.
00:45:05
Speaker
So you can set up the radar to send the pings, send the pulses as many as you like. So you can set one every two centimeters, every five centimeters. You can adjust that. The more readings you have, the more detailed your data is going to be. It's simple, simple. So what happens when you have an object buried underground, your transmitter hits it on the side of it.
00:45:32
Speaker
and then bounce back. And then as you get close to it, the time it takes to bounce back takes less. And then as you move away, it takes longer again. That's why in GPR signals normally look like hyperbolas, because it's this effect. In other cases, when you have like a floor or a geological layer,
00:45:52
Speaker
It's basically just like a line, and it follows like this. Yeah, so as I was saying, two types of reflectors. It's a lot of reflection. So you have point reflectors. It's like individual objects, small objects normally, and they create this kind of hyperbola. And then you have the planar reflectors, which as I say is like floors or geological horizons. You also have like walls, like this typical wall.
00:46:19
Speaker
because it doesn't create this effect, typical of a pipe or a single stone. Walls are more compact. As well, there are filters to avoid, to correct this curving. So if you apply the filter, your object will be right where it should be. Because the actual location, your object will never be as big as it looks like. It's always going to be the actual location of your object will be on the top.
00:46:49
Speaker
of your curving line there. As well, the size of the object you can detect depends on how deep they are. So on the surface, you can detect smaller objects. The deeper you go, the bigger the object has to be to be detected.
00:47:07
Speaker
clear. Then radars, we use different types of frequencies for the radar. They are a whole plethora of range. Radar is not just for archaeology, it's also used for geology.
00:47:25
Speaker
The oil industry is constantly using GPRs. People looking for pipes, services, GPR is not just for archeologists, as many things in this world. So yeah, so depending on the kind of survey you want to do, you will have to choose the type of antenna frequency you would like to use. Normally in archeology, people use between the 500s,
00:47:52
Speaker
They go between 250 up to 1000. I've never seen an archaeology more than that. This is the range. Normally it's around 500, so the most standard GPR you will see in archaeology will be like 450, 500.
00:48:11
Speaker
Some people use 800s. The one we have in the department is 600 and 170, so one frequency for surface objects, one frequency for deeper down, more geological levels.
00:48:24
Speaker
And just to give you an example, I brought here the same profile, the same profile of data collected with two different antennas. So the one here is a bit counterintuitive because the 500 is this one and the 250 is this one. So you can see the 250 goes deeper down.
00:48:44
Speaker
has more range, but it has less detail on each individual anomaly, while the 500 cannot go as deep as this one, but everything in this range has much more detail and is more enhanced. So yeah, as I say, depending on what you want to find, know your place before you do your physics is the moral of the story.
00:49:09
Speaker
Again, some advantages, well, it's super versatile. It can be used in modern and built-up environments. Effective in a wide range of anthropic activity that other techniques won't do it. It's very good for structures, walls, floors. Also, it's the most used for burials as well, detecting burials. People in forensics use it quite a lot as well.
00:49:34
Speaker
And yeah, the radiograms are displayed on real time, so you can see your data as you walk, so you can start having an idea of what you are getting as you go. Of course, it's raw data, you need to process it later for better results. Disadvantages, well, the data processing is very time consuming, because you can go as detailed as you like, because you have several levels of depth, so you can go 10 centimeters by 10 centimeters.
00:50:01
Speaker
You can go as detailed as you like, so it's very, very time consuming to process and interpret the data. But it's a double-edged blade because it gives you much more information, but then you have to deal with that information as well.
00:50:17
Speaker
It doesn't work that well in saturated clays or very moist areas because the water scatters and the clay absorbs the signal. But still, you can get some decent results in these kind of conditions anyway. And then, yeah, well, modern features, like especially like metal, you have like metal objects on the surface, like rubbish, like Coca-Cola cans and metal sheets that someone left in the ground.
00:50:43
Speaker
masks creates this kind of it's called ringing it's a ringing effect it's it's like a it's like a mirror basically the signal bounce back fully so covers the data below this this object so yeah modern modern
00:51:00
Speaker
modern rubbish can match the data below it. This, back to our Roman fort in Wales, this is the GPR data. Again, we have our Principia, we have our Horia, we have the road, we have a wall there, some streets, some roads. And I don't know if you can see, but here, between the roads, we can even see the individual cells where probably the legionaries were living.
00:51:26
Speaker
And then we have the defenses here, the bank and ditch. And hopefully this year we will carry on this way and we will find out if there's a VICUS associated with it. But that's for next year. Another example is Sheldon with the GPR in Sheldon. I showed you Sheldon as well in the LiDAR section as well. It's very clear that you can see here the ditch and the entrance.
00:51:51
Speaker
and some internal weird anomalies that I would like to find out what they are, so we should dig there at some point. But yeah, this summer they did excavations here, they put one trench here, one trench there, some of you were there actually, and the beach is there.
00:52:07
Speaker
So it's real. This is another way of displaying the data. This is like a multi-slice. Sometimes it's useful, sometimes it's not. It just gives you like, instead of showing only one level of depth, it tries to bring you all the depth based on a color ramp.
00:52:25
Speaker
So you can see the differences like the inside, the hinge, and the outside. So it gives you some information. This is from a project in Spain in September. We were looking for... Well, I have to tell you the story. How are we doing in time? We are almost done. We are almost done.
00:52:48
Speaker
Well, in this village, someone in the 50s had the happy idea of getting rid of them... Someone had the happy idea in the 50s to get rid of the ruins of the castle and put a balloon on it. Yes, that's Spain under Franco for you.
00:53:05
Speaker
So they basically put, I think they put dynamite to flood it down. They flooded down the castle and built this bull ring. So we went there, well, before we went there, the winter before, they did some excavations and they found this massive wall.
00:53:27
Speaker
It's like a two meters wide wall from the medieval castle. So they say, oh, why don't we do some GPR to try to find if there's more wall about this castle? Like, oh yeah, let's go, let's do it. So, or any structure associated with it.
00:53:49
Speaker
So yeah, we did that and we have some, so actually it's quite good results. So you can see on the radograms, these are the radograms, this is the slices. So here, this black line here corresponds with this wall. So you can see that the wall goes there and then there's like 90 degrees up, right? And this is how the wall looks like on the profile of the radar. It's nice and square.
00:54:15
Speaker
then we say okay well let's because the castle was square and the wood ring is round so we probably can't get the corners of the castle so let's say instead of we did the survey inside but they say well why don't we do the survey around as well so we went around and actually well there's a line here that is very misleading because this is a pipe
00:54:38
Speaker
But if you look at this dark blob here, this is actually the wall and it aligns very well. And in the profiles you can see very well the difference. This is why the profiles are important. This here, this kind of like rectangular shape here, that's the wall. This curvilinear thing is the pipe. And they go parallel to each other.
00:55:00
Speaker
Well, not quite, but a little bit. Right, and then by the same token, if you follow this line, usually there's a black dot there. And if you project this way, there's another one here. So I think we have parts of the castle still there. Of course, it's in the middle of the town, so I don't know if they will dig it either.
00:55:26
Speaker
Another very important thing is the thing in the middle. This massive thing is right in the middle of the castle. We don't know for sure what it is. We were considering it could be part of the keep. Definitely an underground, because the keep is completely destroyed. Just an underground
00:55:45
Speaker
underground feature associated with the KEEP, probably like a cistern or a well, or if you want to get creative attention, it's probably like a well more than anything. You can see it very well here on the profile.
00:56:01
Speaker
And then a few things about wood rings as well, because I didn't know. I mentioned this myself, I didn't know this. Wood rings have drainage systems as well, because this is more than drainage around the wood ring. Well, I think that's it for today. Thanks for listening. Thank you, Andreas. That was brilliant. Do we have any questions?
00:56:35
Speaker
Is there any particular problem going through tarmac, say for, it's very level tarmac, looking at footings, maybe four foot, six foot down. No, radar should be able to go through tarmac, yes.
00:56:56
Speaker
The problem is if they put reinforcing bars, that's when there's a problem, if they put like metal bars. But I don't think people put metal bars from tarmac, isn't it? Yeah, they should be fine. No, no, because the tarmac has magnetic properties in itself because you need to want... Yes.
00:57:22
Speaker
Matthew, how interpreting images clearly is quite an expert thing. How much will AI help? Quite a lot, and there's people already looking into that. There's people doing research on intelligent identification of features, yes. It's a bit complicated because
00:57:46
Speaker
RQL is human made. So not all RQL looks the same. You can have a barrow. Not all barrows are perfect circles. So IA is struggling a little bit so far, but I'm pretty sure in the future, as the IA gets better and more clever, it will be possible to automatize identification of features. And it's cheaper than you as well, isn't it? Well, exactly. I'm not that expensive. I just need a pint of beer, right?
00:58:14
Speaker
Any other question? Yeah. Which is your favorite method and why? That's a good question. It's like, you know, push it forward, mom and dad, it's like, all of them has a thing, isn't it? Like, everything, each one of them have a thing, like, GPR is nice, but it's very time consuming.
00:58:36
Speaker
I guess magnetometer is the one I've used more in my life, so I guess I'm more attached to magnetometer because it's the one I've done the most. But I recognize GPR as being very useful and very, very resourceful. And then all the aerial techniques as well are very interesting. LiDAR is like, I don't know how can people live without LiDAR these days?
00:59:02
Speaker
Matthew's coming? Back to computing I'm afraid. Are there standards for the publication of data out of the equipment itself and sufficient that you can interchange between different pieces of software or is it all very manufacturer specific?
00:59:21
Speaker
There are some guidances, yes, there are some, they're not set rules, but there are some guidances, yes. Yeah, there are some softwares, like the one we use for interpreting processing GPR data. They are very clever, they don't reverse engineering.
Q&A Session and Contact Information
00:59:39
Speaker
So they take each manufacturer and they adapt their software to the different manufacturers, but it's like a third party software.
00:59:46
Speaker
OK, yes. Did I answer your question? It's also sort of saying it's not the sort of thing that without expertise you can get your teeth into.
00:59:58
Speaker
It's fine. If you're really into it, you can learn it. It's like... I pay for it. If you want to, yes. I know people who have learned coding on their own and they've made like data processing softwares. And they weren't IT people. There's people who learn Python in their spare time. Because it's Python what it's mainly used. Of course, now you've been sacked. You've got plenty of time for this. I'd love to take a picture of this. Any more questions?
01:00:28
Speaker
Right, that's the case. They will say thank you again to Andres.
01:00:40
Speaker
Thank you for listening to Archeology in Ale. For more information about our podcast and guest speaker, please visit our page on the Archeology Podcast Network. You can get in touch with us at Archeology in the City on Facebook, WordPress, Instagram, or X. If you have any questions or comments, we'd love to hear from you.
01:01:08
Speaker
This episode was produced by Chris Webster from his RV traveling the United States, Tristan Boyle in Scotland, DigTech LLC, Culturo Media, and the Archaeology Podcast Network, and was edited by Chris Webster. This has been a presentation of the Archaeology Podcast Network. Visit us on the web for show notes and other podcasts at www.archpodnet.com. Contact us at chris at archaeologypodcastnetwork.com.