Video
Geophysical Prospecting in Archaeology
Transcript
Karen: Good afternoon, everyone. Hello and welcome to the NPS Archeology Program Speaker Series for Fall 2013. My name is Karen Mudar and I'm a senior archeologist in the Washington office. This is the third in a series about geophysical technologies in Archeology. It's very nice to have you all this afternoon. It's a great lineup of speakers and I hope that you can make time for all of them. Two weeks ago, Richard Scott, a retired NPS archeologist talked to us about the use of metal detecting in scientific investigations, and Scott has done pioneering work with metal detecting beginning with his dissertation work at the Battle of Little Bighorn, so it was a special treat to hear him talk.
The date for the next webinar will return to the customary Thursday. Andrew Labounty who is an NPS employee will present “Capturing Cultural Landscapes, GIS, and Historical Imagery at Voyageurs National Park.” The presentation will summarize results of research that he's conducting at the park, and he's combined historical aerial imagery with early shoreline surveys and modern archeological information and digitized thousands of cultural features into a geo-database. His study is a good illustration of the ways that archeologists can use GIS to query and visualize cultural activities, revealing spatial patterns over time and directing future research. Please join us for Andrew's talk on November 21st at 3 o'clock.
Before I introduce our speaker for today, I have some administration to take care of for people who are new to the webinar series. All of the lectures will be recorded, so please be mindful of that when you're asking questions. Please set your phone to mute and remember to unmute your phones when you want to ask a question. The recorded webcast will be posted on the NPS Archeology Program website at the URL at the bottom of your computer screens, and if you can't see the URL, you need to start investigating the problem right away so that you don't miss any of the presentation. I'll also make an announcement in the Archeology E-Gram when the webcasts are posted. If you don't receive the E-Gram and would like to also let me know and I will add your name to the mailing list. If you want to get announcements for this webinar series, also contact me.
Our speaker for today, Kenneth Kvamme, is professor of anthropology at the University of Arkansas where he is Director of the Archeo-imaging Lab, devoted to the detection of archeological remains through geophysical prospecting and remote sensing. Most of Kvamme's career has focus on archeological prospecting beginning with pioneer work in GIS based predictive modeling of archeological locations in the 1980s. From the early 1990s, he has pursued archeological detection through geophysical prospecting. Kvamme is author of over 100 publications and 150 technical reports on GIS, archeological modeling, spatial analysis, geophysical application, and remote sensing. He is associate editor of the journal Archeological Prospection and is on the advisory board of the journal of Archeological Method and Theory.
Although his geophysical fieldwork takes him to projects worldwide, Kvamme specializes in the archeology of the American Great Plains and Rocky Mountain West. Today, he will talk to us about geophysical prospecting which, he says, has come a long way. The instrumentation has become faster, less expensive, and easier to use, the computer software has improved to make data handling, portrayal, and interpretation easier, technical improvements particularly in the representation of survey results now offer views of the sub-surface with unparalleled clarity. And many of us have come to realize the potential that such visualizations offer because they often indicate exactly where buried-archeological structures and features are located, and this information can represent a significant cost savings to projects.
Moreover, these surveys are non-invasive and they leave the ground undisturbed. Unlike traditional excavations which require significant time and investigate limited areas measured only in square meters, geophysical surveys are extremely fast and investigate hectares and can be surveyed in as little as a day. This last feature has proved to be revolutionary to the growth of the true landscape archeology. Kenneth's presentation will examine the four principle geophysical prospecting methods used in archeology, and these are magnetometry, electrical resistivity, electromagnetic induction, and ground penetrating radar.
Each interacts with a sub-surface in a different way and they typically indicate different physical properties of the soil, and what each yields is often complementary and together, when integrated through GIS, these methods offer an increasingly comprehensive view of varied archeaological deposits that Ken will discuss with us today, so welcome and thanks for speaking to us, Ken.
Ken: Thank you very much, Karen, and thank you for the invitation to present today, and greetings everyone out there. It's kind of unnerving, I’m staring at my computer, but I'm talking to a host, so hello. As Karen said, I'll be talking about geophysical prospecting and archeology and it's going to be a 3 part discussion. First, I'll look at the role of geophysics in the larger field of archeology and where it might fit in, and secondly, some basic principles of geophysical surveys in general, and then finally, I’ll close with a review of these 4 principal techniques that were listed, so let me just start here and let's see. Karen, I think I need some controls to ... I don't see those controls for advancing the slides here.
Karen: It's done in the lower left hand corner. Do you see the arrow?
Ken: They're not here, I'm sorry.
Karen: Okay. How about if I advance for you?
Ken: Okay. Try that. What is archeological prospecting? Here is a place to start, and basically it's any technique that will let us gain some information about the sub-surface, informing about archeological conditions, and that would include satellite remote sensing, aerial photography, and ground-based geophysics which is the focus today, but there's also other methods such asLiDAR mapping which gives micro-topography, sub-surface artifact distribution mapping which is highly successful in ploughed fields and desert landscapes where broad distributions of artifacts can indicate something about the archeological record.
Soil coring and probing has been successfully used in a variety of places to peer into the sub-surface and then finally, geochemistry, so there's a host of techniques one might utilize to see into the ground. I was going to focus on ground-based geophysics which probably is the most productive of all of these methods. Next slide.
Here we have a slide that shows what might be considered a traditional view of geophysics as an archeological feature finder. I often work in the Northern Great Plains and a particular target are these corn storage pits because they're time sensitive, they're only used for a few years and they're full of artifacts and ecofacts like pollen and corn kernels and seeds and things like that, bones, and so archeologists want to find them, but they're really hard to find because they're so small and it wasn't until really the advent of magnetometry surveys in the Plains that we could locate these, and we see on the far right, a magnetic radiometry image showing lots and lots of corn storage pits. They become visible because they're filled with magnetically enriched settlement soils and it's kind of like like sticking a bar magnet in the ground making them easy to find with a magnetometer.
With such a map, we can pinpoint excavations exactly now, and prior to geophysics, it was just haphazard and sheer guesswork basically. Next slide.
Here's a traditional view of geophysics and all the methods. Basically, I think most archeologists feel the epitome of archeology is excavation, and that lies at the top and everything else contributes to excavation and knowledge of the site in this way where geophysics would be a subset member akin to something like pollen analysis or lithic analysis contributing to archeological knowledge. What I want to do is go through and look at what geophysics offers and then offer a different view here, so next slide, please.
Broader goals of geophysics might highlight this, and we have some workmen here, I'm sorry. Detecting and locating archeological sites and features is a major activity. Secondly, mapping in, interpreting sub-surface features within archeological sites so we could make broad maps of a site or settlement and work with those maps to figure out what's going on in the site, and with those maps, we could pinpoint features for subsequent excavation, but I think these bottom 2 on the slide are more important or interesting because through geophysics, we could offer primary data for understanding site content, structure, and layout. For example, we could give a map of a settlement showing its key components, and then within a region, we could actually get at settlement pattern studies by looking at distribution of settlements and hamlets for a true landscape archeology.
And I say “these as primary data” because already a number of thesis and dissertations have been pursued using geophysical information as the primary data from which to base inferences, such as site structure, site layout, number of houses and things like that. We see a richer perspective for geophysics here. Then on the next slide, some of the advantages include non-invasive. Once the geophysicist leaves the site, there's no disturbance to the ground, but most of the techniques or many of the techniques are passive, a few are active, injecting radio waves or microwaves, but they leave the ground undisturbed.
Through geophysics, it might be the only way to learn about the sub-surface over large regions. We could survey a huge settlement and show all of its features, whereas it's generally impossible to do with archeology. It's also efficient and cost effective and fairly reliable and it's getting better all the time in terms of locating significant archeological features, and an interpretation is based on patterns we could see in imageries, such as rectangles and squares that are of cultural origin, and then physical principles coming from the theory behind geophysical methods, so together, there's a lot of advantages that geophysics offers archeology.
If we go to the next slide here, a better view might be something like this where geophysics is one data source for knowledge about a site or region, similar to excavation which is a different type of data source that yields material culture and vertical relationships, this kind of thing, so in this view, this is another technique that even could surpass excavation for its information content because we could see what the nature of a whole settlement looks like for the first time. Next slide.
Let me start with overviewing types of geophysical surveys. Basically, there's a dual division I want to just focus on here. There's a few other types that are not so relevant. One would be vertical or profile surveys. On the left here, we see a survey we just completed at Toltec Mounds Archeological Park near Little Rock, Arkansas. This is a state park but it's got some great looking mounds and we've never been able to see into the mounds so we did a vertical, multi-electrical resistivity survey that went meters into the mound and gave for the first time a cross sectional view of this mound. We actually ended up doing about 70 cross sections so we actually have a 3D model of the interior of the mound at this time.
What I want to focus on are lateral or area surveys where instead of going vertically in a profile, we survey horizontally to give plan maps of what might be in the ground, and this is of the most common in archeological application in geophysics because based on the shapes you could in the imagery, you could interpret what's out there, and what we see here is a mapping of magnetic radiometry of Huff Village in North Dakota. This is a state park out there and it's a famous site because there's these rectangular houses that exist in long rows within a fortified network of defenses. It's a huge survey but for the first time, we get a real good layout of the village plus interior contents of some of the houses, and we'll look at some of those a little bit later.
If we go to the next one here, we review some of the basic methods here. One is field methods and a key problem is placing the instruments on the ground and knowing where your measurements come from, so traditionally, we've used long tapes or ropes to indicate our transect lines and basically, you move your instrument along these lines or transects, and then on the tapes, we have meter marks, and you can see on the upper right, these red arrows are pointing to meter marks that indicate where you are in this landscape and then the meter marks can help you place the measurements accurately. Some instruments, such as an electrical resistivity meter which we see in the upper right, you might take or 2 measurements per linear meter, whereas a magnetometer might take 8 or 10 per linear meter and a ground penetrating radar 20 to 50 per linear meter.
The sampling controls the resolution you might see. The big block view in the middle shows how the surveys are organized. We usually break down the servey into smaller blocks, often 20 by 20 or 30 by 30 meters, and this allows us to confront a landscape piecemeal so we do a little bit at a time. In that way, if the day ends, we know which block we ended or if it starts raining, we could finish the block and then pick up in the next block the next day. Through computer software, we just tile together all these blocks to make a composite image of a site or region. Basic field methods are shown there. Let's see the next slide.
Let me talk something about anomalies. Another basic background and basically, anomaly is some kind of measurements that are different than a normal background, so here we have an example in the upper right as some electrical resistivity data and we could see the raw data in, but most of the background, typical measurements are shown with a middle tone reds and whites here, and that's the normal background without anything occurring in it, and anomalies are very high measurement shown in black, so extreme measurements of former anomalies and we could see in the histogram in the center of the slide shows all the measurements from that upper right view, and we would define anomalies as being the extreme measurements in the upper right tail here.
We might classify all the measurements bigger than one standard deviation from the mean as being of interest and the mapping in the middle right shows the key anomalies in this landscape revealing some rooms and house floors and things like that. A problem with the interpretation of anomalies in archeological applications is that many are caused by biological or geological or various soil processes that are out there, so rodent holes and tree throws and pavial channels, all these kinds of things show up in our data, so the real task is to sort out cultural from natural anomalies. In the cultural sphere, we also have the issue of unwanted cultural anomalies, such as recent plow marks. We want to filter those out if we can.
Interpretation, it's a little bit difficult because there's so many layers of anomalies out there and we try to pick out the ones that are relevant to the project and culture and time period at hand. Next slide please.
A little about anomalies, this is from one of the better books out there, Seeing Beneath the Soil. We could see that the top row sketches of living cultures, we might see a native settlement and a Euro-American settlement and eventually, these turn into archeological features in the second row there, or yet, some ditches and post molds and pits and foundations and remains of a kiln, and we might imagine a transect, right? Our area surveys are done on long transects. If we pulled out one transect of electrical resistivity where we're injecting a current into the ground, we might see high resistivity over ditches and pits if the fill is more resistant to electrical current. On the other hand, maybe they're filled with moisture, so it could be lower resistivity and that's why there's like a dual curve shown there.
We also note that the stone foundations of that building on the right show up as very high resistivity readings because usually, stone is highly resistant and it'll show up as some really strong anomalies, and we take a different instrument such as a magnetometer and we might find that the fill is more magnetic in the ditches or pits and we also found that firing builds up magnetism greatly. Where the focus of the kiln was we have a huge magnetic anomaly indicated, so for most of these illustrate how archeological features corresponds to physical differences which are picked up by instruments key to particular physical properties of the ground, one being full of electrical current and another being magnetic field properties. Next slide.
Just a brief mention that data processing's really important. Oftentimes, when we get data out of the instrument, it's not very clear or even muddy looking, and I picked this data set in the upper left which looks terrible and it's because we had a drought and then it rained and it was really wet and all the electrical properties of the ground changed, and so I said, well, let’s just take a slice through all these data profiles and that's what it looks like. After some significant processing which we see in the middle, we did various GPR types of things,we came up with a very nice image of the sub-surface there showing part of a historic town that I'll talk about later.
On the right side, we see some post-processing. I mentioned telltale features that are not wanted. Here we have some plow marks over a prehistoric Native American village in South Dakota, and there's a technique known as "Fourier Methods" where we could process those plow marks out to obtain a clear image by removing the regular periodicities of the plow marks there, making interpretation much easier, and we do see in the bottom right there a fortification ditch with some bastion loops and all the blobby things are indications of individual houses in the village. The next slide.
I want to go to the second part now, some principles and let me just emphasize, if there are questions out there, feel free to chime in. Let's go to the next slide.
I’ll cover some basic principles, one being that using multiple geophysical methods is really useful because one device generally picks up one physical property of the sub-surface and by looking at several properties, you could see different dimensions.
On the left here, we have Whistling Elk Village which is that village in the previous slide, and we could see the magnetometry picked up the outlines of the square house shown in black, and black generally indicates high measurements in geophysics. This house was burned and so it became highly magnetic in the burned zone and the hearth is magnetic as well, right in the middle, and you could see some small dots around the hearth, those are indications of the main support post for the roof. Now if we go to the bottom left, we see an electrical resistivity image of the same house that shows its resistant floor and long linear entryway going to the lower right there, so these really complement each other, filling out different elements of the structure of this house.
On the right side, we see Cougar Bar Village in Idaho on the Snake River and we have a Nez Perce long house with some other houses shown and the magnetic radiometry shows some of the centrally placed hearths down the long axis of the interior on the long house and then a large midden to the north of it, above in the image, and on the right, we see a circular house that probably has an adjoining entryway and then a partition through that house and a midden to the north as well.
The magnetic image is very informative but in the lower part, the resistivity shows that these house floors have a dual nature. We see in black high resistant areas where in the long house kind of going along the hearth line and in the houses B and C, we see that each of those are partitioned to high resistant and low resistant areas and we think this corresponds to use areas where the areas of the floor that were walked on and used regularly show high resistance because of the packing, you have a higher density of sediment there, and then the sleeping or storage areas show low resistance in these houses and this corresponds to some things that are known from to ethnography in this area. Again, the dual nature fills out some more information in the data sets here. Next slide.
A second major principle is get the big picture and I often have arguments with my archeological friends who focus on excavation. You'd be lucky to get a 5% sample and if you take a giant settlement, even a 5% sample, which should be enormous, that’s just like pinpricks into a site, few small trenches, and essentially archeologists would learn through excavation very much about very little of the site. You get good samples of material culture and vertical stratigraphy and dates and things like that which are highly important, but little would be revealed about the site's structure, and that's why geophysics can be such a complement to traditional work where we see on the right, surveyed in the course of a few days, a complete outline of the structure of this village where we have a fortification ditch with evenly spaced bastion loops on the outer rim, the blobby features are houses.
I imagine we have a larger square ceremonial house right in the upper middle and in the lower right, there's hints of a second interior village with fortification ditch and I don’t have a pointer here but you can kind of see it arcing and then a higher density of blobby things which point to houses in the lower right of the settlement. Right now, the outer ditchis being traced which I can see here, and then there's an inner ditch coming across, kind of like the middle. Then, the southern end is a berm against the Missouri River where the Army Corps of Engineers has stabilized the site from eroding further. Let's move to principle 3 in the next slide.
This is one points to interpretation. Features of cultural origin tend to exhibit regular geometric shapes. They tend to be square or rectangular or circular or linear and they have very distinct boundaries. I think all the examples here point to this phenomenon where we see round earth lodges on the right or square rooms of a pueblo in the upper right and some historic features on the left 3 images, including roads and things like that. Whereas in the bottom of the slide, we seenatural features tend to be less regular or highly irregular and with interesting boundaries. A key principle when you look at the imagery is how do you recognize what's cultural? Well, you look for these regular geometries. Next slide, please.
Then a fifth [sic] principle, this is just an example but there's a lot of theory behind each of these methods and if you know some of the theory associated with each technique, it aids interpretation, and with magnetometry, as I mentioned, we know that it catches fire and creates strong magnetism, thermo-remnent magnetic anomalies. In the left, to show burned houses with house perimeters and interior hearths being shown. In the right, I would show an experiment we did at one site where we had our magnetic image and then we excavated the house and we screened all the fired earths which you could see in the photo on the upper right and what we did was we weighed the fired earth weight and you could see weighed by square meter here, the shape of the fired earth weight corresponds exactly with a magnetic anomaly, strongly showing us correspondence. Next slide.
Then the last principle is validation's really important through excavation because people who do this kind of work, we could guess or make informed guesses or estimate through theory what we think is down there, but we really would like some validation, and there’s often a disconnect because I’ll do a geophysical survey and some months later, maybe, the archeologist will excavate and sometimes I never hear what they found, but here's some examples where in the upper left, we have some building footings that were illustrated by excavation wells in a a room that turned out to be concrete in this particular historic site, and then again the corn storage pits, validated that they are, indeed, corn storage pits through excavation and not, maybe, a coyote den or something like that. Next slide.
The final and longest segment here would be an overview of some of the geophysical techniques, so let's start with magnetometry in the next slide. Just a quick look at some of the instrumentation, there's a wide array of instruments made by a variety of companies, and most of the instruments used by archeo-geophysicists today are gradiometers, so it's actually 2 magnetometers housed in a long tube where they constantly difference each other. What the differencing does is it removes the effects of the background magnetic field, which is constantly varying so you get a reading of what's actually going on the ground. What's gone on in the last few years are dual gradiometers, where we see on the right half of the images, you actually have 2 poles so if you walk one transect, you actually survey 2 in one passage, and so this has been a real boon to improving speed of surveys.
We used each one of these instruments in the past and now we're using some of these dual sensors now for much stronger surveys. I might add that nowadays, some of the European colleagues are using tractors pulling large arrays of gradiometers, maybe 8 or even a dozen in one passage at a time, they could do huge fields really rapidly. I haven't seen this in the US yet, maybe 4 sensors at once. Next slide.
Let's just talk about some magnetic principles and I'll focus here more on the theory on magnetometry since I think it's one of the more important methods. Human occupation itself exacerbates or magnifies magnetism in a site, and we've already learned that firing enhances magnetism. By building fires again and again and then cleaning up those hearths and dispersing the fired materials, the soils in a settlement become magnetically enriched. There's also firing a building through abandonment or through sacking of a village for example, which increases settlement magnetism. Fired artifacts are made such as bricks and ceramics. These 2, when they end up in the archeological record contribute to magnetic enrichment, and then there's other processes such as the adding of organic waste and middens or just through settlement.
And organic waste tends to promote bacterial growth, and some bacteria actually aggregate magnetic particles in the soil contributing to a magnetic enrichment. What we see here are just some of the processes that contribute to magnetic enrichment in settlement soils, so they're anthrosols or anthropogenically created soils in the settlement are magnetically enriched, and we see the image on the right is a 3D view of some mounds. These are midden mounds in a site and their magnetic enrichment shown in black on top here. Next slide, please.
Here's a little experiment I did at Larson Village in North Dakota. All these sites are covered in corn storage pits and we surveyed and we have a magnetic image showing, the magnetic measurements across the site, and what we did is we took the validated corn storage pits that we excavated or cored and found the maximum magnetism in each, and what we see are 3 zones in this village. The village core was occupied the longest, maybe as long as 300 years, and then the village mid-zone was occupied less of a period of time, and then the outside village was barely occupied at all, for only a brief period. What happened is most of these villages contracted through a series of smallpox epidemics, so we see early settlement probably in the late 1400s going into the late 1700s here.
If we look at the graphs on the right, we can see, magnetism in corn storage pits which are pretty much all identical - one and a half to 2 meters deep, bell shaped in cross section and filled with settlement soil - that those that were in the longer occupied in the village core, the magnetism is much higher than in the middle or outside zones where the magnetism is quite weak. You can see that in either one of those graphs. This kind of illustrates quantitatively the nature of magnetic enrichment and how it varies across the site. Next slide.
The second principle, people create fires, I've already discussed this one so I'll move to the principle 3 is that people also make fired artifacts. These would be ceramics and bricks are 2 of the most common ones. What we see here, you need to kind of squint but it's an outline of the rectangle of a church that was burned down during the Civil War, but there's bricks and you could see where it's labeled "B," these are remains of bricks beneath the surface that show up in the magnetometer, so we have the outlines of the foundation and then some interior building piers and brick that show up quite well. Given this as kind of enhancement, and this is all validated by the Arkansas Archeological Survey did excavate this particular site so we knew that they were bricks and only about 35 centimeters beneath the surface. Next slide.
Our fourth principle relevant to magnetism here is that when people construct things, they tend to accumulate topsoil on many constructions. On the upper left, we see the Great Bear effigy from Effigy Mounds National Monument in Iowa where the mounding of the soil, right? We find that topsoil itself tends to be more magnetic. The mounding of the topsoil creates this magnetic anomaly. In Figure B there, we have the circular ring of an earth lodge, and an earth lodge is just a dome of wood that was covered with maybe a quarter or third of a meter of soil and that soil will erode off of the roof and build up a ring or berm around the lodge, and we see some magnetic enrichment where that ring occurs because of the thicker mounding of topsoil.
In C, we see a ditch feature, and next to the ditch, the soil used to come out of the ditch is usually piled next to the ditch and we see a little bit of raised magnetism in black next to the ditch right there with the arrow to show. D shows us some fortification ditches at the Double Ditch site in North Dakota, and all 3 of these ditches are filled in so you can't see these on the surface but they're filled with settlement soil which is highly magnetic, so we can see the magnetic outlines of these ditches, and then finally in E, all the red arrows are pointing to filled-in corn storage pits that surround a house, so you can see this rounded rectangular house with its northern and eastern boundaries marked by corn storage pits filled with sediment soils.
In the mounding or accumulation of settlement soil or topsoil creates magnetic anomalies that are highly detectable. Now principle 5 in the next slide shows the inverse of this where constructions also remove topsoil or activities remove topsoil. In the upper left, we see the far left, a two-track, the farmer would drive his truck over this same track and the truck grooves through the magnetic topsoil into the subsoil leaving negative magnetic anomalies. We see a similar two-track but those are twin cattle tracks into the central part of the image. We see there's 2 small white holes near the top of the image. Those are looter's holes where the topsoil has been taken away, so we have negative anomalies there.
The big circle on the left, that's an archeological excavation from 1937 that was never back filled, and you can see the negative magnetism in the middle because all the topsoil's been taken away and mounded around its perimeter, high magnetism because it's mounded, so there's a lot going on with this negative magnetism. Then on the far right of that upper left image, you can see a part of a fortification ditch system which on the bottom image, I show a cross section through. We actually excavated a profile here and we see a fortification ditch that's been partially filled and there's lower magnetism on either end of the ditch with some raised magnetism in the middle of the ditch but it's very subtly raised and that's because of the fill of the ditch.
Some of the inflowing soil has filled the ditch, creating a slightly bump of negatives in the middle, and then on the right side of the ditch is the stacked topsoil taken out and then a hearth, so you could see the hearth, it's fired magnetism there but the mounding phenomenon by the stacked soil and then, generally, where the soil has been removed, a negative anomaly. Going left to right, we have low and then high and low and high going across, showing the nature of the ditch and we often find ditches looking like this as a twin stripe because of the fill sometimes is somewhat magnetic in the middle. The upper right is kind of interesting.
This is courtesy of Jay Johnson, my colleague at the University of Mississippi, but this is the Confederate cemetery at the University of Mississippi, we have rows and rows of graves. Now what happens when you make a burial, you dig a grave shaft, and if you don't replace the topsoil at the top, you would get a negative anomaly over it or low, right? Low being white here and so, evidently, when these burials were placed, the topsoil was not put on top of the grave and we have just going across the magnetic field row by row indicating where all these graves are located. A good example of magnetism in grave finding. Next slide.
Here's another principle that constructions import stone or sediments. The Fort Clark Trading Post on the left, we have a magnetic sandstone used as foundation blocks showing the outlines of this trading post, and on the right, I take this is from the Roman City of Empuries in Spain where the use of limestone in construction that lacked any iron-based particles at all that was not-magnetic, and so we have negative anomalies over all the walls. In the fill, they filled all the rooms with over a more magnetic sand that actually shows up as black here, so kind of the inverse, whenever we walked over a wall with the magnetometer, it would go negative, showing these prints.
Then the final one in the next slide pertains to iron artifacts. So many cultures have used iron artifacts. They really show up to a magnetometer as strong dipolar anomalies so we could see positive and negative poles, and here we have the battlefield of Prairie Grove near Fayetteville, Arkansas, which had over 60 artillery pieces in the battle and much of what they shot was based in iron, and so a key action of the battle was near the boarding house which we see in the slide and just loaded it with iron dipoles. This was validated by the Arkansas Archeological Survey where they did extensive excavation here and found literally tons of burst shells and other iron pieces from the Civil War.
So next slide, let's go to something different, electrical resistivity, our second method. What we're doing here is we're going to inject an electrical current into the earth and look at the resistance to the flow of that current, and what I'm trying to show here is an archeological profile where we see 2 layers and at the interface, more or less, some archeological feature such as a pit, a hearth, and a wall, and if we take electrical radiance going across this, we might find that the pit shows negative or low anomalies because maybe it holds moisture or wetter, and so it would lower the resistance to a current.
The hearth on the other hand might be totally invisible, not having different electrical properties, whereas the wall might be a pile of bricks. Bricks and stones tend to be highly resistant showing a positive anomaly. What causes these resistivity differences is things like density and particle size and porosity and salinity. There's a whole slew of factors but mainly, resistivity surveys are sensitive mostly to stone and brick and then moisture variations in the ground. Some people call these "Mappings of moisture" across the site and, indeed, they can be. Let's look at the next slide and look at just some of the theory.
We have on the left, just a basic circuit showing a battery and current going through it. In Ohm's law, what we have is that resistance equals voltage over current, and with that knowledge we can set up a circuit where on the right, we see the Wenner array with the circuit injecting a current to the ground, so we see the current lines flowing through the soil here and we measure with a current meter what the current is, so it's I in the equation. Then we have a voltmeter in the middle where we measure voltage changes. Voltage divided by current gives us resistance, and what we do, we move such an array over the landscape measuring changes in resistance.
There's a whole bunch of different arrays and this is like the founding array when it was developed originally and there's other ones that are more beneficial, but the nature of these electrodes gives us a geometric sector that you might correct. You see in the bottom, the equation there, it's a simple correction for the nature of that array. In archeology, we use a slightly different one that's shown on the next slide which we call a "Twin-probe array." What it all is is a Wenner array split in half, so we take 2 electrodes and we put them out remotely and then we only move 2 at a time, housed in this rigid frame. You can see our 2 current electrodes setup our circuit down there and we just sample voltage changes.
If we vary the space in those electrodes, we're going to sample the different depths, so closely spaced electrodes shown in the upper right would sample very shallowly, just a quarter of a meter beneath the surface. Then moving the electrodes apart, we could sample a half meter, 1 meter, or even deeper. That electrode stays in control of how deep we're sensing and you might see that on the left figure here is that we moved the voltage electrode T2 further out where we're sensing deeper voltage lines and getting a deeper prospecting by moving the electrode further out. Next slide.
Here's just examples of one of the main resistance meters that are used in archeology, the Geoscan Research instrument. You can have 2 or up to 7 or 8 different electrodes on the reading permitting a multiple depth to be acquired at once or shallow depths and deep depths and pretty good results. Basically, you move these over the landscape, you could see in the upper right figure some of the survey guide takes sampling one or 2 measurements per meter and acquiring data that way. Let's look at some of the applications of this in the next slide.
This is a survey we did in Long Island, New York, Sylvester Manor, and we had very shallow archeology, like a quarter of a meter, and we had this interesting pattern and what this turned out to be upon excavation, it's just a raise of small pebbles, beach pebbles that were used for lanes between warehouses, and this was a place where they shipped off produce for the sugarcane in Barbados, so the supplies were grown here and then shipped south in part of this trade. We have historical evidence through the 1600s that there was a fire at one time and the whole place is rebuilt and I think the resistivity shows 2 grid orientations here so we can hypothesize a pre-fire and then a post-fire type of layout to this warehouse district with the shoreline just immediately to the north of the image where they would reach the ships to gain their cargo for the trade.
Stone, being very resistant, this showed up quite well and we had some good results at this site. Next slide. Here's Bunker Hill National Monument. We did some resistivity here some years ago. I always like this image, you might notice there's a faint circle going around this highly disturbed monument where we have an obelisk and museum and heavy landscaping, but I think that ring is maybe the most promising locus of the famous redoubt where the patriots held off the British in 1775. There's also indications of trails and former walkways and things like this. Next slide.
The previous one is, the fill of that, there would've been a ditch and then a mound for the redoubt and it changed some of the electrical properties so we could see that through resistance. I like this little case study here because it's Cowboy Cabin from the 1870s. We did some post-excavation on it so that we know this tiny cabin, half of it was a stone floor and then the right half was an earthen floor surrounded by a stone foundation. You could see in the interpretations on the right that the stone foundation, the brown earthen floor and the yellow stone floor showing up quite differently and you could see the 3D image really highlights its difference. Next slide.
Just to illustrate some multiple depths, we had a half meter electrical separation at Fort Clark trading post in South Dakota where we see some elements of the super structure foundation on the higher resistivity survey, and then going to one and a half meter electrical separation, we could actually see the builder's trenches that held up the palisade around the trading post, sketched in the upper left there. We could see the 3 episodes of rebuilding are shown and this, too, is validated by excavations by William Hunt of the Midwest Archeological Center. Next slide.
Our third method is we've briefly touched on electromagnetic induction and this one's kind of complicated. It uses radio energy to get at conductivity data which is the inverse of resistivity, and we see the lower right, shows conductivity against resistivity, the R and D, the inverse, so instead of electrodes, you just have radio energy going into the ground through a transmitter. That spaghetti diagram tries to illustrate this but you have a primary radio energy going in in conductive soil that induces a secondary field which generates Eddy currents that are picked up by our receiver, and the strength of those Eddy currents is proportional to the ground conductivity. This instrument actually yields 2 data sets. One is the conductivity of the earth, inverse of resistivity, and another part of the signal picks up magnetic susceptibility which is a component of what magnetometry picks up.
It's a very fast instrument so you can walk really fast if they don't like a resistance meter. Next slide. Let's take a look at some of what this can do. Here's back to Whistling Elk where we see resistivity in lower left and conductivity in the upper left, and on the lower right we can see a cross section across the ditch where we have high resistance means low conductivity, so you can see the inverse right there. In this case, the conductivity was not as clear as resistivity mainly because its target focus is about 0.4 meters and the archeology was fully a meter deep at this site, so the square house in the upper right is quite blurring the conductivity data, but on other sites it's better, so let's take a look at the next slide.
Here's Army City, Kansas, which was a World War I - era town sited to give services to the troops training at the Camp Funston, now Fort Riley, but you can see this is a tremendously dry year, it's a drought year. The resistivity came out pretty good on the lower right. The conductivity couldn't pick up any earth differences at all. It's just pretty much neutral but it did pick up the metal pipes beneath the town, because metal's highly conductive and so the conductivity meter was great for that, so it actually proved to be a useful data set although it didn't show other minor features in the town as well as resistivity did. Next slide.
On the other hand, we have an extremely wet year and in this case, the conductivity came out better than resistivity where in these circular earth lodges at the Fort Clark Village, we could actually see interior features such as you might see that linear feature inside some of the earth lodges, that's actually where the leaning posts were sited to form the walls of the dome-shaped structure, and there are even some indications of interior features like a post hole and things like that, so conductivity, for it to work well, you basically need moist ground and we found great results in that circumstance, so next slide.
I want to turn to the other phase, the so-called in-phase. We have this fine wave signal and part of its in-phase was a primary signal which measures magnetic susceptibility and the outer phase part is conductivity. Here for one instrument in a single survey, we have these 2 different data sets, and we've learned that firing increases magnetism. Well Army City burned down in the early 1920s and you could see on some of the enhanced magnetism which I circled in this, and so we get a fantastic image. It's greatly different than the conductivity data set in this case. Next slide.
This is a recent study I've done. We've taken magnetic gradiometry which measures all sorts of magnetism, this magnetic susceptibility which is the induced part as well as thermal remnants which is from firing, so gradiometry gives us the sum of all magnetism where susceptibility is only the induced part from, say, soil mounding and things like that. We're getting very complementary data sets here with a reasonable moderate correlation between the 2, so I think this highlights that electromagnetic induction meters, they give us 2 very important data sets, both magnetic and conductivity, the inverse of resistivity, so in one survey, you could really nail 2 of these data sets. The drawback with the magnetic susceptibility part is it only responds to shallow depths, half a meter or less, so it has high limitations in that regard. Let's go to the next slide.
Our final technique is ground penetrating radar showing some of the instruments than what we'll talk about here, so next slide. One component, we saw those orange boxes in the previous, so there are antennas and they come in a variety of sizes, physical sizes, and the size correlates with antenna frequency, so our low frequency antennas, they tend to go deep but they give us poor detail. They're also very big, and then our high frequency antennas, they give us a lot of detail on the ground but they only penetrate to shallow depths, so there's a real trade-off. Most archeologists therefore use a medium frequency antenna that gives a little bit of both. I think most of us are using around 400 megahertz antennas these days which is a good choice. Next slide.
As you can see, the antenna shoots microwaves into the earth and they go down and reflect off earth's features and what they do, they reflect off anything that gives a dielectric contrast, and the dielectric property is basically the ability of a material to store electrical energy, and so if you get a good difference, you get a contrast coming back or a reflection. This antenna’s hook up to a computer which records the data which is very much like what we see on the right where the antenna's pulling on a transit and if it's going, you get these reflections coming back, you see in the far right in the antenna, it's like a sine wave, and the sine waves are color coded or grayscaled as you go along and when you have a large reflection, you get all these stripey things.
You could see like a big ditch feature and something else in the same profile, and we pull a profile then move over a half meter or a meter and then pull another profile and so on to generate a lot of reflection data. GPR here, it's really a true 3-dimensional method because the vertical axis here is travel time beneath the surface. It's how long it takes the microwave to go down and come back and it's a proxy for depth beneath the surface. Next slide, please. Let's take a look at some of the details in the profile data.
In cemeteries, we often get these nice hyperbolic reflections over the locus of graves and in general, radar is probably the most productive for cemetery prospecting, finding graves. It's been highly successful in a number of studies. Next slide.
It's also useful at getting a stratigraphy. Here's the Double Ditch site. We have some of these large midden mounds and there was an excavation in 1905 and then we re-excavated the same profile in 2002 and you can see the sloping stratigraphy here, and so this shows that these midden mounds were built up laterally with basket loads of earth that were built up from like, here we see left to right through time, layers of ash and other sediments being piled up, and our radar profiled the same midden here. We could see some of the lateral stratigraphy showing the reflections right here, colored in at the bottom, so the profiles themselves are very useful for getting stratigraphy and other features. Next slide.
Here we see, a further thing we could do is if you take closely-spaced profiles, you could take a slice out of each one. It's somewhat time or depth beneath the surface, right? We call them "Time-slices" initially because they're vertical axes, it's a 2-way travel time and a microwave going into the earth, and if we take a bunch of adjacent slices at the same travel time, we could make time slice maps, so we see on the right, there, is a high, medium, and deep plan map extracted from these GPR profiles showing a house foundation in the middle, bedrock in the bottom, and then maybe a walkway near the top going up to this foundation. With a little bit of knowledge about how fast the microwaves are travelling, we could convert that vertical axis to depth beneath the surface, and generate what we would call a "Depth slice," which we see in the next slide.
Here we have a depth slice through an earth lodge village at Fort Clark. We see these circles are indications of the lodge floors and then a central hearth in the middle, and in between they would throw their rubbish. We have a deep midden in between and in the bottom, we see a corresponding GPR profile showing the household areas in yellow, the midden area with lots of reflections in between, and then a hearth showing up in the right profile, with the transit between A and B show on top, so these time slices are really great for earth lodge archeology to show some of these features, and in the same set we actually have super positioning of houses through time which is quite interesting. Next slide.
There are some issues with GPR. Sometimes I think it's too sensitive and in the upper left here, an aerial view shows some rodent damage, this is rodent spoil dirt. See all these white blobby things, and here's a GPR slice on the upper right images, I colored in these rodent mounds in red and you can see a lot of the anomalies are from rodents, so remember, we're sorting out the cultural from the natural where here's a natural cause for these anomalies and sometimes, it's hard to see the cultural features for all the other affected GPR will pick up, so every little rodent hole and sometimes every tree root and every little rock is shown by GPR and it's oftentimes too much, depending on the study.
Another GPR issues is reflection geometry. In order for something to be detected, those microwaves had to go down, bounce back, and be received by the receiver in the antenna, and the geometry could be totally round for that and sometimes these subterranean storage pits, the microwave bounces round and round and never comes back to the antenna, but often we have V-shaped defensive ditches around the villages which disperses radar energy away from the receiver. The U-shaped ditches however tend to focus the energy on the receiver in the antenna, and we see those quite well. These are some of the issues to contend with. It's not a totally fool proof method.
In the next slide here, we could see that when GPR works, it really does work and this is one of our nicer radar sets from a place called Pueblo Escondido in southern New Mexico on the Fort Bliss reservation, but we have all kinds of indications of pit houses and pueblos and this really shows a pit house and pueblo transition because we subsequently excavated. In the upper image there, these are actually independent pit houses placed adjacently to each other in a long row mimicking this later apartment complex puebloan tradition that goes on. The lower left shows the photo of the site. There's actually nothing on the surface except thousands of potsherds. GPR was a real boon to the site. We actually could focus on where the architecture was. Next slide.
That's what I close with, this. The question is, where does all this lead? Here we have Dr. Spock or Mr. Spock I should say from Star Trek, that was his Tricorder. We're not quite there yet but if you look at the magnetic houses in the upper right, those plan views look very close to excavated plan maps that we see from Raymond Wood’s excavations in the 1960s. We could see interior hearths, storage pits, entry ways and beyond. In geophysics, we could survey the whole village and see what's going on outside the houses. That's something archeologists never did in earlier days, but I think there's a rich future going on out there as sensors get better and the software gets better as well, so let me thank you for your attention and I'll take any questions, if there are any.
Karen: Ken, thanks very much for that very interesting talk. Does anyone have any questions? I have a question, Ken. I was really surprised that you could do vertical survey in mounds. Are the principles the same? That you're dragging your equipment over the surface of the mound?
Ken: It's a different methodology. We know radar will go vertically into the ground but often, these mounds are built of clay and clay is so conductive it disperses energy and in any case, radar often does not go very deep. Where we have these multilateral resistivity surveys now and what we did, we had 96 electrodes and a bunch of graduate students standing in these long lines inserting electrodes at 100 meter long transit. We have a huge box that switches between electrodes to do one profile and we would do a long profile about every 20 minutes roughly and just go on and on and on to the whole mound, but it's a different process entirely.
Karen: Wow. I'm willing to reveal all of my ignorance here, I've never been on a project that used any of the methodologies that you have discussed today and I'm really intrigued, like where do you get the equipment from? Do you build it or is it repurposed from another field or are there actually companies that are making this equipment for archeologists?
Ken: In general, it's bought commercially, so there's a number of companies out there that generate equipment and most of it of course is for geological prospecting in all these instruments, so there are a few companies that build exclusively for archeologists and design for archeological needs and some of the companies, they actually work with archeologists in the design of equipment. Bart Industries for example has worked with archeologists to build a gradiometer in a downhole magnetic susceptibility meter with respect to what archeologists need. There's a lot of options and the equipment tends to be fairly pricey but it's a good investment in the long run. Some of my gear has been working for a dozen years and more and still using it.
Karen: Okay, good. I know a number of people are still on. Do you have any questions or comments that you'd like to make? Ken, have you've done any projects with Park Service lands?
Ken: I've been involved with the National Park Service sponsored workshop that Steve Devore leads every summer. I've probably done that a dozen or 15 times and that's usually on a national park somewhere or a national monument, so I've been on a bunch of those. I thought about it, actually Fort Vancouver, I remember doing an independent survey there ... Hello?
Steve: This is Steve Devore.
Ken: How are you, Steve?
Steve: How are you doing?
Ken: Fine.
Steve: How come you left out susceptibility as a 5th method?
Ken: Well for lack of time.
Steve: I got you, but for Karen, mentioning the workshop, you want to come out? We'll show you how you use the equipment.
Karen: I would love to come out.
Steve: It's going to be at the Aztalan State Park in Wisconsin. It's a Mississipian temple mound site. It's in May 19th to the 23rd.
Karen: Wow. That sounds like a great, great opportunity. Make sure that you send us an announcement for the E-Gram.
Steve: I'm working on those.
Karen: Okay.
Ken: Just to put a plug in it, it's a great workshop because Steve collects a variety of experts and then manufacturers are there and it's a whole week of intensive field work and data processing and lecture and it's a great fun thing.
Karen: I've heard really good things about them. People who've taken those workshops have really sung the praises of the organization and the topics and the instructors that Steve’s put together. If you can make your way to one of them, it's a great opportunity for learning.
Meg: Hi. I have a question for Ken.
Ken: Okay.
Meg: Can you hear me? Hi, Ken. It's Meg Waters.
Ken: I recognize your voice. How are you?
Meg: I'm with the Northeastern office up in Lowell, Massachusetts today and you mentioned the largest scale landscape survey that they're doing in Europe, in England, Austria and those places with multiple sensors. My question is how far away do you think we are here in US with putting together those types of systems and using them to cover even larger landscapes than we currently are?
Ken: I think it rests in the economics and commercial viability of an undertaking. Right now, there's a number of smaller companies and various agencies and universities doing this kind of thing but there haven't been giant projects calling for this or a large number of projects calling for this kind of thing whereas in Europe, I think there's more state sponsored money to sponsor archeology at this time. If you build a large radar array with 8 or 10 antennas, you're talking of significant funding for that and I think most companies aren't prepared to do that at this time.
Meg: Do you think if we had a group of, say, universities or agencies that would come together with the finances, do you think it would be something to have a unit of accessible equipment? Would that be useful or used do you think in the US if we had something like that?
Ken: I sure could have used it. For the last 10 or 15 years, I've been serveying these large earth lodge villages in the Great Plains and it's an enormous undertaking and to think that one of those large arrays could do it in an afternoon whereas it took me weeks and weeks for each one, that would be really pleasing. I just think there's some interest out there in doing such a thing, so we have to be exploring into an economic feasibility.
Meg: Great. Thank you.
Ken: Good hearing from you.
Karen: Do we have other questions? Ken, thank you very much for talking with us today. I'm sorry that it was particularly challenging.
Ken: There's a tunnel underneath my building and they're drilling in in this tunnel underneath and I think right-
Description
Kenneth Kvamme, 2013, ArcheoThursday
Duration
1 hour, 9 minutes, 34 seconds
Credit
NPS
Date Created
11/06/2013
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