Karen: 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 archaeological 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 as LIDAR 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 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 survey 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 ditch is 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 see
natural 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-remnant
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 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 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 surveying
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.
