Hi, I'm Dave Givens, the Director of Archaeology here at the Jamestown Rediscovery project
and I'm standing down in a test unit that we had filled in and lined with sandbags a number of
years ago. So this was an area where we talked to the public about the 1608 extension to James Fort.
Now right here below me, is the palisade, representing that line. It came out of
the north bulwark to the west of me here, and then east over here to a block house,
which has since been excavated and filled back in. Now you'll see around the bottom, we put
down geotextile fabric and protect it, but often we'll leave an area, and we don't dig everything
we find, we'll leave it open and in this case it's the subject of this week's dig deeper video.
Dave: Now what we're doing here with this, and I'd love to backfill this and preserve this for future
archaeologists, is we want to run high frequency GPR on this. Now my colleague, Peter Leach from
GSSI, who makes the equipment that we use here at Jamestown, has flown in to join us here and
we have this theory that we can use the GPR in different methods. Now the 1608 palisade, like
I said, was constructed as an extension onto the original 1607 James Fort and was built by using a
tool actually called a scuppet, a very narrow hoe, and they dug a flat bottom straight sided trench,
put the logs upright in that trench, and then backfilled them. So what we see archaeologically
is the remains of the trench, mix fill, mix aggregate clay e-horizon soils and then stains
of posts that almost look like fried eggs as they go along and then terminate over here
next to the building. So our hopes today are that we can push the science forward of Ground
Penetrating Radar (GPR) and actually be able to see, not only the feature, because we're
confident we can do that, but actually see if we can see different layers and the structure of it.
So the work we're conducting is typifies the ongoing relationship between GSSI and Jamestown
Rediscovery as research, right? So we're not all, it's not always about digging things up, sometimes
it's about finding out about the past through science, right? Archaeology is a science and so
Peter's gonna explain how we conduct that science and how GPR can push that research forward.
Peter: I'm standing on a surface, an excavation surface, here where we can see the top of
the 1608 palisade feature and so we decided to use a high frequency GPR antenna, which is typically
restricted only to use in concrete. And it's very high resolution, but it the trade-off with radar
antennas is that you get real high resolution. You can see small objects, small soil changes,
but you don't get a particularly good depth penetration. We might expect maybe a foot,
foot and a half here, max. But the idea was, we wanted to use a high frequency radar antenna
to scan across the palisade feature and see if we could discern, first of all discern the feature
itself from the natural surrounding stratigraphy, the soil matrix, and to see if we could image
internal characteristics of the palisade feature for use in other situations where the palisade may
not actually have been excavated or exposed and we may be able to view it from the surface down
and not have to excavate. So we used two different antennas in two different configurations for this.
First, we used a StructureScan Mini XT GPR that's a self-contained unit at 2,700 megahertz, that's
the highest frequency antenna that we make and we actually attached a palm antenna to that which is
more archaeologically sized, it fits in your hand, which is excellent for the tight space constraints
inside of an excavation unit. And so we collected data in a configuration called normal polarization
and it's the way that data are usually collected. We collected a grid at one-inch intervals
across a seven-foot expanse along a very well surveyed grid, so that we can control
the line spacing, we can collect straight lines, we can do this very scientifically.
What we next did was we used a technique called cross polarization, where we turned the
antenna 90 degrees to its normal path of travel, and still collected lines in the same direction
and that's never been done for archaeological work to my knowledge and usually it's not done
anywhere except, again, in concrete environments where you're trying to find rebar and conduit.
The idea being that in normal polarization the energy leaving the GPR antenna spreads out with
depth at approximately a 90 degree angle front to back and so that generates the classic hyperbolic
targets that you see in GPR data, these upside down use, looks like a fish on a fish finder,
and that gives you your normal looking data, but to me, that spreading with depth has the potential
to distort soil layers and other targets at greater depths, because the wave front is
spreading out so far. And so in cross polarization mode, we're going in the forward direction
the cone of penetration, the wave front, is really only spreading out at a 45 degree angle
and so the effects of more distant targets and layers is not as, it doesn't have as
much of an impact on the radar data, and it, to me, I hypothesize that will actually
focus the radar data and give us a better representation of soil layering and stratigraphy.
And so we collected those data with the Mini XT in normal polarization and cross polarization
and then we decided that we wanted to use a lower frequency antenna, so we went from 2,300 megahertz
to 1,600 megahertz to try to get deeper penetration. Of course, that antenna has
lower resolution than the other one, but that's also part of the experiment, because
a lot of times, in all walks of life, we find the closer you look at something the uglier it gets,
most of the time. With GPR, it's the same. So the higher the resolution you have
the more stuff you're picking up, but that could be archaeological materials that we're interested
in, but it also is soil clutter wormholes, rootlets, any type of coarse clast in the matrix.
And so we used that antenna in normal polarization and we cross polarized it so on and we collected
exactly the same grid lines, exactly the same length. So now we can compare the two directly
and the idea now is that we have to go and post-process the data, much like a professional
photographer will use Photoshop to enhance and improve their photographs we use a software
that my company makes called Radan7 and it's like photoshop for GPR, in the sense that we can clean
up and optimize GPR data, take away some of the noise, drop down the highs, bring up the lows,
and improve the overall data. The other thing that we can do is since we collected data on a grid,
we know where every single data collection point was, in space. We can funnel that into
our Radan7 software, we can construct a 3D cube of the survey area,
that we can then slice down through like we were using a digital trowel to slice soil layers away,
and what that allows us to do is with profiles we can look at cross sections like looking at
the wall of an excavation, but with the time slices, the 3D cube, we can slice down through
the ground and actually see the geometry, the shape of targets and features and that gives us
one more level of interpretive information that we can use in other situations when we're not looking
directly at the feature with our eyes. Perhaps we're a foot above on the modern ground surface
and so we can use all of this information to build a more holistic interpretation
of the feature and the behavior of the radar in this archeological context.
Peter: So here we have our SIR 4000 control unit and this is what we use to collect
the 1,600 megahertz data that I mentioned,
both cross polarized and normal polarized and what we see here on the screen is a typical,
a representative profile of the GPR data from the 1608 palisade survey. What we have here is
we're looking at a vertical profile. This is very similar to looking at the wall of a backhoe trench
or looking at the vertical wall of an excavation, an archaeological excavation. We have depth on the side,
and we have distance in the along the top, both of them are in feet. And so first when you
first look at GPR data you might be thinking that it's really just a photograph of a pile of dead
zebras or something. It is not, there are actual data, in here and so what we have is, you can see
right in the center here between the one-foot and three-foot marks, there's a very bright area here,
and then on the sides, it's actually quite weak, really low amplitude.
And so, what we have is this, between one and three feet, is actually the 1608 palisade like
in cross-section view like we had excavated, you know, cut a line across it and we're looking at
it from the side. And so, this is the palisade and you might have already figured out that over here
and over here is just natural undisturbed subsoil, and so this is the soil matrix
that is, that's natural soil and here is this, the cut from the trench and so we have
the sides of the trench here, and here, and then inside we have a lot of other layers, we have a
lot of these hyperbolic targets, that suggest that there's maybe some targets in there, like rocks.
Perhaps we've gone over the decayed remnants of one of the original posts that was part
of the construction of the of the palisade. And then, we have, as we get deeper into the ground,
we start to get an area that we call attenuated and so this is really staticky, it's really noisy,
and it's telling me that we're starting to max out the depth of the antenna in this location, because
an antenna will eventually bottom out. You'll, it will not, there will not be enough energy left to
penetrate any deeper and so this is good actually, I want to see this on the bottom of a GPR profile,
because it tells me, without a doubt, that I penetrated as deep as I possibly could
into the subsurface and any additional depth information would have just been all noise and
it wouldn't have been really worth collecting, but I'm very, very satisfied with these data, because
what we're seeing is that the palisade shows up extremely well and the best part is,
one of the most important things in GPR data is contrast. Is something anomalous compared to
the undisturbed soil that does not have any palisade features in it? And in this case,
the 1608 palisade is very, very apparent it's very, very obvious, it contrasts very sharply
with the undisturbed subsoil. And so that is a really good result going forward, because we know
now that GPR behaves quite well in this environment and going forward we can use
this as a diagnostic signature to find other sections of this palisade that have not been
excavated before and then we can map it on the ground without having to do excavation.
Dave: I'm certainly excited about what we found out so far, with just the 2D data and just
running it across and I'd really like to see this apply to other features that we've had out here.
And going forward, I think we'll really be trying it out.
Peter: Yeah, for sure. There's so much research potential out here, I mean radar,
we've already seen in our other research that radar works very, very well out here,
whether it's lower frequency or high frequency. We've had a lot of success and I see more in the
future and I think that we're moving towards a digital trowel kind of method, where we can use
GPR to really learn a lot about features without necessarily having to completely excavate them
and it really is in support of a preservation and place model where
we do need to preserve archaeology for the future and one of the ways that, even if we are going to
excavate it, doing GPR will give us a digital archive that can supplement and augment the
traditional archaeological methods. So this has been really great and I think that
when we start looking at the data and comparing it to other things it's going to be extraordinary.
Dave: Yeah so, our next series on this, we'll process the data and let you all
know what we found, so stay tuned, and thanks for watching our show, guys.