GPR Testing At St. Augustine's Fountain Of Youth Archaeological Park
Not too long ago I had the chance to bring a Ground Penetrating Radar (GPR) unit from Orlando up to the beautiful Fountain of Youth Archaeological Park in one of my favorite Florida cities, St. Augustine. The purpose of our visit was to assist researchers at the site in locating previous test units. That's right, we were looking for old holes! But these weren't just any test units, these were test units that John Mann Goggin had put in at the site decades earlier. The recent discovery of documents concerning these previously unknown/little known test units is allowing researchers to draw a more complete picture of a piece of this amazing site.
Initially, FPAN staff joined Dr. Kathleen Deagan, city archaeologist Carl Halbirt, and members of the SAAA, to excavate particular areas in search of features that might identify Goggin's units. The team was successful, but time and money (as is usually the case in archaeology!) prevented further "ground-truthing" at the site. The mapping is not over, however, because GPR will allow archaeologists to continually investigate and monitor the site without actually digging. So, how does the GPR unit help researchers understand better the subsurface features of archaeological sites?
An important point to note is that GPR does not "see" what is beneath the surface. In reality GPR can only show anomalies in the subsurface material. The antenna's receiver is recording the difference in the electrical conductivity of subsurface material. That is, these electromagnetic waves are going to travel through some material faster than it will travel through other types of material. Air is a great medium for these radar waves because there is little to no hindrance. Wet, clayey soils provide not only a lot of hindrance to the radar wave, but also the presence of water attributes to what is known as attenuation, or essentially the loss of the signal in any appreciable sense for the antenna. This can be dealt with by using different antennas of varying strength, knowledge of the soil matrix so that the GPR Unit can calibrated specifically for a location, and through utilizing processing software.
Initially, FPAN staff joined Dr. Kathleen Deagan, city archaeologist Carl Halbirt, and members of the SAAA, to excavate particular areas in search of features that might identify Goggin's units. The team was successful, but time and money (as is usually the case in archaeology!) prevented further "ground-truthing" at the site. The mapping is not over, however, because GPR will allow archaeologists to continually investigate and monitor the site without actually digging. So, how does the GPR unit help researchers understand better the subsurface features of archaeological sites?
GPR works on a pretty simple principle: Radar waves are propagated or pulsed into the ground via an antenna that is both the source and receiver of these waves. These are the same radar waves utilized by aircraft to locate other craft in the sky, so nothing most folks aren't already familiar with. GPR units used in archaeology are generally pushed on a buggy that is pretty much self contained; the battery, antenna and display screen are all connected together via the framework of the buggy.
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| GPR Unit at Fountain of Youth Park |
On the picture to the left we can see the antenna (big gray rectangular box at base of buggy), the batteries (in the black bag) and the readout screen (by the handles). That yellow box? That's where the magic happens.
There are many different types of GPR units and different antennas which are used for different jobs. On the left we have a 250MHz antenna attached, we also had a 500MHz antenna we tried out as well.
The GPR Unit is pushed over a set grid of transects which allows for x and y control. Think about the Cartesian coordinate systems you had to learn about in school; lay that out on a 40 by 30 meter grid and that's what we're talking about here. This is incredibly important for all of the VERY extensive processing that occurs after data collection.
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| GPR Unit Propagating electromagnetic waves into the soil |
An important point to note is that GPR does not "see" what is beneath the surface. In reality GPR can only show anomalies in the subsurface material. The antenna's receiver is recording the difference in the electrical conductivity of subsurface material. That is, these electromagnetic waves are going to travel through some material faster than it will travel through other types of material. Air is a great medium for these radar waves because there is little to no hindrance. Wet, clayey soils provide not only a lot of hindrance to the radar wave, but also the presence of water attributes to what is known as attenuation, or essentially the loss of the signal in any appreciable sense for the antenna. This can be dealt with by using different antennas of varying strength, knowledge of the soil matrix so that the GPR Unit can calibrated specifically for a location, and through utilizing processing software.
And now for a sneak peak of what we "saw" at the park:
Do you see those downward pointing hyperbolas to the left and at the middle of the reflection profile? That is what someone using a GPR is trying to find! Of course, there's a lot more going on here, but if you've read this far then you deserve to take a break. Go get a cookie and we'll be sure to update when we get the chance to collect more data.
Addendum:
Dr. Lawrence B. Conyers is one of the leading archaeologists utilizing and developing software for GPR. He also has a cool website you can check out if you would like to learn more about GPR applications in archaeology.
References:
Conyers, Lawrence B.
2112 Interpreting Ground-penetrating Radar for Archaeology. Walnut Creek: Left Coast Press, Inc.
Conyers, Lawrence B.
2004 Ground Penetrating Radar for Archaeology: Oxford: Altamira Press.
Johnson, Jay K., ed.
2006 Remote Sensing in Archaeology. Tuscaloosa: The University of Alabama Press.
Text: Kevin Gidusko
Credit to http://www.environmental-geophysics.co.uk/ for gpr diagram.
Pics: Kevin Gidusko