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?
|GPR Unit at Fountain of Youth Park|
|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.