By Tyler Stumpf
University of Kentucky
The worldwide technological boom that has occurred since the 1970s has produced many new technologies we have come to enjoy and rely on: the internet, cell phones, and personal computers just to name a few. These technological advances have also altered how archaeologists go about their research. One of these new technologies is called Ground-Penetrating Radar, or GPR for short. While the modern concept of radar has been around since World War 2, more recently this technology has revolutionized how archaeologists approach learning more about people in the past.
So, what is GPR and how does it work? GPR is a geophysical method, or a method that relies on the physical properties of the Earth. It is used to analyze or understand the characteristics of what is located beneath the ground surface.
GPR works by sending a pulse of electromagnetic energy, in the form of a radio wave, into the ground and recording the strength and time of return of any reflected signal. Radar systems like these include a transmitter, which produces electromagnetic waves, and a receiver which measures potential responses. Reflections occur whenever the wave energy enters a material with a different physical property from the material it left. It can be used to determine the size, shape, or distance of objects in the ground. For example, a wave leaving the air and entering water would slow down, and produce a large reflection of energy. The strength, or amplitude, of this reflection is determined by how large the contrast is between the two materials. In short, GPR measures the difference in how fast a wave moves through the various materials present below the surface.
Archaeologically, this method can be utilized to assist in locating buildings, trash dumps, buried earthworks, mounds, unmarked graves, and many other archaeological features. Over the past decade archaeologists have begun to use GPR to gain insight into the characteristics of the subsurface without digging.
The use of geophysical techniques, like GPR, are important because they are non-destructive methods. Most people recognize archaeologists when they are digging or excavating in order to find information about the past. However, excavation is considered a ‘destructive’ technique because after you excavate a location you cannot put it back in the way it was left. When we dig, we only have one chance to collect as much information as possible – which is why techniques like GPR, alongside detailed notetaking, are important in helping archaeologists learn more about the past.
GPR can show archaeologists a visual representation of the differences present in the subsurface matrix, or dirt. Figure 3 shows a radargram, or a two-dimensional picture of the data produced during a GPR survey. These data can be taken and stitched together to form a top-down view of the data we call a time-slice (Figure 4). In this image (Figure 4) red represents high difference compared to the surrounding soil, whereas blue represents little to no difference compared to the surrounding soil. A good way to think of how this works is to picture using a GPR on the beach. The collected data would show blue everywhere because a beach is primarily sand. If red appeared, it would represent some type of inclusion within the sand, possibly a piece of metal, a rock, a pool of water, or some other material.
GPR is an important step forward for archaeologists who seek to learn more about people living in the past. This tool allows us to understand and gain more information about the past before the excavation begins.
Learn more about Ground Penetrating Radar used at archaeological sites in Kentucky here: