Something new — GPR

We decided to try something new this year, and we have rented a GPR (ground-penetrating radar) unit to survey the field around the main excavation site.

Ground-penetrating radar is a remote sensing method that is not just useful for archaeology. In fact, it is often used for things like locating gas or electrical services, voids, cracks, rock, ice and fresh water and changes in material properties.

David, Andrea, Jolyane, Brianne and Raine (not pictured) are in charge of this exploration.
David, Andrea, Brianne and Raine (not pictured) are assisting graduate student Jolyane (centre-right), who is in charge of this exploration.

The unit consists of a transmitter which emits high-frequency radio waves into the ground. These waves behave in different ways depending on the materials they encounter. For example, they may be reflected, scattered, or refracted. The unit also has a receiving antenna which listens for the return of the signal and the variation in the signal compared to the original signal that was transmitted. The processor unit compares the two signals and then generates a kind of map that you can interpret to read what is going on underground. The depth that can be “seen” under the ground surface depends on the electrical conductivity of the ground, the radiated power and the frequency of the transmission.

We know from historical documentary evidence that there were many structures in this immediate area that are no longer visible. We are hoping that the GPR will give us some targets that we can then explore through traditional excavation. The advantage to this method is we can peer under the surface without having to disturb it, and that helps us to be more efficient and strategic in our excavations.

A ground-penetrating radargram collected on a historic cemetery in Alabama, USA. Hyperbolic arrivals (arrows) indicate the presence of diffractors buried beneath the surface, possibly associated with human burials. Reflections from soil layering are also present (dashed lines). (Wikipedia Commons)
A ground-penetrating radargram collected on a historic cemetery in Alabama, USA. Hyperbolic arrivals (arrows) indicate the presence of diffractors buried beneath the surface, possibly associated with human burials. Reflections from soil layering are also present (dashed lines). (Wikipedia Commons)

Jolyane is a graduate student in the Department of Anthropology. She’s with us this season to supervise a crew to collect the GPR data and then to conduct some test excavations at the areas where we think there are targets. We are looking forward to seeing her results!

Here’s a little preview of the first grid they collected. The image above is an example of the recording of a line walked on the ground. While we certainly could wander around looking at individual lines, we instead have constructed 25m x 25m grids, where we have stacked lines in slices of the grid. These individual lines are then stitched together and we can look at them from a top-down perspective to see the hot spots which might indicate a buried archaeological feature.

GPR grid data, the large diagonal line you see at the midpoint of the screen running from left to right is an old road which is now buried under the lawn.
GPR grid data, the large diffuse diagonal line of mostly turquoise you see at the midpoint of the screen running from left to right is an old road which is now buried under the lawn. We have no idea what that red/orange line running from centre bottom to upper right is. 

Stay tuned, and we’ll follow along with the GPR team the rest of the season.

We’re officially on site as of Monday, so see you then!

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