This morning I thought it might be fun to talk about geophysical scanning techniques.
If you are coming to this blog knowing about what I was up to in the field last year, or having read my CAA poster last week, you’ll know that I attempted a wide-scale Ground Penetrating Radar (GPR) survey at Ferryland, Newfoundland, in May of 2016, with the help of our wonderful curator and GPR tech, Maria Lear. I was looking for anomalies in the results that could indicate a burial in the subsoil, either high-contrast anomalies that might suggest a coffin or coffin hardware or a depression or slump in the soil layers where a grave had settled over time. What were my results? We’ll get to that in a moment!
More and more in archaeology, we are trying to find non-invasive survey techniques that will let us explore a site without having to excavate the entire area to get a good idea of the sub-surface. Because excavation = destruction, if we can use tools like GPR, Resisitivity, or Magnetometery to understand a site then we don’t need to dig the entire thing up. I used GPR for my research in Newfoundland, which transmits high-frequency radar waves while being pushed, pulled, or dragged across a surface, and measurements of the radar waves are taken from the time elapsed between the waves’ transmission to the reflection of the wave back at the machine (Conyers 2004: 1).
In many projects, GPR and other techniques have been used to identify unmarked graves below the surface (Conyers 2006, Patch 2009, to name a few). For all of these techniques, however, there is the little drawback of requiring certain soil and environmental conditions in order to achieve accurate results. Ideal conditions include dry, sandy and/or silty soils, not too many inclusions, fairly flat (for the radar to move across), not heavily saturated with water, and not too cold (also for the radar, they can be finicky!). Now, if you know anything about Ferryland or even Newfoundland in general, you’ll be all too aware that this is often hard criteria to meet!
Prior to my own GPR survey, I participated in two ‘practice’ sessions at other burial sites with Dr. Vaughan Grimes and Maria Lear. First, we undertook a survey of a fake burial ground containing several teaching skeletons at the MUN Botanical Gardens. Part of the 2016 Archaeological field school was to take place at the gardens, and our aim in the survey was to help pinpoint potential excavation areas, which is helpful for excavations with a limited time frame. While a large anomaly was identified, later excavation by the field school did not reveal any teaching skeletons in that area (see Lear and Grimes 2016).
Next, we decided that we should look for evidence of burials in a real burial ground. The field school in 2016 was also focusing on the Tors Cove burial ground and a historic site nearby in the town, so efforts were made to identify burials in the site that might not be marked as well as familiarize ourselves with how burial anomalies looked in the GPR results in a known burial ground. It is likely that the burial ground at Tors Cove, situated on a knoll overlooking the harbour, was in use in the early 18th-century, but has been closed for some time. Due to overgrowth of ground vegetation and a very uneven surface, the GPR had a very difficult time taking accurate readings, and several sections have data missing as a result of bumps and headstones. Results were promising, however, with several hyperbola around 1m deep indicating potential burials (see Lear and Grimes 2016 for images of results).
Finally, Maria and I conducted 3 days of GPR survey at Ferryland. Based the results of my statistical analysis of burial ground / settlement spatial relationships, which I’ll discuss in a later post, I had selected 4 locations which were of potential to contain human burials. We wanted to use the SmartCart with the 250mhz antenna for some areas, but unfortunately the cart malfunctioned on Day 1, so we switched strategies.
As you can tell from the photo, the terrain is quite steep for a machine that slides on the ground, and someone had to follow the machine and Maria the entire time to make sure the wheel didn’t slip as this would ruin the scans. One of the 4 areas had been used as a potato field for ages, so we were only able to run a few transects N/S, but not enough to give a clear idea of what the subsoil was like. However, there were some interesting anomalies in all 4 of the areas, and this helped me plan where I wanted to put trenches! Due to the vast size of the site at Ferryland, digging trenches without a very calculated plan would have taken far too much time, so using a GPR to help narrow down my search area was very helpful!
It was an interesting experience learning how to conduct a GPR survey in a place which has so many inclusions and disturbances in the subsoil. Even places where we knew burials had taken place (real or otherwise) provided mixed results, and my results at Ferryland were extremely loud as a result of all the locations having been used as farm fields and gardens for several hundred years. At the end of the 2016 fieldwork season, me and my amazing team of volunteers had excavated many trenches and units (1m x 1m) in all 4 GPR survey locations at the historic 1621 settlement, and many other spots were tested as well, but unfortunately no indication of human burials were uncovered below surface. The data provided positive results for a post hole in one location, and a strange slope in the subsoil that ended up being a section of the southern defensive ditch which surrounded the town at Ferryland in the early 17th-century but no graves.
I am returning to Ferryland to excavate in a more central location for 4 weeks this summer (2017), which is indicated as high-probability by the statistical analysis as well as proximity to the living settlement and the location of 3 gravestone fragments uncovered in previous years of excavation. Unfortunately I don’t have GPR data for that area because of the sheer amount of slate and gravel in the settlement area, but follow this blog for regular updates on the excavation for the month of July!
Conyers, L.B. 2004. Ground Penetrating Radar for Archaeology. Altamira Press: Walnut Creek, CA.
Conyers, L. B. 2006. Ground-Penetrating Radar Techniques to Discover and Map Historic Graves. Historical Archaeology, Vol. 40, No. 3. Accessed online at: http://www.jstor.org/stable/25617373
Patch, S. 2009. Identification of Unmarked Graves at B.F. Randolph Cemetery Using Ground Penetrating Radar (GPR). New South Associates. Submitted to Historic Columbia Foundation: Columbia, South Carolina.
Lear, M. and Grimes, V. 2016. From Airplanes to Cladestine Burials: A Preliminary Report on Archaeogeophysical (GPR) Surveys during 2016. Provincial Archaeology Office 2016 Archaeology Review. Volume 15. Department of Tourism, Culture, Industry and Innovation, Newfoundland and Labrador. Pg. 151-157. Available online at: http://www.tcii.gov.nl.ca/pao/arch_in_nl/index.html