Subsurface imaging a Viking-Age churchyard using GPR with TDR: Direct comparison to the archaeological record from an excavated site in northern Iceland

Abstract

The results of a ground-penetrating radar (GPR) survey over a Viking-Age Christian churchyard (ca. 1000–1100CE) that was subsequently fully excavated are reported. During excavation, broadband time-domain reflectometry (TDR) measurements of apparent relative permittivity were made of various features which helped to constrain interpretations of the radar data. Although four probes of different lengths were used (0.1, 0.15, 0.2 and 0.3m), the longest waveguide yielded effective frequencies that most closely matched the bandwidth of the radar data, which was collected using a 500MHz antenna.The excavation revealed 25 graves. Comparison of the radar data to the archaeological record indicated that all graves containing skeletal remains produced signatures from the bones, but only half of those were interpreted as such prior to excavation. The skeletons of three adults and two children as well as two pairs of isolated leg bones were identified beforehand, whereas five sets of remains (small child, infants and neonates) were noted only after reanalysis of the radar data following the excavation. Thirteen of the graves had been re-dug and their bones removed prior to 1100CE presumably for interment to a nearby more recent churchyard. The only indication of burial pits in the radar profiles was a break in a sub-horizontal reflector that corresponded to a perforated in situ volcanic tephra sequence. Six of the 25 burial pits were identified prior to excavation, eight more after reanalysis, and 11 had no signature because of the absence of preserved tephra in their vicinity. The TDR measurements confirmed the general lack of contrast between backfill material within pits and soil surrounding pits. This study provides one of the best documented cases for comparison of radar data to the archaeological record for a fully excavated churchyard, and confirms the direct detection of skeletal remains by GPR.