Evidence of attenuation and interference phenomena in GPR signals for archaeological application

Abstract

We conducted an extensive geophysical field campaign using ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) in a former military area at the archaeological site of Nora (South-Western Sardinia). The purpose was to identify a Phoenician, and then Punic and Roman, necropolis of considerable interest, subject of a long-term investigation and currently under excavation. In the same area other buried structures, including a Roman road, were also investigated. The ERT and GPR surveys (the latter conducted with two different frequencies - 200 and 600 MHz), carried out in a sector where the Roman road was known to exist, did not reveal any clear evidence of the presence of this structure. However, the road was then found, by direct excavation, at 0.5 m depth. The failure in the identification of the road at this shallow depth has therefore led to hypothesize possible phenomena of high GPR signal attenuation and/or destructive interference related to the geometry and specific characteristics of the system. We performed synthetic GPR modeling by using the information provided by the excavation and constrained, in terms of electrical conductivity, by ERT measurements. The preliminary results of this simulation confirm the hypothesis of interference and attenuation phenomena, suggesting how the GPR signal may fail to provide an apparently simple piece of information at this archaeological site.