Survey and Sequence Strategies for Full-3D Electrical Resistivity Tomography in Archaeological Sites: A Case Study on a Domus of the Roman Town of Augusta Bagiennorum (NW Italy)

Preliminary geophysical investigations are a cost-effective and efficient way to screen archaeological sites and locate buried structures. Ground-penetrating radar (GPR) is one of the most widely used methods for archaeological prospection, but in some sites, it cannot be employed effectively due to the presence of clay or other electrically conductive materials, which strongly attenuate the electromagnetic signal, or due to bumped terrain, which demands rigorous signal analysis. Alternatively, electrical resistivity tomography (ERT) can be adopted in these situations. However, ERT is not as frequently adopted as GPR for archaeological purposes because it is more time and cost consuming and, generally, has worse resolution. In this study, we aim to test a full-3D ERT approach to improve the imaging quality of ERT surveys for archaeological prospections. We develop specific survey strategies, including a custom open-source quadrupole sequence generator, studied for achieving high sensitivity to archaeological remains within the first metres of subsoil. We performed a test survey on a well-known archaeological site (the Roman town of Augusta Bagiennorum, NW Italy) and compared the results with a state-of-the-art multichannel GPR acquisition. The results showed that both GPR and ERT equally located the outer and inner walls of a complex Roman residential building. Moreover, the ERT could locate two targets, barely visible in the GPR survey, with the antenna used. We also compared the results of our full-3D ERT approach to a more common quasi-3D approach. We found that the full-3D approach overcomes the directional bias found in our quasi-3D acquisitions and provides a more accurate subsurface resistivity model. This methodology is ready to be employed in other archaeological sites and, differently from GPR, can easily operate on bumped terrain, in the presence of clay, and potentially reach greater investigation depths.