Capabilities and limitations of electrical resistivity tomography for mapping and surveying hillfort fortifications

Hillforts are fortified archaeological sites built from the Neolithic to Early Middle Ages within the area of Europe. They were usually surrounded by fortifications consisting of various combinations of ramparts and ditches, which today constitute their most striking remains. Although magnetometry surveys are commonly used for spatial identification of ramparts and ditches, a different method must be employed for directly obtaining depth information. Hence, we evaluate the potential of electrical resistivity tomography (ERT) for surveying hillfort fortifications. Within three hillforts of different ages in the Czech Republic, we investigated various features affecting the imaging ability of ERT, including bedrock type, building material, present-day condition of fortification, impact of past or recent agricultural activities, and field settings of the ERT method. Supported by additional information from magnetometry and electromagnetic surveys, the results show that ERT is most applicable in cases of stony ramparts, ditches carved into rocky bedrock or well-preserved earthen ramparts. Poorer results were achieved upon active and/or recently active agricultural lands, where fortifications have been gradually destroyed by ploughing. The remains of stony ramparts remained distinguishable in the latter case, but mere traces of earthen ramparts and ditches were invisible to ERT due to mixing of fortification material with on-site soil. ERT is a unique method for detailed investigation of both ramparts and ditches by which a structure and its extent can be evaluated to indicate the function of a settlement and obtain information about former environmental conditions, population, land use and/or human–environmental interaction.