Electrical Resistance Tomography (ERT)

ERT is a technique for producing “slices” through undisturbed archaeological deposits to a considerable depth.  Conventional resistivity measures the electrical conductivity of the soil between two probes.  This conductivity changes according to the archaeological materials present; for example, a wall might lead to low conductivity.  ERT uses an array of probes and compares the resistance between each and every pair in order to produce a cross-section or ‘pseudo’-section through the ground. It has proven particularly important at Portus since the early phases of standing buildings are buried to a depth of at least 3.5m and, in combination with GPR, ERT has been able to tell us something of the layout of these structures. 

View of the surface of the residential area of the Palazzo Imperiale, showing collapsed vaults and gridlines laid out for conducting an ERT survey
Photo: Hembo Pagi / ACRG
Profile showing the results of an ERT traverse, with different intensities of response coded by colour