Electrical Resistivity: Applications
Electrical resistivity survey is less widely applied by the Discovery Programme than magnetic gradiometry, in part, due to the slower field survey time. The frame electrodes need to make contact with the soil at each survey point making the survey time for each grid up to 2 - 3 times longer than with magnetic gradiometry. However electrical resistivity is an excellent way of identifying sub-surface stone, such as masonary foundations, which impede electrical current; or organic materials such as middens due to their improved conductivity.
In many projects electrical resistivity has been used in a targeted way to compliment the understanding of sites where extensive mangetic gradiometry has been undertaken. Some examples are given below with links to project pages where appropriate:-
Earthwork Survey, Tulsk, Co Roscommon
Left image: Electrical resistance, Right Image: Magnetic Gradiometry
This example highlights the different strengths of these two techniques. The strong signature (dark tone) identified by the Electrical Resistance survey to the centre right of the mound, suggesting a high concentration of buried stone, is not identified by the Magnetic Gradiometry. This has subsequently been subject to extensive excavation revealing the foundations and collapse of a substantial tower structure. For further details on this excavation go to the Tulsk Excavation page.
Carns Townland, Co Roscommon
This example again highlights the ability of Electrical Resistance survey to identify buried stone features. The remains of a substantial stone structure (dark tone, centre right) can be clearly seen, and contrasted with the lack of evidence on the Magnetic Gradiometry plot. Excavation of this structure has uncovered evidence of a possible medieval church site.