This is an increasingly used geophysical survey technique in which vertical electrical resistivity sections are generated tomographically from measurements along a linear array of equally spaced electrodes inserted at the ground surface. The array is multiplexed to a resistivity meter which gathers one set of all possible independent apparent resistivity measurements.

The software then uses complex processing algorithms to generate a geophysical section or profile. The closer the electrode spacing, the more detailed the image of the sub-surface can be generated, but at the expense of a lower penetration depth.

The Discovery Programme does not have the equipment to undertake this type of geophysical survey but it has been applied by geophysical specialists on selected projects.

Equipment

Electrical resistivity tomography has been undertaken on a number of sites for the Discovery Programme by Dr Paul Gibson, NUI Maynooth.

The equipment used is a Campus Geopulse resistivity meter. Electrical imaging is undertaken using an insulated multi-core cable with a number of fixed interval take off points to which electrodes are connected. The cable is connected to the resistivity meter which in turn is connected to a laptop computer which contains the relevant software to run the process. A parameter file was written to instruct the computer which sets of 4 electrodes to use and the currents to apply. In order to determine how the true resistivity varies with depth the data must be modelled using a computer program. One such program that is widely used in resistivity work is the inversion program RED2DINV.

Electrical Resistivity Tomography occassionally forms part of the overall strategy for geophysical survey. This adds a depth component which can further our understanding of sites where the plan approaches of magnetic gradiometry or electrical resistivity are applied.

Earthwork Survey, Tulsk, Co Roscommon

An important addition to the extensive geophysical investigation of the earthwork at Tulsk was the surveying of a number of electrical resistivity tomography traverses across critical parts of the earthwork. This resulting profiles give a clear indication of the physical compostion of the mound, and the concentrations and depths at which potential masonary evidence may be located.

Combined with the electrical resistance and magnetic gradiometry plans this information helps form the strategy for the Tulsk Excavation.