Effects of a conductive rail on parallel apparent resistivity measurements

Apparent resistivity measurements made in proximity to a rail track are likely to be affected by preferential current channelling through the conductive rails. This report quantifies these effects using a simplified semi-analytic model to represent the rail. The results show that the commonly used measurement configurations are all affected in a similar manner, with greater distortion of the data at larger depths of investigation. The degree of distortion also depends on the contact resistance between the rail and the ground. The effect is least, however, for the Dipole-Dipole configuration, which is therefore recommended for resistivity surveys undertaken parallel to conductive rails. It is also recommended that the rail-ground contact resistance be measured during repeat resistivity surveys to allow the effects of the rail to be evaluated.