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.