A perturbation in the flow of a telluric current. In what is assumed to be an isotropic homogeneous medium at or near the Earth’s surface, telluric current flows uniformly and there are no discernible potential gradients. The presence of a subsurface geologic structure (e.g. a salt dome) perturbs the line of flow of the telluric current, forming a potential gradient which can be measured (the telluric anomaly) using non-polarizable electrodes spaced several hundred metres apart.