The question of the eastern extent of the Variscan externides in the Polish Lowlands is a subject of ongoing scientific debate. The key to resolve this problem is to provide a good seismic image of the pre-Permian strata. Unfortunately, the industrial seismic profiling brings sparse information from below the Zechstein, thus in 2003, a special seismic experiment called GRUNDY 2003 has been organised. The acquisition system was based on the deep seismic sounding equipment and was targeted at the recognition of Palaeozoic strata. The results in form of the 3D tomographic velocity field and the migrated depth section form the basis for the geological interpretation. Deduced geological model suggests the existence of two different geological media below the Permian strata. The likely interpretation is that it is the contact zone of the Variscan externides with their foreland. Obtained results are of a great importance for the hydrocarbons exploration perspectives.