The Gothic Cathedral of Tortosa (1346), is superposed over a Romanesque building (1178) as a palimpsest, which there is no physical evidence. The geophysical survey (2102-13), has provided some relevant data from the previous seo, which have enabled the location of some of its main features: apse, pillars of the nave and façade. With these data it is possible to understand the fitting of the process, constructive/ deconstructive of both buildings. The study concludes that the main structural elements of both buildings fit as a gear, and the possible dimensions of the Romanesque Seo are deducted through the evidences found in the subsoil.