GPR investigation to map the subsoil of the St. John Lateran BAsilica (Rome, Italy).

The St. John Lateran Basilica is the Pope's Cathedral and the first public building constructed for Christian worship. The complex has been the focus of sundry excavations since the 1730s. These have revealed traces of the earliest phases of the building, along with parts of the Castra Nova of the Imperial Horseguard, a bath complex and palatial housing. Interpretation of these excavations is, however, difficult; and most are either undocumented or only partially recorded. The geophysical prospection is generally considered as the attempt to locate structures of archaeological interest buried in the natural subsoil, but in many cases, when applied in urban centres, this attempt could fail due to the effect and disturbances caused by recent man-made structures in the subsoil, covering any signal related to possible archaeological structures. In the present paper the GPR surveys carried out in the urban archaeological site of St. John Lateran Basilica, in Rome, characterised by different targets and environmental conditions, are presented and discussed. This site is characterized by artificial medium as road pavement, outside the basilica, and ancient buildings, below the current basilica. The paper illustrates the ground penetrating radar (GPR) surveys and the obtained results.