Ground Penetrating Radar Survey in the Saint John Lateran Basilica

An exceptional building in its own right, the Archbasilica of St John Lateran also lies within an area of intense archaeological interest. As explored elsewhere in this volume, extensive excavations beneath the Basilica have revealed its Constantinian phases, together with elements of the Severan Castra Nova and still earlier elite residences. Further investigations in the vicinity have exposed not only some of the earliest elements of the Basilica's baptistery, but also part of the Oratory of Santa Croce, itself overlying a major bath complex of the Severan period. An integral element of the Lateran Project's attempt to characterise these features better and to set them within the topography of the Lateran area has been a programme of Ground Penetrating Radar (GPR) survey. This paper outlines the methods employed in that survey and considers some of the key results to emerge. Advanced interpretation of GPR results is best undertaken in conjunction with material derived from a range of other sources and specialist fields; discussion of relevant material is, therefore, incorporated into the interpretation of survey data by area. Given the complexity of the archaeological deposits beneath and around the Archbasilica the team sought to focus GPR work on the following questions: 1.Private residences of the imperial period: Could the extent and layout of any of these buildings be identified? What might radar reveal about road systems and property boundaries? 2.The Castra Nova: How far might it be possible to identify the wall circuit of the Castra? How did it relate to the buildings around it? What did it replace when it was constructed between AD 193-6? 3.The Archbasilica: How far much could be learnt about the form of the Constantinian Basilica? What could GPR reveal about the later structural history of the building? 4.The Patriarchium: Could the layout and extent of the buildings cleared by Sixtus V by identified? These concerns in turn determined the areas selected for GPR survey, but always the team remained conscious that other features could emerge in any data sample which would in turn require further analysis.