GPR survey to study the restoration of a Roman monument

This paper describes the ground-penetrating radar (GPR) survey carried out on the Roman theatre of Sagunto (Valencia, Spain) following recent restoration work in 1991. The structure has been substantially altered a number of times: it was remodelled during the Roman and Moorish periods, partially destroyed during the Peninsular War (1808–1814) and the Carlist War (1833–1839), and extensively modified during the 1930s and 1950s. Major reconstruction work was carried out in 1991 to convert the building into a working theatre. The GPR survey was performed to detect the possible effects of the 1991 remodelling on the historical remains and to identify the point of contact between the present-day modifications and the Roman remains. We took several on-site measurements of the wave velocity in the different materials in known areas to determine as accurately as possible the depth of the contact point. The velocity was calculated by measuring the depth in these areas and the two-way travel time of the wave. The measurements were taken from the walls and the tiers as the thicknesses of the materials were known in these areas. The recorded values were compared with the velocities reported by other authors and with the information from diffraction hyperbolas recorded in the radar data, which are caused by small objects inserted in the medium. We used these velocities to determine the exact point of contact between the Roman remains and the materials used in later restorations. The contact points cause the reflections that can be seen in the GPR data. The results also reveal considerable variations in the surface of the Roman tiers, which can be attributed to partial erosion in these areas caused mainly by water.