Application of Ground Penetrating Radar (GPR) analysis on San Giovanni's Baptistery in Florence.

Among the non-destructive and non-invasive techniques, the Ground Penetrating Radar (GPR) is widely used to investigate shallow subsurface. GPR analysis is based on the propagation of electromagnetic waves and reconstruction of the medium is based on measurement of the elapsed time between transmission and reception of the impulse. Resolution and investigation depth are closely related to frequency of the signal, as the frequency increases, resolution increases and depth of exploration decreases.

Since, several decades, GPR was mostly used for the reconstruction of the geological features of the shallow subsurface and the detection of pipes, tunnels or hidden objects. More recently, thanks to the applications of higher frequency antennas, GPR has been used successfully to reconstruct structural features of buildings and masonry structures, providing critical information especially for historical buildings that underwent multiple construction processes and that are commonly missing detailed architectural information. For this reason, the application of this technique on historic buildings becoming more and more popular.

In this study, we present GPR analysis performed in the San Giovanni’s Baptistery in Firenze, that was built during the XI-XII centuries and totally covered by white and dark-green marbles and was object of conservation measurements through centuries under the supervision of the Opera del Duomo (OPA), with the most recent major conservation works performed around 1930. The Baptistery’s interior cover consists of a pseudo-dome, and its external roof is shaped like a pyramid with eight pitches.

Due to the lack of detailed documents concerning its masonries structural and textural assemblages, the GPR was performed with multiple antennas (by IDS Georadar, part of Hexagon) in single or array configuration (copular and crosspolar antenna disposal) on the entire structure of the edifices and focusing particularly on the roof and the extrados of the inner dome. The data were processed using GRED HD software, allowing to produce 2D and 3D pictures (tomographies) and allowing to define thickness and structure of the roof, inner dome and walls, so providing new information required to correctly plan focused conservation intervention.

This study is performed in the framework of the HGP (Heritage Ground Penetrating Radar) project (CUP:B55F21007810001) funded withing the Next Generation EU program.