- Copyright © 2002 Society of Exploration Geophysicists
A geophysical study of the Scarsella vault, the high altar of St. John's Baptistery in Florence, was conducted to determine the internal structural features and thus ensure its proper restoration. The Scarsella's crossvault is covered with precious mosaics dating back to the Middle Ages. The vault is 10 m long and 5 m wide, varying in thickness 5–2.5 m.
To determine the extent of some fractures known to exist on the internal surface and to define the structural features of the vault itself, three different geophysical techniques were applied: electrical resistivity tomography (ERT), seismic transmission tomography, and ground-penetrating radar (GPR). Data reduction involved both 2D processing and inversion of the ERT and GPR sections, 3D inversion of seismic tomography, and 3D rendering of ERT and GPR images. An overall interpretation was obtained by correlating the different results, leading to a more detailed knowledge of the vault's internal structure.
Data acquisition was done with extreme care to prevent any possible damage to the mosaics while still ensuring high-quality measurements. A high-precision topographic survey of the vault was necessary to process the data, especially in the case of seismic tomography, where a precise location of sensors and shotpoints is crucial. The 3D image of the investigated structure is shown in Figure 1.
Electrical resistivity tomography
Electrical resistivity data were acquired using slim 2.5-cm long needle-shaped, AgCl-coated silver electrodes, which were carefully placed in the joints between the tesserae of the mosaic. The contact resistance between the electrodes and the vault material was minimized by using medical conductive gel.
ERT was carried out along seven profiles from R1 to R7 (Figure 2) located on the internal surface of the vault, both longitudinally (R1 to R5) and transversely (R6 and R7). The survey resulted in about 2500 …