Decay Detection in an Ancient Column with Combined Close-Range Photogrammetry (CRP) and Ultrasonic Tomography

Abstract: This study presents the integrated application of a few non-destructive techniques, i.e.,
Close Range Photogrammetry (CRP), and low frequency (24 kHz) ultrasonic tomography complemented
by petrographical analysis. The aim here is to assess the conservation state of a Carrara
marble column in the Basilica of San Saturnino, which is part of a V-VI century Palaeo Christian
complex in the city of Cagliari (Italy). The high resolution 3D modelling of the studied artifact was
computed starting from the integration of proximal sensing techniques, such as CRP based on the
Structure from Motion (SfM) technique, which provided information on the geometrical anomalies
and reflectivity of the investigated marble column surface. The inner parts of the studied body were
inspected successfully in a non-invasive way by computing the velocity pattern of the ultrasonic
signal through the investigated materials, using 3D ultrasonic tomography. The latter was optimally
designed based on the 3D CRP analysis and the locations of the source and receiver points were
detected as accurately as possible. The integrated application of in situ CRP and ultrasonic techniques
provided a full 3D high resolution model of the investigated artifact, which made it possible
to evaluate the material characteristics and its degradation state, affecting mainly the shallower
parts of the column. The 3D visualisation improves the efficiency, accuracy, and completeness of the
interpretative process of data of a different nature in quite easily understood displays, as well as the
communication between different technicians.