3D damage imaging of Carrara marble sculptures by acoustic tomography coupled to photogrammetry

Abstract

Marble is known for its sensitivity to weathering. Therefore, marble sculptures can develop superficial and inner alterations when subjected to weather conditions. Addressing a complete condition report of these alterations is essential for defining the conservation conditions and planning restoration. However, while superficial alterations can be reported by simple visual inspections, the identification of inner defects necessitate more complex inspection methods. Besides, the evaluation of the conservation state of cultural heritage artefacts requires the use of non-destructive techniques. This research thus aims at investigating the influence of exposure to weather conditions on the degradation state of two Carrara marble sculptures from the Roman theater of Arles, France, as well as at answering specific conservation issues coming from curators. To do so, acoustic tomography coupled to photogrammetry was carried out to document the superficial state as well as to probe the inner state of the objects under study. A new method is tested to locate the transducer positions (without any sticky material): the 3D photogrammetric models of the sculptures were projected directly onto them to mark the transducer positions immediately on the model. The 3D models were then used to measure the distances between each emitter and receiver position. This approach also allows saving the transducer positions on the 3D model, which provides a digital record of measurement positions in case of additional future measurements. Overall, the degradation state of the sculptures studied coincides with their degree of exposure to weather conditions during Roman times, highlighting marble sensitivity to climatic conditions. Besides, depth profiles were estimated for visible cracks with the time-of-flight diffraction method and showed that the most worrying crack had not propagated inside the sculpture as much as curators feared, pointing out that acoustic techniques can be support tools for conservators.