Innovative Techniques for Sub-surface Investigations

Diagnostics and monitoring of cultural heritage sites are mandatory as part of the preservation of artistic artifacts. The more delicate and prone to degradation a work of art is, the more efficient and informative a diagnostic technique must be. The innovative techniques described in this paper were applied to the study of walls, whether painted or unpainted, to determine the moisture content and degree of salinity to gather information about the porosity in the sub-surface, and to detect the presence of voids and anomalies at a depth of several tens of millimeters. The techniques used (evanescent field dielectrometry [EFD], unilateral nuclear magnetic resonance [NMR] and holographic radar) were largely complementary. EFD and unilateral NMR can measure the water present in a wall from a few to approximately 20 mm beneath the surface. EFD can detect the presence of soluble salts before efflorescence takes place; unilateral NMR can estimate the porosity of a wall; and holographic radar can provide information about the texture of a wall and detect any large detachments and cavities present therein. The above techniques were applied to the ancient San Rocco Church located in Cornaredo, a small town near Milan, Italy. These techniques were applied on a very degraded fresco painting, with the main objective of quantifying the increasing dampness. As a result, moisture and salinity images of the wall were obtained by EFD and unilateral NMR, determining the moisture distribution from a depth of 5 to 20 mm inside the wall, and the distribution of soluble salts in the sub-superficial layer. Furthermore, the presence of detachments and material anomalies were detected by means of a holographic radar survey.