An innovative method for in situ monitoring of the detachments in architectural coverings of ancient structures

Detecting the presence, the position and the extension of architectural coverings separations, both on horizontal and vertical layers – such as mosaics, plasters, slabs or other functional and decorative layers – is at the base of the analysis of the state of conservation of ancient structures. Non-destructive investigations used to find detachments, like Laser Doppler Vibrometry (LDV), are hardly executable in situ. They are also influenced by the boundary conditions and often need the intervention of onerous technical consultations. For this reason, traditionally, conservators rely on the technique of auscultation, which entails gently knocking with the hand, listening to the sound produced by the surface and perceiving the vibration of it. Together with the thermographic analysis, the results of these examinations are translated into the graphic documentation of the discontinuities of the structural elements, detachments and cracks between layers, in order to achieve the conservative diagnosis and outline a project of intervention. Analysis carried out with such method is accurate, but introduces a strong subjective conservator's connotation, which makes it not repeatable. In this contribution, after having briefly examined the state of the art of in situ technologies, an innovative acoustic-vibrational technique for the detection of detachment phenomena between layers and discontinuity is introduced. The proposed technique also allows tracing an accurate map, using an image correlation method between an acoustic “snap” of the point of interest and a “reference snap”. The analysis of the entire surface produces a graphics map of the detachments. This graphic documentation represents the “acoustic signature” of the structure, i.e. the “status quo” of the surface during the analysis. This representation can be used to monitor the state of conservation of the structure over time. In this paper, the method is developed and tested in laboratory using a personal computer and a hardware device specifically designed and fabricated, with the aim of providing a qualitative yet repeatable assessment of the state of conservation of a covering. The proposed method could be implemented in an easy-to-use and inexpensive portable version of the system using a smartphone, to which the developed hardware device would be connected through the audio jack to acquire the acoustic information, making the new system suitable for “in situ” operation. It can be also used for monitoring the evolution of the conservative scenario. The first results achieved on a test object are showed.