Integrated non-invasive sensing techniques and geophysical methods for the study and conservation of architectural, archaeological and artistic heritage

In the last two decades, the use of non-invasive methods for the study and conservation of cultural heritage, from artefacts and historical sites to recent architectural structures, has gained increasing interest. This is due to several reasons: (i) the improvement of performance and information resolution of sensors and devices; (ii) the increasing availability of user-friendly data/image analysis, and processing software and routines; (iii) the ever greater awareness of archaeologists and conservators of the benefits of these technologies, in terms of reduction of costs, time and the risk associated with direct and destructive investigations of archaeological sites (excavation) and monuments (i.e. masonry coring). The choice of diagnostic strategy depends on the spatial and physical characteristics of the cultural objects or sites, the aim of the investigation (knowledge, conservation, restoration) and the issues to be addressed (monitoring, decay assessment, etc). This makes the set up and validation of ad hoc procedures based on data processing and post-processing methods necessary, generally developed to address issues in other fields of application. This methodological perspective based on an integrated and multi-scale approach characterizes the papers of this special issue, which is focused on integrated non-invasive sensing techniques and geophysical methods for the study and conservation of architectural, archaeological and artistic heritage. In particular, attention is given to the advanced application of the synthetic aperture radar (SAR) from the satellite-based platform for deformation monitoring thanks to the innovative differential SAR interferometry (DInSAR) technique; Zeni et al show the significant possibilities of the proposed methodology in achieving a global vision not only of cultural heritage but also of the embedding territory. This collection also deals with the application of non-invasive diagnostics to archaeological prospecting, and cultural heritage monitoring and diagnostics. The paper by Conyers et al gives a very good review of the limitations and advantages of ground penetrating radar (GPR) in archaeological prospecting, where the focus is on aspects of data processing and image visualization. Kadioglu et al describe a case that exploits GPR to define buried archaeological remains at the Zeynel Bey Tomb in the ancient city of Hasankeyf, Turkey. In particular, attention is given to a new three-dimensional visualization of the scene which provides a clearer representation compared to the usual constant depth slices. The paper by Bavusi et al is concerned with the use of GPR as a tool to monitor an important recent architectural masterpiece and critical piece of infrastructure, the Musmeci bridge in Potenza; in particular, the complexity of this civil engineering structure is tackled by adopting advanced data processing tools such as microwave tomography. A microwave tomographic approach is also exploited for GPR data processing in cultural heritage diagnostics by Brancaccio et al, where their survey is concerned with the masonry diagnostics at St Carmine's Church, one of the more ancient and important monuments of Naples. This special issue also gives a good overview of state-of-the-art methodologies and appealing cases of data integration and correlation. Nordebo et al present a new and advanced approach to information fusion, used here to combine ERT data from different orientations. The approach is general, suitable for sensor integration, and opens up a new route to data fusion for diagnostics and monitoring. Some very interesting cases of data integration in archaeological prospecting and cultural heritage monitoring are also presented. Loperte et al present an interesting case of an integrated geophysical approach based on magnetic, GPR and geoelectric techniques used to investigate the Greek and Roman settlement of Paestum in southern Italy; their aim is to undertake preventive archaeological research on an area of the ancient settlement affected by new construction. Another interesting case of on-field integration of GPR and seismic techniques is presented by Leucci et al; their aim is to characterize the deterioration status of the pillars of the cathedral of Tricarico, in the Basilicata region of southern Italy, where the diagnostics have provided crucial information that is helping to drive the restoration work. Another example of sensor integration is presented by Carlomagno et al, which is focused on the joint exploitation of infrared thermography and GPR for inspection of architectonic structures. Two important studies are presented: the first aims to evaluate the conservation of frescoes as well as the state of the underground masonry structure at Pompei (Naples); the second aims to evaluate the state of degradation of remnants of a theatre at a site in Nora (Cagliari). The integration of different diagnostic techniques is shown to be important also for movable pieces such as the sculpture of the Pharaoh with the god Amun, one of the most important pieces in the 'Museo delle Antichit? Egizie' of Turin. In particular, Sambuelli et al demonstrate the joint use of GPR ultrasonic 2D tomographies to assess the integrity of the base of the statue. Capizzi et al propose the integration of a pacometer, high frequency GPR and ultrasonic techniques to investigate the internal extension of all the visible fractures (and to search for hidden ones) of a marble slab (second to third century AD) from the archaeological museum of Rome, which was damaged in transit for an exhibition. Finally, this special issue also presents cases of surface and material characterization, which is a field of increasing interest in monumental heritage monitoring. Camaiti et al present a survey that uses a portable hyperspectral device to characterize the alteration affecting the marble facade of the Santa Maria Novella church (13th century) in Florence. Their analysis shows how hyperspectral instruments may be considered a powerful tool for characterizing historical surfaces in a non-destructive and non-invasive way. Fort et al describe an advanced petrophysical analysis of several types of natural stone used in Spain that can be used to determine their anisotropy, which is one of the key factors affecting the quality and resistance to decay caused by a variety of different agents, such as water.