## 3 Innovative SHM Techniques
### Interferometric Sensors
The system, installed in 2001, consists of a network of different FBG sensors to measure displacement, crack opening, strain, and temperature. The sensors' efficiency was successfully tested during an 8-month demonstration period. [PERSON] et al. [112] described the design, implementation, and data acquisition of an SHM system based on FBG displacement and temperature sensors to investigate the structural behaviour of the Church of Santa Casa da Misericordia of Aveiro (Portugal), which presented some relevant cracks in the triumphal arch. The sensors registered displacements associated with both thermomechanical and structural effects. The effect of an earthquake that hit the region in February 2007 was also measured, showing that some displacement sensors measured a structural change unrelated to temperature variations. One of the main requirements of the project was to exert minimum visual impact and damage in the structure; thus, the sensors were bonded to the stone blocks using epoxy resin instead of by drilling methods. After the installation of the sensors, only the small supports could be seen, as the fibres were almost invisible. The authors concluded that this SHM system was optimal when planning restoration and conservation interventions. A reversible and non-invasive bonding method was developed by [PERSON] et al. [113] to install FBG sensors on delicate surfaces (such as frescos) to comply with restoration and conservation requirements. The study demonstrated that FBG sensors could be successfully used to monitor structures having architectural and artistic value. The possibility of using FBG sensors to identify the fracturing patterns of falling frescos was more recently explored by [PERSON] et al. [114]. The study was carried out by testing various plaster moulds equipped with short and long-gauge FBG sensors glued on the surface or embedded in the plaster, showing the pros and cons of the two systems. Tests led to the conclusion that using FOS to detect cracks and track their propagation  
Figure 6: FBG operating principle. can improve the global understanding of fracturing patterns and complement high-speed camera systems. However, some limitations and room for improvement were also reported. The first limitation was related to the limited range of the FBG peak-tracking algorithm, which led to missing measurements. Secondly, a complete fracture network could not be detected, as it would require a denser array of FBG sensors, which would involve high costs. [PERSON] et al. [115] investigated the effectiveness of FBG sensors for the static and dynamic monitoring of structures made of adobe, one of the most common materials used worldwide in the construction of ancient buildings with high historical and cultural value. The study was carried out by testing a full-scale adobe wall subjected to cyclic horizontal loading. The specimen was equipped with a network of 13 FBG displacement sensors and an FBG accelerometer to monitor damage and deterioration evolution during the destructive cyclic test, as well as to identify dynamic properties. The network of sensors proved to be simpler to install, with less cabling and low physical and visual impact on the structure, than traditional techniques. The first natural frequency determined using the FBG accelerometer was compared with that obtained from an electronic accelerometer and a seismograph, showing a low relative error (between 0.74% and 2.08%).