## 2 Conventional SHM Techniques
### Dynamic Monitoring Systems
Then, the structure was instrumented with nine triaxial strong  
Figure 4: Different types of dynamic sensors: force-balance accelerometers (**a**), from [38], and MEMS accelerometers (**b**), from [40]. motion accelerometers, registering a minor earthquake of 4.8 magnitude. The vibration shapes and frequencies obtained from ambient vibration were used to calibrate a linear FEM for preliminary seismic analysis of the structures in the linear ranges. The data from the earthquake response showed a drop in frequencies of the first two vibrational modes, even if the earthquake was small and no visible damage was detected in the structures. The authors concluded that microcracking diffused throughout the whole building might be the reason for this change in the dynamic properties. [PERSON] et al. [22] presented a methodology based on ambient vibration tests and operational modal analysis for the damage identification of masonry structures. The method comprises four phases: data collection, simplified health monitoring, detailed health monitoring, and local complementary non-destructive testing. The procedure proved to be suitable, even for complex historical constructions, with the analysis of two case studies, the Clock Tower of Mogadouro and the Church of the Jeronimos Monastery in Lisbon. Dynamic SHM systems have also been used to monitor two monumental Roman heritage structures, the Roman Arena of Verona and the Flavian Amphitheatre (the Colosseum) in Rome. [PERSON] et al. [44] presented the results of the 1.5-year monitoring of the Roman Arena of Verona, which involved static displacement measurements and tracking the fundamental modal parameters (natural frequencies, damping ratios, and mode shapes). The dynamic monitoring system, comprising 16 uniaxial wired accelerometers, detected moderate seismic events following its implementation in 2012, allowing for the identification of the peak ground acceleration at the foundation, maximum acceleration at the monument's top, and modal parameter variations. The authors developed and implemented robust routines for automatically processing monitoring data, transmitting them online to a central server. This processing software provided nearly real-time information on the structural health condition, acting as an early warning tool  
[PERSON] et al. [74] presented the results of the first two years of dynamic monitoring (2014-2015) of the Colosseum in Rome. The system comprised a wireless network of accelerometers and a backend server allowing users to access measures remotely. The system monitored the long-term vibrations induced by vehicles and subways, thus providing experimental support for determining acceptable acceleration levels that would not damage the structure. [PERSON] et al. [41] described the application of a dynamic SHM system to three medieval towers, focusing on understanding the effects of temperature variations on natural frequency changes over time. The authors observed that natural frequencies increase with increased temperature, which can be explained by the closure of superficial cracks caused by the material's thermal expansion. The same result occurred in freezing conditions: indeed, the freezing of the structural system and the presence of ice filling the cracks made the structures stiffer. [PERSON] et al. [75] presented the results of long-term dynamic monitoring campaigns regarding different monuments in urban areas. The study examines the impact of various vibration sources, especially wind and human activities, on the dynamic behaviour of the monitored structures. Besides the daily and seasonal fluctuations resulting from environmental factors variations, the authors also investigated the impact of reduced high-frequency ambient noise during the SarsCov2 pandemic lockdown on one of the monitored buildings. The authors observed a significant decrease in average noise levels while implementing safety measures.