## 1 Introduction  
Rehabilitating any building requires inspection prior to intervention in order to collect as much information as possible about existing pathologies or damage. Based on that information, a precise diagnosis can be made to facilitate taking appropriate decisions about its restoration. Such analysis and diagnosis of the state of preservation is fundamental in the case of listed heritage buildings, especially of its structural safety. In order to ensure its structural performance is correctly assessed, the materials used in the various elements that provide its resistance, as well as the structure's overall behaviour must be characterised. Numerous techniques have appeared in recent decades that offer information about the condition of a building's structure and the quality of the results obtained has progressively improved. Non-destructive testing (NDT) is significant among technologies designed for structural assessment. Unlike conventional inspection techniques that may cause damage to heritage buildings, NDT provides detailed information about them without altering or affecting their earlier and highly valuable characteristics in any way. Prominent among the non-destructive tests commonly applied to buildings are impulse radar/ground penetrating radar, endoscopy, ultrasound, moisture measurement or infra-red thermography [1]. Numerous papers cite optimal success with these systems in obtaining substantial data for suitable decision-making in rehabilitation processes [2; 3; 4]. However, these survey techniques also entail certain negative aspects, one being the huge volume of data they can produce and secondly, the expertise required by technicians to analyse and interpret that data [5]. Consequently, more in-depth examination is required of how these various techniques can be applied and the outcome analysed in order to facilitate their calibration, interpret their results and reach a subsequent diagnosis to support decision-making. From the point of view of historical buildings with load-bearing walls, regardless of the type of construction, one issue that tends to cause controversy among experts is identifying the parameters needed to characterise their mechanical behaviour properly [6; 7; 8]. Defining the mechanical properties of the materials depends on parameters such as the modulus of elasticity (E), shear modulus (G, \(\upmu\)) or Poisson ratio (\(\upnu\)), all of which need to be defined accurately in order to assess their structural safety using numerical models properly [9; 10; 11]. Certain simple procedures based on assessing the construction of the masonry define those parameters, such as the masonry quality index (MQI) [12], which provides a simple and systematic approach whereby, in view of the wall's construction configuration, it is possible to specify an approximate range for compressive and shear strength (\(\mathrm{f_{m}}\) and \(\uptau_{0}\)), and for the elastic modulus or tangent/Young's modulus (E). These values are sufficiently accurate to effectively estimate the masonry's structural behaviour, as ratified by various researchers [13]. Apart from the above procedure using quality assessment criteria and in view of the complexity of the problem given the large number of variables, other techniques need to be used to obtain empirical information about the masonry under assessment. One such technique that provides substantial data on the state of the structure relates to the fundamental period/frequency, based on which, each case study can be individually analysed and diagnosed, enabling specific areas with the probability of resonance to be identified and the relevance of existing damage to the structure or its seismic vulnerability to be estimated. The natural period (\(T\)) of a system can be defined using Equation (1) as a function of the system's total mass (\(m\)) and its lateral stiffness (\(k\)).