## 4 Results and Discussion  
Figure 4 shows the DEMs obtained in 2018 and 2020 and Figure 5 shows some details of the last one transparently overlaid onto the processed orthophoto. The DEM analysis in 2018 (Figure 3a,b) highlighted depressions and reliefs relative to a hypothetical plane passing through the point of origin of the reference system (P = 0,0,0) in a spatial range of 3.66 cm. The reliefs were mainly located on the frame of the panel where the swellings responsible for the detachment of some mosaic tiles were clearly highlighted. The depressions, on the other hand, affected the central and right portions of the mosaic. The presence of an internal iron frame at the edges of the panel could, on the one hand, have favored the onset of bubbles and swelling as a consequence of the natural oxidation processes of the metal; on the other hand, it could have resisted deformations due to volumetric variations of the mortars in response to changes in temperature and humidity and/or deformations triggered by kinematic stresses (natural or artificial). This could explain the existence of depressions detected in the central part and the onset of lesions, especially on the edges (most resistant points) and the points of greatest weakness of the panel (central and right part). In order to identify the surface variations from 2018 to 2020, the point clouds of the surveys were metrically compared through the CouldCompare software (Version 2.11 beta, Telecom ParisTech and the R&D division of EDF, Villeurbanne, France). Figure 6 shows the absolute distances between the point cloud obtained by subtracting the 2020 data from  
Figure 3: Grid data acquisition (**a**) with an IDS georadar (**b**). those of 2018. On a global scale, a uniform swelling of the central part of the mosaic was estimated to be in the range of about 4 mm. Figure 4: DEMs obtained in 2018 (**a**) and 2020 (**c**) and the same drawings transparently placed on the orthophoto (**b**,**d**). The values in the legend are expressed in meters. Figure 5: Details of the orthophoto obtained in 2020 (**a**–**c**) and DEMs transparently overlaid on the them (**d**–**f**). Figure 7 shows the images obtained in 2018 from the visual analysis of the orthophoto in which the areas affected by restorations, the detachment of tesserae and fractures are highlighted. The indication of the fractures was just quantitative and therefore they were not classified by taking into account the elevation, thickness and dimensions. The visual analysis of the orthophoto made it possible to detect in detail every single fracture and the detachment of tesserae present on the surface. The latter phenomenon, caused by multiple factors, was aggravated by the vertical position of the mosaic, which, compared with the original horizontal position, determined the displacement on the ground and the irreversible loss of the individual tiles or portions of the mosaic. The fractures had two main alignments: one vertical/horizontal, which mainly affected the edges of the mosaic, and one diagonal to the mosaic, which concerned the central/right portion of the panel. The latter was particularly relevant and fell into the areas of greatest depression recorded in the digital surface elevation model. The presence of restored and consolidated areas on the left side may have increased the resistance of the surfaces, avoiding the onset of superficial lesions. Regarding the GPR results, anomalies due to the presence of the iron support reinforcement present under the frame stood out in the different time slices (Figure 8). The degrees of amplitude variation in the time-slices were assigned a color scale that was chosen in order to show sufficient contrast to make the anomalies easily recognizable: light green corresponded to low amplitudes, while red corresponded to high amplitudes. The anomalies visible in these representations depicted the spatial distribution of the amplitudes of the reflections at specific depths within the grid.