## 3 Material and Methods
### Ground-Penetrating Radar (GPR)  
GPR is based on the diffusion of electromagnetic pulses into the soil and the recording of those re-radiated by buried targets characterized by sufficient dimensions and electromagnetic properties different from those of the surrounding ground. The quantities that are measured are the time required for the wave to travel the path from the transmitting antenna to a discontinuity and return to the surface (double time or two-way time) and the amplitude of the reflected wave. The double travel time depends on the speed with which the wave propagates within the material and provides information on the depth at which the reflectors are located. However, the amplitude, which represents how much energy returns to the surface after reflection, depends on the initial energy of the sent wave, the quantity that is dissipated along the way and the contrast of the electromagnetic properties of the materials that comprise the surface of the reflection. A complete description of the method is available in textbooks, such as [31; 32; 33; 34; 35].  
The factors that influence the performance of the system in terms of the detectability of existing targets are the electromagnetic properties of the propagation medium, which determine the depth of investigation that can be reached, which varies from point to point. Since the attenuation of the means is a function of the radiated frequency, the use of high-frequency antennas generally allows for enhancing the resolution power, but at the expense of the penetration depth of the signals.  
The georadar surveys were carried out on the entire surface of the mosaic in its actual vertical position. An IDS georadar RIS-K2 with a 3000 MHz high-resolution antenna was used for the data acquisition, whose features were considered sufficient and suitable for an overall assessment of the conservation status of the mosaic with respect to the type of target to be investigated and the thicknesses of surfaces to be analyzed.  
Technically, data acquisition took place on 114 lines, from top to bottom every 5 cm, where instrumental readings were executed in continuous mode (Figure 3). In addition, several horizontal profiles and some profiles on the sides of the mosaic were acquired.  
Raw data were processed using GPR-SLICE 7.0 software (Screening Eagle Tecnologies Ag, Zurich, Switzerland) [36] using standard methodological approaches. In the first step, data and trace editing were executed, inserting information such as the temporal and spatial sampling intervals (time window, 20 ns; samples/scan, 512; scan/mark, 25; unit/marker, 1). Data were recorded and processed as 16-bit data and were converted by subtracting out the DC drift (wobble) in the data and, at the same time, adding a gain with time. A time-zero correction was determined to designate the starting point of the wave and the center frequency of the antenna was matched. Then, a bandpass filter and background removal were respectively applied to reduce noise from the oscillating components that had a regular frequency cycle in the frequency domain and to remove striation noises that occurred at the same time. Processed radargrams were subsequently corrected with an automatic gain control (ACG) function [37] that was applied to each trace based on the difference between the mean amplitude of the signal in the time window and the maximum amplitude of the trace. In the final step, the resulting filtered radargrams were inserted into a three-dimensional matrix from which sections were obtained at a particular double-time interval (measured in nanoseconds). Considering the complex layering and the reduced length of the profiles, we preferred to not apply the migration filter and we avoided presenting results converting time to depth using a mean value with the possibility to obtain an arbitrary and inaccurate estimate.