The document contains the results of GPR survey perfomred on July 12th, 2017 and May 8th, 2018.
The first image shows the area investigated by means of the Ground Penetrating Radar (GPR), which covers a widespread crack pattern well visible in the cross hall of the Loggia where also a Linear Variable Displacement Transducer (specifically the LVDT1) was located. It pictures the fracture, which is located between two types of walls, a stone wall (left), which is part of the external perimeter of the Palace, and a brick wall (right), which is an internal wall. The crack pattern occurs at the junction area of these two walls. The figure shows also the presence of three main cavities, due to the fall of some bricks due to wall fracture, and the arrangement of stones and bricks. In particular, it is possible to observe that the left part is made up by alternating stone blocks whose lengths are 0.40 m and 0.70 m, respectively.
Description of the performed GPR analysis and results
In order to assess and monitor this crack pattern two Ground Penetrating Radar (GPR) measurement surveys were performed in July 12th 2017 and May 8th 2018, respectively, by using the IDS manufactured RIS K2_FW GPR system (https://idsgeoradar.com/products) equipped with a 2 GHz single fold shielded antennas.
The raw data, collected during both the GPR campaigns, were processed by means of the same processing chain. This latter was designed in the frame of Heracles project and it is based on a microwave tomographic approach, which faces the imaging as the reconstruction of the contrast function, i.e. the function accounting for the unknown dielectric permittivity variations occurring into the region under test with respect to the known reference dielectric permittivity. Accordingly, the output of the GPR data processing chain is the 3D spatial distribution of the amplitude of the reconstructed contrast function, as normalized to its maximum value, and it is represented by means of constant depth slices, which for brevity are referred to as tomographic images.
Figure 2 (left) shows the sequence of the tomographic images at increasing constant depths, from the surface towards the interior of the wall, referred to the GPR survey performed in July 12th 2017 and obtained by means of the GPR data processing chain developed during the Heracles project.
Figure 3 (right) shows the sequence of the tomographic images at increasing constant depths, from the surface towards the interior of the wall, referred to the GPR survey performed May 8th 2018 and obtained by means of the GPR data processing chain developed during the Heracles project.
The qualitative comparison of the tomographic images, referred to the two GPR surveys and at the same increasing depths, does not highlights significant differences. In fact, both the sequences of the tomographic images allow us to infer that the crack affecting this zone reaches a depth z of about 0.10 m, while the first, second and third cavities reach depths about 0.13 m, 0.11 m and 0.07 m, respectively. In addition, from the depth slices z = 0.50 m is possible to retrieve information about the three stone blocks, which are z = 0.70 m long and whose arrangement is similar to that of the surface stones (see Figure 1).
