The assessment of the building conditions with NDT techniques is necessary to estimate their<br>safety and to plan their restoration. In the last few years, the GPR technology has been improved to<br>meet the requirements needed for this application, e.g., many manufacturers have developed new high<br>frequency shielded antennas to ensure very high resolution images. 3D GPR reconstructions are<br>particularly interesting to investigate the complex geometry of structural elements. To execute a 3D<br>GPR survey it is necessary to acquire a regular matrix of GPR traces. Accuracy in antenna position and<br>orientation is very important for a successful application of a 3D migration software to produce images<br>that can be directly interpreted by the end-users. An array of antennas helps to ensure a regular spacing<br>in the cross-line direction of the survey but the array is heavy and large so that it might be inappropriate<br>to investigate vertical elements like walls, pillars and columns and to adapt to the shape of the surface<br>decorations especially in historical buildings.<br>For these reasons, we developed the PSG (Pad System for Georadar), patented by Politecnico of<br>Milano. It is a new easy-to-use and low cost positioning system to execute 3D GPR surveys with a<br>monochannel equipment. The system can work both on horizontal and vertical elements. We produced<br>some prototypes with different materials and we found the system really useful to solve many problems<br>posed by the restorers of historical buildings such as the analysis of beam-wall connections, the<br>reconstruction of the internal structure of vaults, the detection of wooden frames hidden inside masonry<br>walls, etc.. The system is also quite useful to speed up the surveys finalized to produce effective 3D<br>maps of the reinforcements in concrete structures.


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