A case study where dual-polarization was essential for correct interpretation of GPR results

The 19th century restoration works of the Basilica of Sant'Andrea in Mantua consisted of important modifications of pronaos and facade. According to historical documents, the use of metal reinforcements was planned to tie the facade with the counter-facade but designs and details are missing and since no external evidences of the tie-rods can be observed, the existence of these reinforcements was a fundamental open issue. The problem has been recently addressed with a GPR survey. Some tie-rods have been confirmed. The location of some of them has been problematic because of plaster decorations that reduced the survey area to narrow corridors. The dual-polarization was a benefit because some reinforcements, oriented parallel to the survey corridor, were detected only by the parallel-broadside polarization. In addition, for the exact location of these elements, 3D migration was needed because the reflection was generated out-of-plane. Some other unexpected reinforcements were also detected within the counter-facade wall. The complex geometry of these reinforcements was finally understood by composing the results of GPR investigations on orthogonal faces of the counter-facade. However, a tricky question arose on one side of the wall where an unexpected anomaly in GPR response seemed to reveal the presence of an additional set of metal bars. Again the dual-polarization was essential to exclude this misinterpretation and to understand the real origin of these unexpected signals.