Ground penetrating radar is a powerful tool to perform studies on cultural heritage buildings and historical remains because of its non destructive applications and its high resolution. We performed two GPR surveys on a roman theater restored by using modern tecniques and materials and under an old stone gothic bridge where a main sewer was designed. In the first case, we used 900 MHz and 500 MHz center frequency antennas; the purpose of the study was to investigate the contact between the remains and the new used materials in order to detect damages in the old stones caused by the restoration actions in the theater. We were able to determine the dielectric parameters of the involved materials (old and new) because in some areas of the theater the size and the surface between the old and new construction were visible. In this way we were able to characterize the location and dimensions of the investigated anomalies. In the second case, we used 100 MHz and 500 MHz centre frecuency antennas; the main purpose was to determine the quality and geometry of the bridge foundations in order to avoid that the construction of the main sewer caused damage in the bridge. We were able to detect wood into the foundations, several trunks had been inserted between ground and the first stones used in its foundation. Some dificulties found during these two surveys and in the interpretation of the obtained records will be discussed. The main goals achieved in this work will be presented.