Accessing a historic wall structure using GPR. The case of Heptapyrgion fortress Thessaloniki Greece

In this work we study part of the Northern wall structure of the Heptapyrgion fortress of Thessaloniki (UNESCO monument) that was identified to face moisture and structural problems. Moisture damage is a critical issue in the preservation and integrity of historic walls, thus it is necessary to detect it and to develop restoration plans. Heptapyrgion is one of the most important monuments in Thessaloniki so it is very crucial to use non-destructive techniques. To achieve that, a pilot GPR survey was conducted. The data obtained with 250 and 500MHz antennas were processed and interpreted in relation to the existing problem. To further assist the interpretation 2D GPR model simulations were carried out. GPR data was also compared to geoelectrical ERT data obtained at the same section on the wall. Overall GPR images are in good agreement with existing information and proved very useful in accessing the wall structure.