(Laboratory of Geophysical-Satellite Remote Sensing & Archaeo-environment, IMS-FORTH Rethymnon, Greece)

Urban archaeological geophysics (U.A.G.) is an emerging field that focuses on the effective geophysical exploration of urbanised areas in order to provide specific solutions concerning the preservation of cultural monuments located in urban territories. U.A.G. can be applied in different urban environments to investigate the integrity of structures, to study the geological stability of the settings where monuments are located and map the archaeological relics. Taking into account the extensive construction works that accompany the inevitable development of new and the expansion of existing modern urban infrastructures it becomes obvious that all these may threaten important archaeological monuments, which are still buried in the subsurface of the urban territories. This work presents an integrated application of the Ground Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT) methods to effectively map the complex changes of the physical environment below and around the area of Saint Andreas church in Loutraki (central Greece). Saint Andrea’s church is a relatively small three-aisled basilica with cupola. The cultural value of the monument is significant, as according to the legend, the stairs at the north side of the church lead to the entrance of a small cave, which was used as a hideaway by Saint Andreas when he was pursued by Romans. The church has been severely damaged by the Alkyonid earthquake in 1981 and the Ministry of Culture is studying a restoration plan of it. The main purpose of this geophysical investigation was to provide the necessary feedback concerning the ground subsurface properties for the future restoration of the monument.
High resolution ERT data (electrode spacing of 0.5-1 m) were collected along profiles and in a grid employing two different arrays (Dipole-Dipole & Wenner-Schlumberger) in order to increase the vertical and horizontal resolution. Bentonite contact electrodes were used for the maximum protection of the monument. A dense grid of parallel GPR sections (DX=0.5m, DY=0.1m) was completed in various parts inside and outside the area of the church. Two and three dimensional inversion algorithms were used to reconstruct the true subsurface resistivity. Specific filters (first peak estimation, DC shift, trace-to-trace averaging filters) were employed to enhance the GPR possible reflectors.
The geophysical campaign indicates that a) a uniform stratigraphy lies below the surface in the interior of the church with the exception of a few areas that are filled with parts of rock masses, b) the most of the high resistivity anomalies and strong reflections were caused by rock masses which probably come from the limestone side detritus, c) the limestone bedrock appears in depths larger than 6 m d) the upper surface of the water table is located 1.2-1.5 m below the road surface.
These results can provide significant information in order to design the future restoration works of the church. The combined use of ERT and GPR seems to be adequate to reconstruct the complex subsurface properties encountered in this urban setting. In general urban geophysics provides a valuable tool at the stages of restoring monuments of significant cultural value, constituting a unique and most cost-effective method of approaching the technical characteristics of the subsurface.

Keywords: Electrical Resistivity Tomography, Ground Penetrating Radar, urban geophysics, Saint Andreas church, Loutraki, Greece