Geophysical investigation of the Temple of Apollo (Hierapolis, Turkey)

The ruins of Hierapolis are located a few kilometres North of Laodicea in the Lycus Valley (western Turkey). This area is characterised by a large geothermic field that produces several hot water springs along tectonic structures (fractures and faults). Inside Hierapolis is the Temple of Apollo, the principal deity of the city. While the foundations of this temple go back to late Hellenistic times, the present remains of the upper structure are from the 3rd century AD. Next to it there is an underground chamber (called the Plutonion) from which poisonous gases emerge. This paper presents the results of a geophysical survey carried out to explore firstly the buried cavities or structures beneath the Temple of Apollo, knowledge of which is important to understand whether the Plutonion could be entered from the inside of the temple for completing the sacrifices; secondly to contribute to the ongoing evaluation of ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) as tools for research into subsurface archaeological features (voids, walls, etc.). Two-dimensional ERT imaging was used to detect the presence of an active normal fault passing under the Temple of Apollo, as shown by geological, geomorphological and archaeoseismological observations. The resistivity profiles reveal the presence of conductive material (clay) covering the archaeological structures. The presence of active normal faults is indicated by the displacement of the bedrock and the conductive material on top of it. Three-dimensional GPR imaging was used to detect man-made structures located under the Temple of Apollo. The results of the two survey methods applied were compared, assessing the relative merits and demerits of each technique. Their combined use is discussed in terms of providing enhanced views that are more informative than a single method