Mediterranean Arhaeology and Archaeometry, Vol. 13, No 1, pp.9-20 Copyright @ 2013 MAA Printed in Greece. All rights reserved. GEOPHYSICAL PROSPECTION AT THE HAMZA BEY (ALKAZAR) MONUMENT THESSALONIKI, GREECE Gregory N. Tsokas 1, Nectaria Diamanti 1*, Panagiotis I. Tsourlos 1, George Vargemezis 1, Alexandros Stampolidis 1, and Konstantinos T. Raptis 2 1Aristotle University of Thessaloniki, School of Geology, Department of Geophysics, GR-54124, PO Box 352-1, Thessaloniki, Greece. 2 9th Ephoreia of Byzantine Antiquities, Eptapyrgio, GR-54003, PO Box 18432, Thessaloniki, Greece Received: 1/7/2012 Accepted: 28/8/2012 Corresponding author Nectaria Diamanti ([email protected]) ABSTRACT A two-phase geophysical survey is presented, whose aim was to investigate parts of the Ottoman Hamza Bey mosque located at Thessaloniki, Greece. Along with ground penetrating radar (GPR), which is a fully non-destructive method, a number of electrical resistivity tomographies (ERTs) were carried out. Bentonite mud electrodes were used instead of metal stakes, in order to comply with the non-destructive character of the whole operation. Our aim was to study the subsurface geological structures at the location of the mosque, and to detect and possibly map any ancient remains concealed under the monument. Both ERT and GPR results indicated a distinct, near surface horizontal discontinuity which was attributed to the presence of an ancient floor at the atrium area of the mosque. This floor was then revealed after a subsequent excavation. Moreover, high resistivity anomalies and distinct GPR signals were observed deeper at the atrium area. They are attributed to possible voids, remains of ancient walls, or other man-made structures concealed under the floor of the monument. KEYWORDS: Archaeological prospection, geophysical survey, non-destructive testing, ground penetrating radar, electrical resistivity tomography 10 GREGORy N. TSOkAS et al INTRODUCTION asymmetrical three-sided portico was added to the northwest forming a In this work we present the geophysical quadrilateral atrium. Repairs and survey which was carried out in the Hamza alterations would continue so that the Bey mosque, a significant ottoman building could be adapted from time to monument, situated in the city centre of time to the needs of the muslim community, Thessaloniki (N. Greece). This survey was which operated there till 1923 (Raptis, 2012). conducted in the framework for the From 1928 to 2006, when the edifice came to construction of the new underground the property of the Ministry of Culture, the Metro line in the city of Thessaloniki, and monument was used as a market place. The commenced contemporaneously with an dilapidated state, to which the edifice had extensive maintenance project of the come during that period, was treated by an monument (Fig. 1). extensive maintenance project, financed by the 3 rd Community Support Framework and performed by the 9 th Ephoreia of Byzantine Antiquities (Raptis et al. , 2007; 2008; Raptis and Xanthos, 2010; 2012). At the same time, the static inadequacy of the monument, farther affected by the cutting of the tunnels of the new Metro line, made necessary the investigation of (a) the building’s foundation depth and (b) the underlying anthropogenic layers or pre- existing constructions that could affect the adequacy of the foundation masonries; parameters examined by both conventional Figure 1. Site location map and the Hamza Bey excavation methods (Raptis, 2009) and monument (Raptis and Xanthos, 2010). geophysical surveying. Geophysical surveying for detecting and Hamza Bey mosque (Fig.1), also known mapping buried archaeological remains has as Alkazar after the cinema that was been very widely used over the past operating for a long period in the decades (Gaffney, 2008), with electrical monument, was built in 1467-8 by Hafsa , the resistivity being one of the most popular daughter of the ottoman military methods for doing so (Tsokas & Liritzis, commander Hamza Bey, as a district muslim 1990; Tsokas et al. , 1994; Diamanti et al., house of prayer. The edifice consisted 2005, Papadopoulos et al. , 2007, Tsokas et al. , originally of the domed prayer hall that was 2008a; 2009). However, a geophysical to become the nucleus of the later application within monuments is a special architectural complex, with a porch, revak , demanding field which requires a fully non- along its northwest façade. In the mid 16 th destructive approach. Given the century, a Π shaped ambulatory with a significance of the monument, a major minaret was annexed to the mesçid , prerequisite for any geophysical survey is signifying its conversion to a mosque. The that this should be fully non-destructive. As edifice was completed during the last such the ground penetrating radar (GPR) decades of the 16 th century, when along with method is preferable. Several applications the rearrangement of the colonnade and the of GPR surveys in standing monuments superstructure of the ambulatory, an have been reported in literature (Savvaidis et al., 1999; Leckebusch, 2000; Lorenzo et al. , GEOPHySICAL PROSPECTION AT THE HAMZA BEy MONUMENT IN GREECE 11 2002; Leucci, 2002; Binda et al., 2003; Piro et geotechnical, geological and environmental al., 2003; Linford, 2004; Nuzzo, 2005; Tsokas subsurface detection problems (Ward, et al., 2007). 1989). The goal of resistivity methods is to Another technique that only recently has measure the potential differences on the been used in a fully non-destructive mode is ground surface due to the current flow the electrical resistivity tomography (ERT) within the ground. This measured potential method. Despite its widespread use in drop reflects the difficulty with which the typical field surveys, the application of ERT electrical current flows within the into standing structures was understandably subsurface and hence, it gives an indication not very popular due to its destructive of the earth's electrical resistivity. nature since typically spike electrodes have Knowledge of the subsurface material to be used to establish galvanic contact. resistivity is used for distinguishing However, relatively new developments existing underground features, such as suggest that the application of non-spike layering, voids, man-made structures etc. electrodes to perform ERT surveys in a fully ERT can be considered as the non-destructive manner is possible. development of the standard geoelectrical Systematic studies of different materials method. The advent of fully automated ranging from cooper flat base electrodes measuring instruments with electrode (Consentino and Martorana, 2001) to multiplexing ability in combination with conductive gel or bentonite electrodes the development of advanced interpretation (Athanasiou et al ., 2007) suggest that they can algorithms, allows the collection of a large be a surrogate to spike electrodes and can be amount of data and the production of used successfully to obtain valid galvanic electrical resistivity images of the resistivity measurements in a fully non- subsurface. The rapid advanced in destructive manner. As a result they can be computer technology during the last used in urban environment and also inside decades, allowed the development of existing monuments or for investigations in automated algorithms, known as inversion the walls of the monuments (Carrara et al. , algorithms, which are able to create precise 2001; Tsokas et al., 2008b; Tsourlos and images of subsurface resistivity distribution Tsokas, 2011). (Tripp et al. , 1984; Li and Oldenburgh, 1992; The aim of the presented geophysical Tsourlos, 1995). Moreover, the development survey was (a) to study the subsurface of automatic measuring systems facilitates geological structures at the location of the acquisition of a large number of mosque and (b) to detect and possibly map measurements in a limited time. buried antiquities under the monument. ERT has proved to be a successful The case presented here comprises an method in providing fast and reliable example of a combined geophysical survey shallow subsurface geoelectrical property for investigating the area of interest. The images under various field conditions, even geophysical methods employed were: ERT in complex geological environments. and GPR. Further, it is also an example of indoors surveying. 2.2 Ground Penetrating Radar (GPR) GPR is the general term applied to 2. GEOPHYSICAL METHODS techniques which employ radio waves, 2.1 Electrical Resistivity Tomography typically in the 1 to 1000 MHz frequency (ERT) range, to map structures and features Resistivity techniques are established buried in the ground (or in man-made and widely used to solve a variety of structures). A GPR unit sends out radio 12 GREGORy N. TSOkAS et al waves that partially reflect back when they thickness equal to ~0.6 m, and revealed the meet objects electrically different than their ancient flooring at the atrium area. And surroundings, and partially propagate in lastly, after the revelation of the ancient deeper layers. A transmitting antenna emits floor, we conducted geoelectrical and GPR the radio waves and a receiving antenna measurements again on the atrium floor. records the reflected ones. By recording the This geophysical survey was part of a more time interval between the instant at which extensive campaign of geophysical the pulse is emitted by the transmitter and investigations at the entire area of the is recorded by the receiver, and by knowing monument. its propagation velocity in the subsurface, it is possible to map underground reflectors.