GPR and EM surveys to investigate the Archaeological Area of Pollentia (Alcudia-Mallorca, Spain)

Environmental Semeiotics (2011) 4(3), 46-54 DOI 10.3383/es.4.3.2 © diaRnet® 2011 GPR and EM surveys to investigate the Archaeological Area of Pollentia (Alcudia-Mallorca, Spain) TROGU A.1; RANIERI G.1; CHÁVEZ Mª E.2; ORFILA M.3 1 Land Engineering Department, University of Cagliari, Cagliari (Italy) Prehistory, Anthropology and Ancient History Department, University of La Laguna, La Laguna (Spain) 3 Prehistory and Archaeology Department, University of Granada, Granada (Spain) Email corresponding author: atrogu@unica.it 2 Abstract: In the roman city of Pollentia (Alcudia-Mallorca, Spain), a geophysical survey was performed, both using EM and GPR methods. Geophysical prospection was required to solve some of the problems regarding the topography and the extension of the city and the layout of the city walls too. GPR and EM surveys were carried out in two different areas, one inside the archaeological site, near the forum and the other close to the southwestern boundary of the known Roman city. Because of a high noise level in the recorded data, the EM prospection do not has produced good results. On the contrary, the GPR survey has permitted to highlight some anomalies that are more probably ascribable to archaeological remains. Keywords: Geophysical prospection, GPR, EM, Pollentia, Roman city 1. INTRODUCTION The Roman town of Pollentia (Alcudia) is located in the northeast of Majorca, south of the present Alcudia (Figure 1). The city has a strategic position, controlling two bays, the bay of Alcudia in the south and Pollença in the north (Orfila 2000; Orfila et al. 1999, 2005, 2006). This city, with the legal status of colony according to Mela (Chorographia, II, 124-125), was founded as a consequence of the invasion of the Balearic Islands by Quintus Caecilius Metellus in 123 BC. (Strabo, Geography, III, 5, 2). Until very recently the date of founding of the city was considered the 123 BC. However, a recent work developed in the Forum has provided a complete stratigraphic sequence of its occupation and a better GPR AND EM SURVEYS IN POLLENTIA 47 understanding of the urban organisation of the city. The excavations undertaken in the city since 1996 have revealed stratigraphic sequences that indicate that the first Roman constructions in the area of the forum did not take place before approximately 70 BC. (Orfila et al. 1999; Orfila 2000; Cau and Chávez 2003; Orfila et al. 2006). This date coincides with evidence from the residential area of Sa Portella, where the oldest architectonical examples built using Roman techniques are dated around the turn of the decade 70-60 BC. (Arribas et al. 1973). Figure 1: Site location. The forum of the city functioned as such until the late third century AD, when a large fire destroyed it. The archaeological surveys have identified the remains of this fire and recovered the objects abandoned by its inhabitants, who were crushed in situ by the collapse of the roofs. This is the case of the Insula of tabernae to the west of the Forum, where powerful levels of destruction caused by fire have been excavated (Room Z and porch of West Street, Room AA, etc.). All these levels show the destruction of structures in a moment that would be dated to the 270-280 AD. However, this will not mean the abandonment of this area, although diaRnet® 48 TROGU ET AL. there will be a change in its configuration, now restructured, being occupied in Late Antiquity and Medieval Age (Orfila 2000; Orfila et al., 1999, 2005, 2006). Archaeological investigations at the site date back to early XXth century, being driven especially since the 50's by the Foundation W. Bryant (Vermont, USA). As a result, different areas in the city are now known, such as part of the Forum -with the Capitolium, various temples, an Insula of tabernae, streets, etc-, the residential quarter of Sa Portella with three domus, the House of Polymnia, numerous remains in Can Viver, Can Basser, the theatre, several necropolises and at least two portions of two city walls, among other remains. Many other buildings or construction elements were brought to light in former excavations carried out mainly at the beginning of the XXth century. Unfortunately, these remains cannot be seen now as they were covered up at the end of the fieldwork seasons (Orfila et al. 2006) (Figure 2). Currently, archaeological researches in the area continue making new excavations, especially in the east and north sides of the forum. However, knowledge of the general topography of the city continues to be problematic, especially regarding the limits of urban space and the location of its ports, mentioned in some medieval documents (Orfila 2000; Orfila et al. 1999, 2006). That is why for several years has been carrying out a progressive and systematic geophysical survey of the site from different areas of the city in order to solve some of the issues raised in relation to the topography and the boundaries of the city, design of its walls, and the location of its ports. It is in this background that survey was carried out in 2008 in order to investigate five areas near the forum and a sixth area at about 500 meters south where, particularly, the survey pointed out a series of wellorganized anomalies (walls), which are located at a depth of about 0.5-1 m. This was being a very significant result, in order to determine the boundaries of the Roman city (Ranieri et al. 2008, 2010). Under the SOTOER Project -System Used to Make a Geo-reference for Orthogonal Layouts at Roman Age-, there was a new geophysical survey in 2010 in order to check both the extension of the archaeological remains beyond the northwest boundary of the known Roman city, and the diaRnet® GPR AND EM SURVEYS IN POLLENTIA presence of urban pattern in an area close to the forum (Figure 3). Figure 2: General map of the Roman city of Pollentia (Alcudia, Majorca). diaRnet® 49 50 TROGU ET AL. Figure 3: Sketch map of the prospection carried out during the year 2010. A) Archaeological area; P) Parking area. 2. METHODS We performed the surveys by means both of a GPR SIR-3000, by GSSI Inc., and of an EM CMD1, by GF Instruments. For the GPR survey, we used antennas both of 200 MHz and of 500 MHz, while for the EM survey we used the CMD1 with two antennas at an effective depth range of 0.75 m and of 1.50 m respectively. Surveys were carried out both one and one half metres spaced along two orthogonal directions. Figure 4 shows the distributions of the profiles for the parking area while in the Figure 5 are reported those for the archaeological site. Thanks to the high density of measures (about 170 scans/m2 in the area A and 100 scans/m2 in the area P), we are able to carrying out a 3D data processing from the GPR survey by means both Radan6 and Reflex6. The processing of the EM data was carried out at 0.75 and at 1.50 diaRnet® GPR AND EM SURVEYS IN POLLENTIA cm depth both as apparent conductivity and as phase maps. Figure 4: Profiles distribution in the parking area. a) GPR, b) EM. Figure 5: Profiles distribution in the archaeological area. a) GPR, b) EM. diaRnet® 51 52 TROGU ET AL. 3. RESULTS AND CONCLUSIONS In the archaeological area, the GPR time-slices obtained from the 3D data processing shown straight anomalies that appear to be in good agreement with the know archaeological remains. In fact, it is possible to recognize some features with a pattern that could match to an urban layout. These anomalies are in good alignment to the existing remains as it is illustrate by the black dotted lines in the Figure 6. Figure 6: Archaeological area: GPR slice at 55 cm depth. The amplitude values are normalized with respect to the maximum peak. Only the contour lines above 45% of the maximum are shown. Less information was obtained about the presence of archaeological remains in the parking area. The cause of this behaviour may be searched in the presence of a thick layer of filling material above the archaeological level, or it can be put in relation to the reduction of the urban pattern, that maybe can be associated to the borderline of the Roman city. diaRnet® GPR AND EM SURVEYS IN POLLENTIA 53 Here we can put in relationship only few anomalies to the remains, as it is shows by the black dotted line in Figure 7. Figure 7: Parking area: GPR slice at 170 cm depth. The amplitude values are normalized with respect to the maximum peak. Only the contour lines above 35% of the maximum are shown. The EM data were very noisy, likely due to the presence of scattered metallic object. Furthermore, the unknown thickness of the filling material in the parking area, joined to the limited depth of penetration of the CMD1, might have reduced the possibilities to investigate the archaeological layer. Acknowledgements Thanks to Luigi Noli, who has gave a fundamental contribute to the diaRnet® 54 TROGU ET AL. realization of this study. References Arribas, A., Tarradell, M., Woods, D. (1973) Pollentia I. Excavaciones en Sa Portella, Alcúdia (Mallorca). Excavaciones Arqueológicas en España nº 75, Ministerio de Educación y Ciencia, Madrid. Cau, M. Á., Chávez, M. E. (2003) El fenómeno urbano en Mallorca en época romana: los ejemplos de Pollentia y Palma, Mayurqa 29, 29-49. Orfila, M., Ed. (2000) El fòrum de Pollentia. Memòria de les campanyes d'excavacions realitzades ente els anys 1996 i 1999. Ajuntament d'Alcúdia, Àrea de Patrimoni, Alcúdia. Orfila, M., Arribas, A., Cau, M. Á. (1999) El foro romano de Pollentia, Archivo Español de Arqueología 62, 99-118. Orfila, M., Cau, M. Á., Chávez, M. E. (2005) La ciudad romana de Pollentia (Alcudia, Mallorca). Resultados de la investigación entre 1996-2004, Tabona 14, 93-115. Orfila, M., Chávez, M. E., Cau, M. Á. (2006): Pollentia and the roman cities of the Balearic Islands. Early Roman Towns in Hispania Tarraconensis, Journal or Roman Archaeology, Supplementary Series 62, 133-145. Ranieri G., Loddo F., Godio A., Stocco S., Cosentino P.L., Capizzi P., Messina P., Savini A., Bruno V., Cau M.A., Orfila M. (2010) Reconstruction of archaeological features in a Mediterranean coastal environment using non-invasive techniques, Making History Interactive. Computer Applications and Quantitative Methods in Archaeology (CAA). Archaeopress: Oxford, 329-336 Ranieri G., Loddo F., Godio A., Stocco S., Cosentino P.L., Capizzi P., Messina P. (2008) Synergy of geophysical methods on the reconstruction of the archaeological features of Pollentia (Majorca), 27° Convegno Nazionale GNGTS, Trieste, 6-8 Ottobre 2008, Extended Abstract, 372-375. diaRnet® View publication stats