Geophysical and Topographical Survey of the Theatre at Ancient Sparta by Helen Goodchild | Papers by Helen

GEOPHYSICAL AND TOPOGRAPHICAL SURVEY OF THE THEATRE AT ANCIENT SPARTA Chris Gaffney, Helen Goodchild and Scott Harrison Birmingham Archaeology Report PN. 1643 May 2007 GEOPHYSICAL AND TOPOGRAPHICAL SURVEY OF THE THEATRE AT ANCIENT SPARTA CONTENTS 1 Introduction ................................................................................................ 1 1.1 1.2 1.3 1.4 2 Location and topography....................................................................... 1 Archaeology .......................................................................................... 1 Aims of the survey ................................................................................ 3 Summary of results ............................................................................... 3 Methodology ................................................................................................ 3 2.1 2.2 2.3 Resistance Survey ................................................................................. 4 Topographic Survey .............................................................................. 4 General considerations.......................................................................... 5 3 4 5 Results......................................................................................................... 7 Conclusions.................................................................................................. 8 Bibliography ................................................................................................ 9 Geophysical and Topographical Survey of the Theatre at Ancient Sparta, 21st-25th May 2007 GEOPHYSICAL AND TOPOGRAPHICAL SURVEY OF THE THEATRE AT ANCIENT SPARTA Chris Gaffney, Helen Goodchild and Scott Harrison 1 Introduction Resistance and topographic survey were undertaken in May of 2007 on the area of the ancient theatre at Sparta. This survey was carried out on behalf of the British School at Athens under Professor Catherine Morgan in preparation for excavation in 2008. 1.1 Location and topography Grid reference: UTM 34N, 626542 4104939 The area under investigation lies directly to the north of the modern town of Sparti (ancient Sparta), in the Peloponnese of Greece (Figure 1). The area surveyed was the slope of the ancient theatre, originally the seating area. As a result the topography is fairly extreme with, on average, slopes of 36%. The study area was bounded within the area covered by the theatre. Figure 1. Location of Sparta in the Peloponnese, Greece (Image from Google Earth). 1.2 Archaeology Known archaeology included not only the heavily-robbed ancient theatre seating, but also a number of Byzantine period buildings built around the top edge of the theatre. Three open excavation trenches are located within the area of interest, all containing structural features (Figure 2). 1 Geophysical and Topographical Survey of the Theatre at Ancient Sparta, 21st-25th May 2007 Figure 2. Plan of the theatre showing excavation trenches (Nigel Fradgley 2001) 2 Geophysical and Topographical Survey of the Theatre at Ancient Sparta, 21st-25th May 2007 1.3 Aims of the survey A primary aim of the survey was to map the potential for buried archaeology within the near surface of the ancient theatre seating area. In particular it was hoped that the surveys would discover the extent of Byzantine period buildings that overlie part of the theatre. In order to achieve these aims both resistance and topographic survey were undertaken. This work was conducted in advance of excavation, which will be undertaken in 2008. 1.4 Summary of results Despite the difficulty of unfavourable ground conditions, resistance survey showed anomalies of archaeological potential. It is hoped that these anomalies indicate the location of structures near the top of the slopes, and that they are likely to relate to the Byzantine period buildings. Results on the remainder of the area were not as clear, though ground conditions during the survey are likely to have disguised potential archaeological features. Topographic survey was successful, showing potential areas of un-disturbed, or little disturbed seating areas, implying that these areas may not have been reused in the Byzantine period. Additionally, a small rectilinear structure shown in relief at the top of the western bank has been potentially identified, though this may be an artefact of the data. 2 Methodology The approximate location of the survey is shown in Figure 3. Figure 3. Location of resistance survey area on the site of the theatre (Image from Google Earth) 3 Geophysical and Topographical Survey of the Theatre at Ancient Sparta, 21st-25th May 2007 2.1 Resistance Survey The resistance survey, carried out using a Geoscan RM15, was limited to an area of approximately 2,600m2 (Figure 4). The Twin-Probe resistance array was used to measure the earth resistance at discrete points over the area of the ancient theatre seating. This array uses two pairs of electrodes, one is static while the other is mobile and is used to measure the variation in resistance over the area of interest (Gaffney and Gater 2003). Each pair comprise a current and a potential probe which are separated by 0.5m on the mobile frame, and a variable distance between the static probes. The distance between the latter is nominally 0.5m, but this varies due the process of ‘normalisation’ when the static position is changed due to the finite length of cable. The area of investigation was divided into squares with sides of 20m using tapes, though this proved difficult due to the nature of the terrain. The 20m grids were then sub-divided and data collected at 0.5m intervals along 1m collection traverses. The individual grids were then fused into a single data set for processing, analysis and display using specialist software. 2.2 Topographic Survey Topographic survey was carried out using a Leica 530 Differential Global Positioning Satellite system. Differential GPS works by using a static base station as a control to increase the accuracy of the roving receiver. The rover is used to take static or kinetic readings in order to locate points of interest or capture topographic information. As can be seen in Figure 4, the topographic survey covered the majority of the theatre area (approximately 5,940m2), and was only limited by tree coverage around the top of the seating area or areas sloping too extremely to safely take readings. GPS points were imported into a GIS and interpolated to create a topographic surface. Base heights from this surface were then used to create a 3-dimensional model of the theatre area over which the resistance data could be draped. N Figure 4. Map showing theatre structures plus approximate extent of theatre seating in black outline, with topographic survey and resistance results overlaid. Outlines derived from Google Earth Satellite image shown in Figure 3. 4 Geophysical and Topographical Survey of the Theatre at Ancient Sparta, 21st-25th May 2007 2.3 General considerations The ground conditions were generally poor due to the variable nature of the surface that included many exposed rock fragments; additionally the ground condition was extremely dry though this was punctuated by heavy rain (Figure 5). Both resistance and GPS surveys were also complicated by the slope of the ground, with some near vertical areas where soil had eroded (see Figure 6). GPS survey was further limited by the tree canopy to the north of the theatre which interfered with satellite reception. It was clear during the initial measurements that the surface of the ground was fine dust, beneath which a hard, compact surface of earth was found. Later heavy rain saturated the dust layer. Neither situation is ideal with problems due to contact resistance and, after the rain, anomalous current paths along the ground surface. This required a significant amount of processing to suppress these data defects. A number of large tree stumps restricted the survey. Where these have not been entered as dummy readings, static points were taken by the GPS to aid interpretation of subsurface features. Two excavation trenches also further restricted the survey - one to the west and one to the north-east, both at the top of the seating banks. A further excavation trench was located on the far east of the seating, though this area was not surveyed using the resistance. The general nature of this area is evident from the topographic survey (Figure 7). Figure 5. Theatre seating area, facing north-east towards the excavation trench in the topcentre of the theatre. 5 Geophysical and Topographical Survey of the Theatre at Ancient Sparta, 21st-25th May 2007 N Figure 6. Slope map derived from the GPS survey overlaid onto theatre extent and standing structures (black outline). The paler areas show the most extreme slopes, whilst the dark areas are flatter. N Figure 7. Topographic survey with tree-stumps and excavation trenches overlaid onto theatre extent and standing structures (black outline). 6 Geophysical and Topographical Survey of the Theatre at Ancient Sparta, 21st-25th May 2007 3 Results The topographic survey showed that faint traces of the seating could still be detected, even though not observable on the ground (Figures 8 and 9). The western seating area shows a series of vertical features which are likely to be a product of the collection strategy and are not archaeological features (Figure 8). Exaggeration of the results, by turning all heights into integers (decreasing positional accuracy by removing decimal places), appears to show clearly the original structure of the seating, and particularly areas of likely robbing where the seating is interrupted (Figure 10). This, however, must be treated with extreme caution, as coarser resolution elevation models are known to create a stepped appearance under certain circumstances (Cadell 2002, 5). Figure 8. 3D rendering of the topographic survey facing north-west, showing faint traces of seating plus the collapse of the overburden into the lower tier. Faint traces of nonarchaeological vertical features show to the west. Standing theatre structures are shown in grey. Figure 9. 3D rendering of the topographic survey facing north-east, showing faint traces of seating plus the collapse of the overburden into the lower tier. The excavation trench on the eastern seating bank shows clearly. Standing theatre structures are shown in grey. 7 Geophysical and Topographical Survey of the Theatre at Ancient Sparta, 21st-25th May 2007 Figure 10. Exaggerated results from the topographic survey showing possible seating and robbed areas. Standing theatre structures are shown in grey. Results of the resistance survey were limited, though do show some areas of high resistance, probably related to building material at the top of the western seating area (Figure 11-13). This area was known to have yielded structures from the excavation trench here. The high resistance anomaly completely surrounds this trench implying further intact structures continuing along the top of the seating bank and slightly down-slope. Other areas of high resistance appeared to follow the direction of the original seating, albeit in fragmentary form. Results from the eastern area were problematic due to the ground conditions (dry weather followed by heavy rain not penetrating sufficiently deeply) and so any archaeological features may be disguised. The resistance results are difficult to interpret, and the digitised polygons of the results from high-pass filtering show a number of anomalies (Figure 14). Those anomalies most likely to represent archaeological features are shown in black in Figure 15. Other areas of high resistance are most probably accumulations of stone, potentially related to the structures. Combined with the topographic data the western seating area shows a number of rectilinear features, which are probably later structures (Figures 16 and 17). The topography also assists in showing potential underlying structures. In Figure 17 it is possible to see a rectangular structure extending from the top of the seating across the flatter area, though this might be a product of the collection rather than a true feature. The coverage of the GPS in this area was limited due to the tree canopy, and may have caused such artefacts. 4 Conclusions Whilst conditions were not ideal for resistance survey, the combination of the data with topographic survey have resulted in a number of potential later structures being identified, particularly on the western bank of the theatre seating. The eastern bank, though not providing a significant amount of data, is also likely to contain archaeological features but, due to the collection conditions, these are not particularly visible in the results. 8 Geophysical and Topographical Survey of the Theatre at Ancient Sparta, 21st-25th May 2007 5 Bibliography Cadell, W. (2002) Report on the generation and analysis of DEMs for spatial modelling. The Macaulay Institute. http://www.macaulay.ac.uk/LADSS/documents/DEMs-for-spatialmodelling.pdf (Accessed May 2007). Gaffney, C. and J. Gater (2003) Revealing the Buried Past. Geophysics for Archaeologists. Stroud. Tempus. 9 Geophysical and Topographical Survey of the Theatre at Ancient Sparta, 21st-25th May 2007 N Figure 11. Results of the resistance survey. Grey-scale plot of low-pass filter results. Black shows areas of high resistance, blue indicates unsurveyed areas. N Figure 12. Results of the resistance survey. Grey-scale plot of high-pass filter results. Black shows areas of high resistance, blue indicates unsurveyed areas. N Figure 13. Results of the resistance survey. Dot-density map of high-pass filter results. Black shows areas of high resistance, blue indicates unsurveyed areas. 10 Geophysical and Topographical Survey of the Theatre at Ancient Sparta, 21st-25th May 2007 N Figure 14. Interpretation of features from the high-pass filtered results overlaid onto theatre extent and standing structures (black outline). N Figure 15. Interpretation of features from the high-pass filtered results overlaid onto theatre extent and standing structures (black outline). Areas in black show most likely archaeological features. 11 Geophysical and Topographical Survey of the Theatre at Ancient Sparta, 21st-25th May 2007 Figure 16. Geophysical interpretation draped over topography. Outlined area shows potential later structures Figure 17. Topography shows potential structures from the resistance results continuing at the top of the theatre 12