Geophysical Surveys for Archaeological Research in Urban Areas: The Case of the Roman Theatre in Padua

Abstract

The Roman theatre of Padua stood in the area now occupied by Prato della Valle, a huge elliptical-shaped square with a central green part (Isola Memmia) surrounded by a canal, built in the second half of the 18th century and part of the modern urban city center of Padua. Some still-preserved remains of the Roman theatre stand at the bottom of this canal. Recently, in 2017, emptying and cleaning the canal and excavating these remains, which had been known in the past, new geometrical and archaeological information has been collected. To date, however, there are no specific indications about additional preserved buried parts of the Roman theatre and its overall extent between the central and the outer part of Prato della Valle. Therefore, several electrical resistivity tomographies (ERT) and ground penetrating radar (GPR) sections were collected in 2017 to gather new information. The results of geophysical prospecting with recent archaeological evidence and historical documents, even the complexity of the urban environment, provide further details on the possible extent and location of additional buried remains of the Roman theatre, opening new archaeological considerations and issues related to the use of ERT and GPR in urban contexts.

1. Introduction

The remains of the Roman theatre of Padua stand under the present-day Prato della Valle elliptical square (Figure 1).

The theatre, built in the southeastern area out of the Roman urban center of Padua (Patavium), probably was affected already at the time of its construction by standing water problems [1]. The old name “Pratum cum Valle” is found as early as 1077. According to the toponymy that is still preserved today, “Valle” probably reminds an ancient depressed area with marshes and swamps, perhaps related to the hydrographic upheaval that interested the Po Valley at the end of the sixth century and to the wetlands documented in this area at the end of the 16th century. From a geomorphological point of view, recent studies established the alluvial nature of the sediments in this area [2,3,4], predominantly silty and clayey [5], also confirming their low hydraulic permeability. Undoubtedly, the regimentation and removal of water, as well as the idea to realize a modern and significant arrangement and enhancement of this area, were at the basis of the intervention, which led Andrea Memmo to want and realize the current arrangement of Prato della Valle [6] in the second half of the 18th century. (Figure 1).

The remains of the Roman theatre were first identified in 1775, during the cited rearrangement of Prato della Valle, when Simone Stratico and Angelo Ciotto thoroughly described and drew the remains today buried below this square [7] consisting of a rectangular structure (probably the stage building) with the semicircular platform at the base of the cavea. After that, only the part standing at the bottom of the canal in Prato della Valle was documented in the 1960s and 1980s during the cleaning of the canal [8]. More recently, in 2017, the same portion of the submerged remains was again brought to light, described and geometrically reconstructed thanks to the excavation project carried out by the University of Padua, the municipality, and the local Superintendence [1,9,10]. This archaeological activity allowed for the detailed analysis of the part of the foundations of the cavea still preserved below the canal (Figure 2), consisting of a concrete foundation platform (about 2.4 m in thickness), five semicircular structures, and ten partially visible radial walls [10].

The area around this part of the canal was then investigated by geophysical prospecting to verify the possible presence, depth, and extension of other structures related to the Roman theatre described at the end of the 18th century and to define its overall dimension. The use of geophysical prospecting for archaeological purposes in urban contexts is conventionally associated with the ground-penetrating radar (GPR) application [11,12,13,14,15,16,17] and rarely finds examples of other non-invasive techniques [18,19,20,21,22,23]. The possibility of applying different geophysical methods for archaeological investigations in rural sites (e.g., magnetic, electromagnetic (EM), resistivity) [18,24,25,26,27,28,29] in modern urban areas is strictly limited by paving stones, cobblestones, asphalt, underground utility networks, and, more generally, EM or magnetic noise. In addition, the frequent occurrence of specific types of subsoils, in particular high clayey and silty soils or a mix of incoherent materials, as well as those usually present in the shallow subsoil of urban centers, produce signal attenuation or no coherent noise disturbs and related problems in the GPR interpretation and in the easy identification of possible archaeological targets [30,31,32,33,34,35]. Prato della Valle in Padua provides an apparent example of the easy logistical applicability of GPR and ERT for archaeological research in urban areas. The remains of the Roman theatre would insist for a substantial portion below the green spaces inside and outside the canal surrounding the Isola Memmia and partly under the outermost partially paved area. The application of magnetic and electromagnetic in frequency domain (FDEM) techniques, on the other hand, were both rejected a priori because of the high presence of electromagnetic and magnetic noise that generally characterizes urban areas and, in particular, this one because of the depth of investigation required to intercept the structures of interest. The archaeological structures here should be about 3 m deeper than the ground level [10]. These depths appear excessive for applying the magnetic method, also considering the excessive noise caused by the underground utilities and installations in the area, including the street lights.

On the other hand, for the FDEM method, although the investigation depth could be overcome, the same reasons related to the presence of magnetic and EM noise led to considering this technique inappropriate in this context. The prior information about the conductive sediments [2,3,4,5] under and outside the archaeological remains (supposed resistive), and different, maybe less conductive soils in the upper part drove them to the use of the GPR and ERT. Thus, in this case study, we showed how the highly favorable logistic, particularly for ERT measurements, made it possible to integrate this technique with GPR acquisitions, helping the identification of archaeological remains overcome the unexpected problems encountered by GPR measurements.

2. Materials and Methods

2.1. Historical Cartography and Data Merging

To select the areas of interest for geophysical prospecting, firstly, old documentation, mainly drawings and reliefs of the Roman theatre, was examined and reviewed. This operation helped place structures that emerged in the past in contemporary cartography and speculate on the possible original extent and orientation of the Roman building. Three different maps were placed in the same file and docked to the regional technical map. The first considered document (Figure 3) reported the drawings by Angelo Ciotto of the remains found in 1775 during the construction of modern Prato della Valle, published in 1795 by Simone Stratico [7].

This map produced a complete and fundamental overview of the Roman theatre remains (stage building, orchestra, and cavea) found in the 18th century, with specific information about their size and orientation. Unfortunately, the profile of the actual buildings in Prato della Valle did not perfectly match the one represented here, probably because of recent modifications. The second (unpublished) old map considered here by Giuseppe Jappelli (1823) [36] provided some other new, more precise, but not exact indications about the proper location of the Roman remains. The still-existing urban elements (e.g., canal, bridges, etc.) represented here with the same remains found in the 18th century and described before helped in the proper position of the ancient building in the area of Prato della Valle. The third map considered here was realized by the Municipality of Padua during the emptying and cleaning of the canal in October 1963 [8]. This last document reproduced the remains at the bottom of the canal, part of the foundations of the cavea, partially investigated during the 18th-century excavations: twenty radial walls around five concentric semicircular structures, different by materials and construction techniques. All structures appeared cut by the banks of the canal. Thanks to this third document, an overall plan of the Roman theatre remains was reproduced in the context of the present Prato della Valle. This new complete map represented the background for the planning geophysical prospection, emptying, cleaning and archaeological excavation carried out in 2017.

2.2. Geophysical Measurements

To verify the right location and extension of possible remains of the Roman theatre in Prato della Valle in the area identified by the analysis of historical cartography, several ERT and GPR sections were planned (Figure 4): five overlapping GPR and ERT sections (L1–L5 in Figure 4) in the inner green area, two more GPR and ERT sections (L6–L7 in Figure 4) in the outer green belt, and six more GPR sections (L8–L13 in Figure 4) to the East between the outer green area and the asphalt.

For the acquisition of ERT lines, an IRIS Syscal Pro 72-channel resistivity meter was used with a dipole-dipole configuration and 48 electrodes spaced 0.5 m apart (Figure 5).

L1, L2, L3, and L5 ERT lines were performed with a roll-along acquisition [37], overlapping 24 electrodes on two consecutive 48-electrode lines totaling 35.5 m in length. L4, L6, and L7 used only 48 electrodes for a 23.5 m total length. The same maximum survey depth achieved for all these configurations is less than 5 m. For ERT data inversion, we used the ProfileR software, based on the Occam approach [38]. Data pre-processing consisted of rejecting values that exceeded the quality factor considering direct and reciprocal measurements. This process implied a loss of approximately 10% of data points but ensured the maximum control for the data used in the inversion process. To allow adequate comparison between the different sections’ anomalies, we finally adopted the same log10 resistivity range (0–2.3 log10 Res), plotting the results with Golden software SURFER 15.

An IDS RIS MF Hi-mod system with 400–900 MHz antennas was used for GPR surveys. The system was moved on the field by a cart equipped with an odometer for handling and correctly identifying distances traveled and locations of anomalies in the investigated system (Figure 6). The GPR sections were processed in different steps through ReflexW software, performing dewow, timing correction, manual y gain, background removal, and data migration (Stolt). Given the depth of archaeological remains, also considering the noise and attenuation problems of the higher frequency, the 900 MHz data were not considered here, opening the discussion only for 400 MHz GPR sections.

3. Results

The integrated map obtained by historical cartography and modern reliefs (Figure 4) showed that the Roman theatre remains stand inside and outside the canal in Prato della Valle. In particular, the stage building would be partially under the road along the eastern side of Prato della Valle and the paved area just to the west out of the canal. In contrast, the orchestra would be located partly under the same paved area and partly under the canal’s turf. The lower portion of the cavea represented in Ciotto’s and Stratico’s map (Figure 3) extends partially under the green area and within the canal, where some radial walls are also present. However, these structures apparently can continue outside the western limit of radials inside the green part of Isola Memmia. Considering the length of the radial walls brought to light within the canal, which is about 13.5 m, we considered this distance as the outer limit of these inner structures, as represented by semicircular dashed-lines in Figure 4. However, this limit would not represent the real outer limit of the theatre, which probably consisted of an outer ambulatory and a facade portico, considering some findings in the last century [7,8] and in the last excavation in 2017 [10]. In fact, on both occasions, a brick structure was brought to light beyond the end of the radial walls (e.g. an outer corridor). This presence indicated that the outer limit of the monumental building must lie well beyond the outer visible and considered limit of the radial walls and, thus, according to our reconstruction, thanks to ERT results, at a distance greater than 13 m (Figure 7). Following data processing, contrary to what might be expected from an initial assessment related to field logistics, the GPR produced disappointing results, probably due to the strong shallow noise and signal attenuation associated with the type of mixed soil covering the remains of structures in this area. The a priori unexpected high electrical conductivity of upper soils, in the first 2–3 m above ground level, was confirmed by ERT surveys, which instead contributed new important information on the extent and possible location of the Roman theatre remains (Figure 7).

The black dashed boxes in ERT sections indicates the overlapping GPR section. The pink boxes represent the maximum extent of each radial wall crossed by ERT and GPR sections. The dashed white lines and boxes represent respectively the expected depth of remains and the possible extension of outer structures in the Isola Memmia.

Thanks to the identification of resistive anomalies compatible with the structures of interest in terms of extension and depth (Figure 7), it was found that in the central green area surrounded by the canal (Isola Memmia), the archaeological remains should extend for about less than twenty meters starting from the edge of the canal in this area and moving toward from East to West. The ERT anomalies would seem to gather a probable foundation slab and the overlying remains of radial walls, which are difficult to distinguish individually, probably because of their poor mutual contrast in electrical properties (similar resistivity). In this area, the resistive anomalies extended from 3 m above ground level and beyond the maximum depth reached by ERT investigations. Other structures, probably related to the lower portion of the cavea and orchestra, were also identified in the eastern part outside the green zone. In this area, the archaeological remains would appear to be shallower and about 2.5 m deeper than the current ground level. Finally, the GPR acquisition performed along the outermost area of interest, covered by asphalt, showed not clear that the possible remains of the stage building should be located here if still preserved (Figure 8).

The high level of EM noise recorded here due to underground utility networks made it difficult to recognize the remains of any structures of interest, effectively postponing future investigations. Perhaps investigations with a series of ERT lines by drilling the asphalt in this area are more significant.

4. Discussion and Conclusions

The geophysical measurements conducted in 2017 in Prato della Valle, in parallel with the new emptying and cleaning of the canal, allowed obtaining important information on the extent and location of the remains of the Roman theatre of Padua. In fact, it was possible to verify the presence of structures belonging to the theatre inside and outside the area surrounded by the canal (Isola Memmia). The results obtained with ERT measurements made it possible to see that the remains of the building extend in the area inside the canal for more than 4 m below the current ground level. It seems more superficial in the outer sector, according to the different topographic levels in the two areas now covered by non-invasive measurements. Another significant result concerned the extent of the Roman theatre and its outer limit. Indeed, ERT measurements indicated that the theatre structures extend internally to this area up to about 20 m, starting from the western bank of the canal. Considering the position, arrangement, and length of the radial walls, this measurement would seem to confirm the existence of a surrounding structure, as already supposed, based on old [7,8,36] and recent (2017) [10] archaeological data (Figure 9).

According to evidence from geophysical prospecting, this structure should have a width of about 9 m. These data are of great interest for the reconstruction of the architectural form of the monumental building and for the definition of its dimensions. Considering the presence of this new outer limit, the Roman theatre of Padua would reach about 114 m in diameter and would therefore be counted among the largest buildings in Roman Italy. Finally, from a methodological point of view, the combined use of the GPR technique and ERT highlighted the lack of effectiveness of the GPR technique in this unexpected shallow conductive and noisy urban environment and the power of the ERT, particularly effective in defining the location, extent, and architectural layout of the Roman theatre of Padua. The stimulating results provided by this preliminary non-invasive investigation led to plans to collect additional ERT lines to investigate the system more deeply by defining the boundary of the structures identified in their shallow part. An important part will also be represented by future planned archaeological excavations following these non-invasive measurements and to assess the actual location and extent of the ancient structures potentially identified at this stage in the subsoil of Prato della Valle, both inside and outside the canal that today defines the Isola Memmia.