A18 Integrated Full 3D Geoelectrical and GPR Tomographies in the Ambulatory of the Roman "Villa Del Casale", Piazza Armerina

Transcription

1 A18 Integrated Full 3D Geoelectrical and GPR Tomographies in the Ambulatory of the Roman "Villa Del Casale", Piazza Armerina P.L. Cosentino* (University of Palermo), P. Capizzi (University of Palermo), G. Fiandaca (University of Palermo), R. Martorana (University of Palermo), P. Messina (University of Palermo) & I. Razo Amoroz (University of Palermo) SUMMARY The Villa del Casale is a wonderful Roman Villa excavated in the archaeological site of Piazza Armerina (Sicily) in Its interest is mainly to be referred to the floor mosaics, among the largest and most beautiful ones of Roman times, of superlative quality. The floor of the Corridor is covered by a wonderful mosaic depicting the hunt and capture of wild animals, so that it is called the Corridor of the Great Hunting Scene. The Villa is now under restoration, and during the planning of the restoration a 3D electrical resistivity tomography was performed on the corridor by using an acquisition grid composed by 704 electrodes. Furthermore 25 GPR profiles with 400 MHz antenna were acquired. The 3D ERT survey was carried out by utilizing a new 3D array, named Maximum Yield Grid (MYG), that allows a great decrease of the acquisition time and of the invasivity of the ERT investigation in respect of classical arrays. The results show shallow anomalies, that correspond to the canalizations for water drainage and to the concrete basement built below the corridor during a previous restoration intervention, and a diminution of resistivity with depth, related to the aquifer below the Villa.

2 Introduction The Villa del Casale is a wonderful Roman Villa excavated in the archaeological site of Piazza Armerina (Sicily) in The villa covers an area of about 3,500 m 2, and was probably once Maximilian Hercules hunting house. He was one of Diocletian's collaborators both of whom governed the Roman Empire. Its interest is mainly to be referred to the floor mosaics, among the largest and most beautiful ones of Roman times, of superlative quality. The floor of the Ambulatory is covered by a wonderful mosaic depicting the hunt and capture of wild animals, so that it is called the Ambulatory of the Great Hunting Scene. The Villa is now under restoration, and during the planning of the restoration a 3D electrical resistivity tomography was performed on the ambulatory by using an acquisition grid composed by 704 electrodes. The 3D ERT survey was carried out by utilizing a new 3D array, named Maximum Yield Grid (MYG) (Fiandaca et al., 2008), that allows a great decrease of the acquisition time and of the invasivity of the ERT investigation in respect of classical arrays. Furthermore 25 GPR profiles with 400 MHz antenna were acquired. Previous geophysical surveys, involved low frequency (100 MHz) and high frequency (1600 MHz) GPR, as well as refraction profiles, TEM soundings and seismic well loggings (Cosentino et al., 2007). 3D ERT Survey The applicability of DC electrical tomography to media covered by precious and vulnerable surfaces, like mosaics, is limited by problem related to the current injection. To overcome this problem we utilized electrocardiogram (ECG) electrodes as potential electrodes (Cosentino and Martorana, 2003) and small stainless steel nails, among the mosaic tesserae, to inject the current (Fig. 1). The survey was performed by using an acquisition grid of 704 electrodes, 60 cm spaced, but only 32 electrodes were used to inject current (and then only 32 small nails were inserted in the interstices among the tesserae of the mosaic) (Fig. 2a). In fact with the MYG array only a few current injections (in this application 108 injections) are needed to ensure resolution comparable to those of the better classical 3D arrays, choosing potential measurements between all the electrodes of the acquisition grid along the current lines for each injection (Fig. 2b). The MRS256 georesistivity-meter (GF instruments, Czech Republic) was utilized to perform measurements. This instrument works with 256 potential channels and, in conjunction with the MYG array that utilizes all the electrodes of the acquisition grid for each current injection, allows a large decrease of the acquisition time. In this application three hours were needed to locate and connect cables and electrodes and about one hour to acquire data. Special software, developed to recognize and exterminate bad data, allowed us to analyze data in few minutes. The final step was the inversion (more than apparent resistivity values), performed by using RES3DINV software (GEOTOMO software).

3 Figure 1 (a) Acquisition of 3D ERT measurements; (b) Stainless steel nail used as current electrode. Figure 2 (a) Blue dots: potential electrodes of the acquisition grid; black dots: current electrodes of the acquisition grid; red lines: current dipoles; (b) Example of potential measurements for a particular current injection. The black lines represent the current dipoles utilized for the inversion process. Figure 3 Green lines: GPR profile; blue dots: 3D ERT electrodes. GPR Profiles 11 GPR profiles 59m long, having a spacing of 30 cm, were acquired parallelly to the ambulatory, while 14 GPR profiles 4.2m long were acquired perpendicularly to the ambulatory, in the first 17.5 m (Fig. 3). The GSSI SIR-2 Georadar, equipped with a 400 MHz antenna was used to perform measurements. The acquired data were processed with background removal and band-pass filters and then elaborated in terms of time slices. Results The inversion model of ERT data reaches a depth of 12 m and presents a large variation of resistivity along the vertical direction, because of the presence of the aquifer (Fig. 4a). The piezometric level was measured during the ERT acquisition in a well near the ambulatory, and the depth of the water table corresponds to that inferred from the ERT inversion, but only in a portion of the investigated volume. In fact in the first 25 meters of the model, along the ambulatory direction, the conductive layer below the surface is shallower, maybe because of the presence of superficial water or of an alluvial clay lens. Certainly below the ambulatory floor there are two canalizations for water drainage, built during the first restoration of the Villa in the 1950s (Gentili, 1999): the inversion model shows resistive superficial anomalies along the canalizations described by Gentili, but presents also a linear resistive anomaly, not related to known structures, that starts at the fountain of the peristilium near the ambulatory (Fig. 4b, Fig. 4d). Furthermore the inversion model shows a superficial resistive anomaly related to the concrete basement that was posed below a portion of the ambulatory floor during the restoration in the

4 1950s. The anomaly is not uniform, but it presents a more resistive portion in the left side, maybe because of a reinforcement of that side of the concrete basement, where the ambulatory presented (and still presents) a huge subsidence phenomenon (down to 1.2 m). Figure 4 (a) Inversion model of 3D ERT data with the superimposition of the piezometric level; (b) Inversion model of 3D ERT data: top view. Blue lines represent the canalizations of the water drainage under the villa constructed during the restoration in the 1950s (Gentili, 1999). Gray rectangles represent the canalizations interpreted from the ERT data. Pink rectangle represents a portion of the ambulatory with a concrete reinforcement; (c) Time slice of GPR data, 20 ns; (d) Map of the villa with the superimposition of geoelectrical acquisition grid, GPR profiles and of the embedded structures. The red circle locates the well utilized for the measurement of the piezometric level (pink line of fig. 3a).

5 The radar data show superficial anomalies compatible with those highlighted by ERT (Fig. 4c), but reach a smaller depth of investigation. The concrete basement below the portion of restored mosaic is clearly identifiable, even though the corresponding time-slice is not shown here. Conclusions The two techniques, one of which is here proposed after the first application test, seamed to furnish interesting results, giving pieces of information regarding: 1. locations of recent pipelines and discovery of an ancient canal; 2. characterization of the concrete basement below a portion of the ambulatory; 3. the pattern of the water table below the Villa. References Fiandaca G., Cosentino P.L. [2008] The new Maximum Yield Grid (MYG) Array in 3D resistivity tomography. Near Surface Proceedings of the 14th European Meeting of Environmental and Engineering Geophysics. 14th European Meeting of Environmental and Engineering Geophysics, Extended Abstract, (Submitted). Cosentino P.L., Capizzi P., Fiandaca G., Martorana R., Messina P., Razo Amoroz I., Pellegrino L. [2007] Diagnostica per il consolidamento del mosaico pavimentale dell ambulacro nella villa romana del casale (Piazza Armerina). Proceedings of XXIII International Meeting: Bressanone 10-13/07/2007: Il consolidamento degli apparati architettonici e decorativi conoscenze, orientamenti, esperienze tav 7. Cosentino, P. Martorana R. [2003] High-resolution micro-geophysics: electrical tomography for walls, Proceedings of 3ª Asamblea Hispano-Portuguesa de Geodesia y Geofísica, Valencia 2002, Gentili G.V. [1999] La Villa Romana di Piazza Armerina Palazzo Erculio. Fondazione Don Carlo, Osimo. Vol. 1, 286 pp.