Ground Probing Radar prospecting and urban cultural heritage: the case study of San Pietro dei Pescatori church in Cagliari, Italy.

PIRODDI L.1, CALCINA S.1, TROGU A.1, BAKINOWSKA W.2, CASNEDI M.L.2, COCCO O.2, FAEDDA R.2, NETTEKOVEN T.3

1 DICAAR, University of Cagliari, Cagliari (Italy) 2 student, University of Cagliari, Cagliari (Italy) 3 student, University of Berlin, Berlin (Germany)

Email corresponding author: lucapiroddi@yahoo.it

Abstract: The present study deals with the goal to underline the potentialities of Ground Penetrating Radar in order to evidence the various kind of archaeological features potentially present in sites of many historical frequentations like urban centres and churchs in particular. In this paper a case study of San Pietro dei Pescatori church in Cagliari, Italy, is presented.

Keywords: GPR, georadar, cultural heritage, archaeology

1. HYSTORICAL OUTLINES

The church of San Pietro dei Pescatori ( Fig. 1) , which is still home to the “Gremio” of fishermen, is located in the current Viale Trieste in Cagliari, along the coastal stretch of the Santa Gilla lagoon , to the west of the city. Acquired the title Sancti Petri de Piscatore before 1090, as evidenced by a document in which the Judge of Cagliari - Costantino-Salusio II de Lacon-Gunale it also confirmed the membership of the Order of Victorines .

The pre-Romanesque apse can be ascribed to workers in the Lombard- Catalan judged Cagliari around the eleventh century, while the front (8.27 m.) probably rebuilt, shapes and presents findings related to Gothic typical of Tuscan workers active in Sardinia around the thirteenth century. The building is constructed of large blocks of limestone with several regular elements of plunder . The facade is divided into three areas by a series of four pilaster strips that centrally branch describing a

Environmental Semeiotics (2013) 6(2), 18-27 DOI 10.3383/es.6.2.1

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projecting molded arch with the inner bezel . Above, the facade is dominated by a large central molded arched opening. Inside, the church has a large single- aisle hall that closes deep with a large exedra (Coroneo R, 1993).

Figure 1: The front of the church of San Pietro dei Pescatori

Inside, the church has been much modified over time, presenting

restoration rather craft both as regards the finish of the walls as regards artifacts such as marble altars and furniture, moldings, staircases, balustrade in reinforced concrete and so on. (Figure 2). The overall result shows a clear disregard of the most basic precepts of conservation and restoration.

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Figure 2: The balustrade in reinforced concrete and two lateral modern niches.

The Church of San Pietro dei Pescatori is located in one of the historic districts of Cagliari, Stampace, near the district known as Marina which was the port of the city since the Roman period, important center of maritime trade routes in Rome, Africa and Spain. In the port area the popular and mercantile districts were located, as evidenced by the findings made under the Church of St. Eulalia. A large thermae building was built in the current Largo Carlo Felice, not far from the church of San Pietro. However, the heart of Carales , the Roman Cagliari, was constituted by the Forum, where there was a monumental temple with a large yard and the auditorium and many other related buildings, including tanks, water tunnels , remains of portions of road and an area for thermal plant. The area is also rich in other Roman remains, such as the so-called Cave of the Viper and the Villa of Tigellio. In this area, Roman Cagliari overlaps with the Phoenician of the previous era that had its heart in the lagoon of Santa Gilla.

A series of discoveries occurred starting from ‘800 in the area where the church stands and in the surrounding areas (Figure 3) indicating the presence of an extensive necropolis on the site. These findings consist, in fact, in a series of funerary inscriptions dated between the fifth and sixth centuries. A.D., currently housed at the National Archaeological Museum of Cagliari.

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Figure 3: The areas surrounding the church.

2. INSTRUMENTS AND METHODS

The church has been investigated with two georadar equipments, one for the inner and narrow areas and a second for the parking areas surrounding the church. The GPR method is in fact a reliable non invasive method for monitoring of underground presences, which was the main goal of this prospecting.

The G.P.R. used to investigate the corresponding area within the main room, the sacristy and the churchyard is a TerraSIRch SIR System-3000 manufactured by GSSI. The portable control unit is equipped with a full-screen which allows to see in real time the subsurface sections that are being analyzed, and a memory in which data is stored during the prospection.

The distances through radar profiles have been gauged by means of a manual marker procedure. The antenna used has been a single-channel equipment from IDS with nominal frequency of 200 MHz, very versatile and small (30x45 cm) (Figure 4).

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Figure 4: The G.P.R. TerraSIRch SIR System-3000

The areas investigated by means of the SIR 3000 instrument were covered along two orthogonal directions according to the following scheme (Figure 5).

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Figure 5: Acquisition schemes for the main room prospection.

The G.P.R. used, to investigate the outer perimeter of the Church is

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one STREAM X 200 System product from the IDS - Ingegneria Dei Sistemi. This system is a multi-channel equipment, being sell in two juxtaposed modules for a total of 16 antennas (central frequency 200 MHz) and 15 channels in a configuration optimized for a car or quad towing and able to cover wide free areas in very little times with good maps representation (Figure 6).

Figure 6: The G.P.R. STREAM X 200 System in its original configuration.

For the survey here illustrated, a modified version was chosen, using only 8 antennas and 7 channels. This modified setting was pushed by hand in the case of exploration carried out at the outer perimeter of the church of San Pietro dei Pescatori because of the relatively narrow areas.

The complete system is in the case of STREAM X a little more complex than SIR 3000, being the antennas linked to a control unit, the DAD, which in turn is linked to a portable computer for acquisition parameters set up, real time user control of GPR sections by monitor and data storage. The distances through radar profiles have been gauged by means of an automatic doppler system.

The biggest areas outside the church, actually with a parking main function, were investigated along parallel profiles according to the scheme in Figure 7.

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CHIESA DI S.PIETRO DEI PESCATORI

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Figure 7: Acquisition schemes for the outside areas prospection.

The data processing was performed on RADAN and GRED HD3 softwares according to standard procedures:

• Normalization, where required; • First bang correction; • Notch filter at 50 Hz; • Low-pass filter at 600 MHz; • Gain correction; • Migration and conversion time-depth through the hyperbola

analyses.

3. RESULTS

The depth-slices of the main room prospecting show some clear reflections, starting from 20 cm depth where a strong linear reflection is present (Figure 8.a). Looking at 125 cm (Figure 8.b) some strong

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reflections are still present but without a preferential linear shape. At bigger depths reflections disappear, and we have new strong reflections at 200 cm depth (Figure 9) where the layout seem to indicate an artificial square or rectangular structure in the area between the two niches and continuing under one of them.

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Figure 8: Depth slices at 20 cm (a) and 125 cm (b) of the main room. The entrance is on top-left edge while the main altar is on the opposite edge.

Figure 9: Depth slices at 200 cm of the main room. The orientation is the same as

previous Figure 8.

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Looking at the outside areas, the main parking area is the more interesting. At small depths, 44 cm (Figure 10.a), some reflection are clearly detectable: some linear shapes are present corresponding to the utilities networks but the most interesting is a strong and wide reflection, represented in orange, large approximately 2 by 4 meters.

This strong reflection is also confirmed at bigger depths, 124 cm (Figure 10.b).

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Figure 10: Depth slices at 44 cm (a) and 124 cm (b) of the parking area. The orange arrow indicate a strong reflection present at various depths.

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4. CONCLUSIONS

The georadar survey has been able to recognize many potential archaeological features of the studied site.

In particular inside the church some anomalies can be traced to ancient walls, the deepest to potentially very ancient walls. Some isolated anomalies can be traced to the cemetery use of the site and to underground graves.

The areas surrounding the church have shown an interesting very big object which, thinking to archaeological features, can be traced to a columbarium tomb.

Acknowledgements

Authors gratefully acknowledge the Gremio dei Pescatori for the possibility of carrying out the survey and for the useful informations on the site.

Authors gratefully acknowledge Luigi Noli for the assistance in the acquisition phases.

References Coroneo R. (1993) Architettura Romanica dalla metà del Mille al primo '300. Nuoro, Ilisso.