Fast Geophysical Prospection to Map the Archaeological Site of Cocciano: Preliminary Results

ANNALS OF GEOPHYSICS, 60, Fast Track 6, 2017; DOI: 10.4401/ag-7406

Fast geophysical prospection to map the archaeological site of Cocciano: preliminary results

VINCENZO SAPIA*, FEDERICO FLORINDO**, MARCO MARCHETTI *, MARIA DI NEZZA *

*Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy **Gruppo Archeologico Latino “Latium Vetus”, Monte Porzio Catone, Roma, Italy

[email protected]

Abstract

We present preliminary results of a multidisciplinary geophysical investigation applied to the subsoil im- aging of the archaeological site of Cocciano, near Rome. We acquired capacitive coupled resistivity data along two parallel profiles and we performed a magnetic survey over a small subset of the survey area. The recovered resistivity models suggest the presence of a shallow, sub-horizontal, resistive layer (ρ > 350 Ωm), of slightly variable thickness (2 – 3 m), which we interpret as the response of ancient substructions overlying a relatively low-resistive layer, which we ascribe to the geological substratum. Processed magnetic data show a clear magnetic signature aligned to form a curve-shaped anomaly right at the prosecution of a nearby, partially exposed, ancient wall.

I. INTRODUCTION • Lower substructions which constitute the north-western front of the Spinetta terrace, The study area (Latitude N41°49'11", Longi- composed of a sequence of 36 vaulted rectan- tude E12°41'02") is located in Cocciano, hamlet gular spaces (Figure 1); of the town of Frascati, inside the Colli Albani • Upper substructions which consist of further Volcanic District about 20 kilometers southeast substructural spaces with vaulted ceilings lo- of Rome. The investigation area regards the cated at the base of a late medieval (?) tower- north-west-facing terrace in the property house; called Spinetta, which is most notably charac- • Caementicium wall, about 0.6 m thick, only terized by the presence of a sequence of sub- partially emerging from the soil and vegeta- structional spaces opening on its front. The tion and clearly showing a curved trend whole Spinetta area was place of a series of ar- (Figure 2). chaeological findings, which are generally rec- With the aim to discover new buried archaeo- ognized as the remains of a vast Roman villa of logical features, non-invasive, fast and easy- the imperial age, notably attributed to Emper- deployable geophysical measurements were or Tiberius (Valenti 2003a). As already report- performed using the magnetic and the capaci- ed from Valenti (2003b), some relevant archae- tive coupled resistivity (hereinafter CR) meth- ological features characterize the site: ods (see Figure 3 for surveys location). 1 ANNALS OF GEOPHYSICS, Fast Track 6, 2017; DOI: 10.4401/ag-7406

Archaeological-related magnetic surveys are quired electrical data, which overlay a sub- frequently reported in the literature of the past horizontal (3 m thick) relatively conductive 50 years (Weaver, 1961; Bevan, 1994; Schmidt, layer. The extension of the surficial resistive 2007; Di Mauro et al., 2014). Conversely, CR layer is comparable to the observed substruc- measurements applied for similar studies, are tions forming the front side of the Spinetta ter- far less used and only few scientific articles are race as shown in Figure 2. The conductive layer available in the recent literature (Bottacchi et is interpreted as fallout pericalderic scoria- al., 2009; Alfouzan et al., 2016) cone deposits of the final stages of the Villa Two CR profiles were acquired over the Spinet- Senni eruption cycle (Madonna degli Angeli ta terrace, while the magnetic survey was con- succession, from 365±4 to 351±4ka) which crop ducted in an area where, an ancient wall, inter- out in the investigated area (Giordano et al., rupt its circular trend and still, presumably, 2006; Marra et al., 2009). These deposits, espe- below the ground surface. cially along the Alban Hills caldera walls are made by alternating lava, scoria and welded scoria fall deposits. As for the magnetic data, the vertical gradient magnetic map highlights the presence of a clear curve-shaped magnetic anomaly resembling the trace, at depth, of the exposed wall structure trend.

II. METHODS

A scalar magnetometer G-858 by Geometrics (sensitivity 0.05 nT) in gradiometer configura-

Figure 1. Example of the substructions located at the tion was employed with sensor separation of northwestern side of the survey area and forming 0.8 m and lower sensor placed at about 0.3 m the front part of the Spinetta terrace. above the ground. Data were acquired in bi- directional mode along parallel paths about 2 m spaced, using a sampling rate of 1 Hz. Ac- quired magnetic data were geo-referenced by means of an integrated GPS receiver (UTM projection, WGS84 coordinate system). The vertical magnetic gradient is computed by sub- tracting the measured bottom sensor magnetic data to the top sensor response. Since magnetic investigations depend on the contrast in the magnetic properties between the target and the enclosing soil matrix, a rapid magnetic susceptibility survey was performed. We used porta- ble magnetic susceptibility meter (sensitivity 1 Figure 2. Ancient wall with a clear curved trend and x 10-3 SI units) to complement the magnetome- an average thickness of about 0.6 m thick, only par- ter survey and help with the interpretation of tially emerging from the soil and the vegetation. the recovered magnetic anomalies. Ground resistivity data were measured using a Geomet- Preliminary geophysical results are encourag- rics OhmMapper system, using a multi- ing. We observe a high resistivity anomaly, of receivers array and 5 m transmitter and receiv- nearly constant thickness, throughout the ac- er dipole. To increase subsoil investigation 2 ANNALS OF GEOPHYSICS, 60, Fast Track 6, 2017; DOI: 10.4401/ag-7406

depth the two CR profiles, approximately et al., 2013). Despiked and corrected apparent west-east oriented, 15 m spaced apart and 150 resistivity data are inverted with res2Dinv m long, were collected using three different software using a smooth constrained least- rope lengths of 5 m, 10 m and 15 m respective- squared inversion algorithm (Loke and Barker, ly. Raw data are corrected using the effective 1996). dipole lengths for line antennas (Oldenborger

Figure 3. a) Location map of the area. The red dots indicate the survey area near the village of Frascati, Rome, Italy. b) Close up view of the survey area with the location of the geophysical measurements. The red lines show the CR profiles and the yellow box indicate the magnetic survey area.

III. RESULTS ed to the presence of a continuous magnetic sources, with less magnetic susceptibility than The magnetic survey provided us valuable in- the surroundings, buried at very shallow dications to speculate on the presence of a sub- depth. In fact, magnetic susceptibility meas- surface trace of the wall structure as the sub- urements were carried out to characterize the surface prosecution of the exposed wall. Data magnetic properties of the exposed wall and processing consisted of the removal of data the ground. Data were collected along the ex- spikes and other errors introduced during the posed wall and to the adjacent soil with a 2 m survey, such as striping effect between lines. mark spacing. Measured data revealed that the Among high intense, sparse and small-scale ancient wall exhibits magnetic susceptibility magnetic anomalies, the presence of a distinct values ranging from 12 – 17 x 10-3 SI while the magnetic signature in the vertical gradient surrounding ground ranges from 5 – 9 x 10-3 SI magnetic map is evident. As shown in Figure units. Although very small, the difference be- 4, this vertical magnetic gradient anomaly tween these values evidences a magnetic sus- reaches an intensity of -200 nT/m and is ar- ceptibility contrast between the exposed wall ranged to form an approximatively east-west and the soil. oriented curve pattern. Due to its lateral conti- Inversion results for the despiked CR data nuity, this magnetic anomaly is probably relat- along each profile are shown in Figure 5. 3 ANNALS OF GEOPHYSICS, Fast Track 6, 2017; DOI: 10.4401/ag-7406

IV. DISCUSSION

This short paper presents the preliminary results of a geophysical survey conducted in the Cocciano-Spinetta archaeological site. The aim of this work was to check the feasibility of two fast and easy-deployable geophysical methods as a tool for detecting the presence of subsurface archaeological targets. The success of any magnetic investigation strictly depends on the existing magnetic contrast between the feature of interest and the soil. In this case, the ancient wall is made of leucitite shards, an effusive Figure 4. 2-D contour map of the magnetic vertical volcanic rock type, mixed to a fine concrete gradient. Data are pre-processed using the Mag- matrix. The magnetic susceptibility contrast Map2000 software and then interpolated by the between the volcanic rock and the surround- Kriging method using a regular grid of 3 × 3 m cells. ing soil supports the recorded magnetic data and facilitates the interpretation of the ob- In general, the recovered resistivity models served magnetic anomalies. clearly show two distinct electrical units. From The magnetic results allowed us to confirm a the surface down to about 3 m depth, we ob- circular trend of the wall, in accordance to serve a relative higher resistivity response of what proposed by Valenti (2003a). The recov- the subsurface materials. In particular, profile ered negative vertical magnetic gradient P1 (Figure 5a) shows a resistive layer of slight- anomaly, right at the prosecution of the wall, ly variable thickness with resistivity values can be due to: a) the magnetic properties of the above 400 Ωm throughout its entire length. wall are different at depth, b) the wall has been This layer overlays a sub-horizontal, 3 m thick entirely replaced with materials characterized low-resistive layer with minimum resistivity by different magnetic properties. The first op- values of about 60 Ωm. Below this layer, the P1 tion will involve the partial substitution of the CR resistivity model hints to a high resistive wall with soil of significantly lower magnetiza- layer at depth. Similar results come up also tion. The second option implies that the wall from the CR profile P2 (Figure 5b). The shal- has been entirely removed from its original lower portion of the subsoil clearly shows a position and replaced with less magnetic sus- high resistive layer, about 3 m thick, which ceptibility soils. Both options would justify the overlays a sub-horizontal 3 m thick low- link between the recovered negative vertical resistive layer up to a depth of 7 m. Again, the magnetic gradient anomaly and the lack of the recovered CR model indicates that the resistiv- wall (which showed higher magnetic suscepti- ity increases at depth, although we are aware bility), thus ascribing the recovered magnetic that the resistivity features at the bottom of gradient response to the presence of its trace at each CR profiles are in a lower sensitivity re- depth. gion and they could reasonably be explained as numerical artifact of the inversion.

4 ANNALS OF GEOPHYSICS, 60, Fast Track 6, 2017; DOI: 10.4401/ag-7406

Figure 5. CR subsurface resistivity inversion models for the a) P1 profile with RMS = 0.8% (with left being south-west and right north-east), b) P2 profile with RMS = 1.1% (with left being south-west and right north- east), Same resistivity colour scale is adopted for the two CR profiles. Dashed white line hypothesizes the substructions/bedrock boundary.

We considered the CR survey as a very useful Monti Tiburtini, visually dominating the tool for a preliminary investigation of the sub- whole eastern “Ager Romanus”. soil prior to set up a more laborious electrical This preliminary study indicates that both resistivity tomography measurements (Sapia et magnetic and CR techniques can be used for al., 2017). In fact, the OhmMapper instrument archaeological investigation purposes in Coc- was found to be very fast and accurate to map ciano. In the near future the geophysical sur- the subsurface resistivity up to about 7 m vey will be extended to other sectors of the ar- depth. We were able to acquire two parallel chaeological area. In particular, electrical resis- electrical profiles, each 150 m long, using three tivity tomography (ERT) will be acquired ei- rope lengths for a total path of almost 800 m in ther over the terrace or to image the subsoil be- less than one hour (see HRA-P1 and HRA-P2 low the circular wall structure. The ERT data in Figure 6). The overall resistivity distribution collected over the same profile as the CR can evidenced the presence of a high resistive layer be also used to cross-check both recovered re- up to a depth of 3 m, which is coincident with sistivity models. In addition, new CR profiles the thickness of the north-western substruc- will be performed parallel to HRA-P1 and P2 tions (see Figure 1). In our opinion, the high sections in order to cover the entire terrace ar- resistivity could be the response of such a mix- ea. By this way, CR and ERT data could then ture of bricks and concrete as observed from be used to build up a 3D resistivity model of the exposed substructions which is in contrast the whole Spinetta terrace up to a depth of with the hypothesis of decorative front wall as about 20 m. A combined use of magnetic and reported from Valenti (2003a, b). ground penetrating data (GPR) will be The Spinetta terrace, although taking advantage planned over two, infrastructure-free, subset of the natural slope of the hill, is clearly artifi- areas located both at the eastern and western cial. Facing north-west it overlooks a very im- side of the archaeological site with the aim to pressive landscape (today partly covered by image the presence of further buried targets constructions and the Rome-Frascati railway (i.e. buried walls, floorings, roads). artificial slope) ranging from Rome to the 5 ANNALS OF GEOPHYSICS, Fast Track 6, 2017; DOI: 10.4401/ag-7406

Figure 6. General planimetry of the study area adapted from Valenti (2003a) showing the substructional structures, with schematization of the High Resistivity Areas (HRA) resulted from the two CR profiles (P1 and P2) and results from the magnetic survey carried out in the area of the caementicium circular wall.

ACKNOWLEDGEMENTS Bevan, B. W. (1994). The Magnetic Anomaly of a Brick Foundation. Archaeological Prospec- The authors kindly thanks Enrico Devoti and tion, 1, 93. Giovanna Cappelli for their valuable assistance Bottacchi, M. C., Colonna, T., Mantovani, F., during the fieldwork. We also thanks the re- and Medri, M. (2009). Application of the viewer for the constructive comments and OhmMapper resistivity-meter to detect the helpful suggestions which helped us to im- theatre of Sentinum Roman town by using prove the manuscript. 3D resistivity model. ArcheoSciences. Revue d'archéométrie, 267-269. REFERENCES Di Mauro, D., Alfonsi, L., Sapia, V., and Urbi- ni, S. (2014). A neighborhood revealed by ge- Alfouzan, F., Zhou, B., Bakkour, K., and ophysical prospection: An example of urban- Alyousif, M. (2016). Detecting near-surface ization at the Phoenician–Punic settlement of buried targets by a geophysical cluster of Mozia (western Sicily, Italy). Journal of Ap- electromagnetic, magnetic and resistivity plied Geophysics, 104:114-120. scanners. Journal of Applied Geophysics, Giordano, G., De Benedetti, A. A., Diana, A., 134:55-63. Diano, G., Gaudioso, F., Marasco, F., Miceli,

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