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13.1 Introduction

The need to have preliminary information to identify archaeological site or manage cultural heritage without invasive excavation activities represents a great challenge for the scientific research. An approach based on the use of geophysical techniques could offer an effective support for the resolution of this challenge. Thanks to their characteristics of non-invasivity and repeatability, geophysical techniques offer benefit to detect contrasts of physical properties of the subsoil associable to the presence of archaeological buried structures allowing the integration of different techniques reducing uncertainties of the interpretation investigating archaeological structures placed in different contexts without interfering with economic and social activities.

An integrated geophysical approach based on the use of electrical and electromagnetic data provides great advantages to overcome resolution and depth investigation limits of each techniques and for this reason it is strongly recommendable (Chianese et al. 2004, 2010; Rizzo et al. 2005, 2010, 2014; Nieto et al. 2005; Bavusi et al. 2009; Lasaponara et al. 2011; and reference therein). Generally the term used to indicate the use of geophysical techniques in the archaeological field is archaeogeophysics that includes all the archaeological prospection techniques that can help archaeologists for the remote sensing of the subsoil in the shallower layers interested by anthropic activities occurred in the past (Piro 2009).

The success of archaeogeophysics is mainly related to the existing physical contrasts between the properties of the hosting subsoil and archaeological buried remains. Furthermore presence of noise caused by anthropic activities as heterogeneity of subsurface material, building artefacts, underground utilities or electromagnetic interference or high attenuation phenomenon due to the characteristics of the soil can reduce the possibility to identify archaeological structures.

In this chapter there is a short discussion of the main geophysical techniques used in archaeology, namely ground penetrating radar, magnetic surveys and resistivity methods. Then, in order to improve the knowledge on the geophysical signature coming from buried remains characterized by low physical parameter contrasts, experimental tests realized on an analogue archaeological full scale laboratory case addressed on the assessment of the capability of geophysical techniques to detect archeological remains placed in humid/saturated subsoil are described. Finally, several interesting real cases where archaeogeophysical techniques, based on the use of Geomagnetic (M), Ground Penetration Radar (GPR) and electrical resistivity (DC) measurements acquired in different contexts are showed.

The application of GPR for archaeological research had become largely accepted. There are many reasons of this success that made the GPR the most widely used geophysical techniques in the archaeological field: high resolution, an acceptable depth of investigation for archaeological targets: a wide application of the GPR in urban archaeological areas where magnetic method struggles; finally the possibility of integration with the data obtained with other geophysical methods.

The electrical imaging is a geoelectrical method, widely applied in geological research, to analyze electrical behavior of the subsoil and its use is increasing in the archaeological field, in particular in lacustrine and water saturated scenarios as well as in marine and coastal areas. The great advantage of this method is that it is effective in conductive soils and in area characterized by electromagnetic noise supporting for example GPR measurements in clay soils or in marine areas (Passaro 2010; Simyrdanis et al. 2015) or noisier urban context archaeogeophysics (Osella et al. 2015; Papadopoulos et al. 2009).

Magnetic surveys have been often used in the last three decades in the archaeological fields to individuate buried remains and structures providing to have excellent results. Generally in the preliminary phase of archaeological investigation, magnetometric surveys are very able to delimitate archaeological areas of interest with an acceptable resolution reducing time of acquisitions without, nevertheless, give information on anomalies depth (Chianese et al. 2004; Drahor et al. 2011).

13.2 Archaeogeophysical Test in the Full Scale Laboratory

GPR and ERT are two of the most used geophysical techniques in archaeology but each of them suffers of important limitations. In particular this is true for the GPR that, although provide a high resolution, suffers from attenuation of EM energy in conductive scenarios. The EM velocity in soil is influenced by soil water content, at the same time greater is the water content greater is the conductive behaviour of the medium. But the most important role in the attenuation phenomenon is imputable to the presence of salts in solution of different nature and the clay content able to produce heavy attenuation losses (Tosti et al. 2013). For this reason GPR is less effective in clay or water saturated soil. The resistivity method could be a good solution to overcome limitations related to GPR. Indeed electrical resistivity tomographies (ERT) are capable to individuate targets placed in conductive soils where the EM signal is absorbed. So geophysical experiences based on the use of GPR and ERT were made to investigate and increase the knowledge about the influence of water content on geophysical response of archaeological remains (Capozzoli et al. 2015). The archaeogeophysical test was made reconstructing an archaeological framework of Roman times at the Hydrogeosite Laboratory of CNR-IMAA, a pool shape structures of 252 m3 filled with homogeneous silica sand (95% SiO2) characterized by an average diameter equal to 0.09 mm (very fine sand), a porosity of about 45 ÷ 50% and a hydraulic conductivity in the order of 10−5 ms−1. The archaeological site was composed by different kind of structures as burials, paved road and stone wall partially collapsed located at different depths (see Fig. 13.1). The test was realized in two main phases: in the first one, the top of the shallower structures was at a depth of 0.70 m; in the second phase this depth was limited to 0.20 m.

Fig. 13.1
An orthophoto of various excavated objects in a reconstruction of an archaeological site. Four sections indicate the depth and other measurements of the involved objects.

The archaeological framework focused on the Roman age reconstructed in laboratory, orthophoto (a), plan of remains (b) and sections (c). The circles shown in the maps represent the positions of the piezometers. The simulated structures, described clockwise, are a column, capuchin tomb, rectangular tomb and enchytrismos, paved road and stone wall (modified from Capozzoli et al. 2015)

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The geophysical surveys were performed using a GPR System SIR-3000 (GSSI-Instruments) coupled to a 400 MHz antenna and survey wheel and a Syscal Pro Switch 96 (Iris Instruments) georesistivimeter with the water table placed at different heights. GPR data were acquired in continuous and reflection mode with a time window of 70 ns, samples per scan set at 512 with a resolution of 16 bits and a transmit rate of 100 KHz. GPR surveys were carried out using a reference grid where the distance from each line was of 0.20 cm and the investigations, covering an area of 4.2 × 6.2 m, were made in both main directions. The geoelectrical acquisitions were performed in both the phases but in the first one, for the presence of concrete walls of the pool, the length of the profile was too little to give information about the test site.

After the adoption of an outright scanning, 2D radargrams of the site allowed us to identify the archaeological structures in dryer conditions as showed in Fig. 13.2 where depth slices with no interpolation are plotted. When the water level increase upon 0.20 m from the surface the attenuation phenomenon of electromagnetic signals didn’t allow to identify the deeper remains.

Fig. 13.2
A diagram exhibits 16 depth time-slices under 0.20, 0.80, 1.20, and 2.0 columns of depth of the water table from the surface and 0.95, 1.20, 1.35, and 1.55 rows of the estimated depth of investigations.

Depth timeslices with water level fixed at the different heights. The depth of the maps was estimated analysing the shape of the hyperbolas present in the 2D radargrams (modified from Capozzoli et al. 2015)

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In the second phase, after the removal of the first 50 cm of sand, different geoelectrical acquisitions were made adopting several types of arrays to carry out 2D and 3D acquisitions, also using loop shape grid. Some acquisitions were made directly on the water. 2D ERTs, based on use of electrodes placed every 0.20 cm according to Wenner-Schlumberger and Dipole–Dipole arrays, have showed their capability to identify, with an acceptable resolution, the archaeological buried remains. While 3D acquisitions, obtained with a grid of electrodes placed according a square mesh of 0.60 m have allowed to identify only the bigger structures as the stone wall. Strong uncertainties with soil characterized by high water content are related also for ERT acquisitions. The reason is imputable to the decrease of the electrical contrast between the background and the structure, especially in correspondence of the graves where the voids were filled with water (Fig. 13.3).

Fig. 13.3
A set of 3 D electrical resistivity images of a structure, along the z versus y versus x axes, with highlighted areas via a spectrum that ranges from 5.00 to 200 ohm in meters. The Roman road, stone wall, rectangular tomb, and Capuchin tomb are marked.

3D electrical resistivity images obtained with a 3D ERT acquisition according to an equatorial dipole–dipole array. The high values of resistivity are due to presence of structures (modified from Capozzoli et al. 2015)

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13.3 Integration of Geophysical Techniques: Real Cases

13.3.1 Archaeogeophysical Investigation at the Site of Masseria Nigro

During the construction of an oil-pipeline, some important evidences of the past came to light and various geophysical prospections were made. In particular an integrated geophysical survey to detect buried remains was realized into the archaeological site of ‘Masseria Nigro’, Viggiano, southern Italy, to identify ancient structures developed between the fourth and the third centuries BC, destroyed after the Roman conquest. GPR, ERT and MAG surveys were carried out to study the extent of the archaeological site (Rizzo et al. 2005). Regarding the magnetic measurements, an area of about 800 mq was investigated with a caesium vapour magnetometer G-858 GEOMETRICS (see Fig. 13.4). The mark spacing and profile spacing were set at 0.5 m while the sampling rate was 10 Hz. The acquisitions were made with a gradiometer configuration with two sensors placed at a vertical distance of about 1 m apart. GPR investigations were made with a SIR-2000 (GSSI Instruments) and the whole area was subdivide in two regular sectors. Fifty four parallel profiles equally spaced every meter were acquired and processed with Radan NT software. Finally, on a limited area three short parallel electrical resistivity tomographies (ERT) were carried out with the Syscal R2 multielectrode system (32 channels). A dipole–dipole array layout with an electrode spacing of 0.5 m was adopted.

Fig. 13.4
A wide shot of Masseria Nigro adjacent to an open field with scattered trees. One side is labeled as the magnetic investigation area where site 1 has G P R and geoelectrical investigation areas while site 2 only has the former.

Localisation of the investigated site with indication of the investigated areas with the different geophysical techniques (modified from Rizzo et al. 2005)

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Thanks to the high electric and electromagnetic contrast between the hosting soil and archaeological remains, it was possible obtain interesting information about shape, dimension and depth of the buried structure before the excavations activities supporting the archaeological research. In particular electrical resistivity tomographies (ERT) showed an increase of conductivity values in correspondence of the stone walls. GPR, in addition to recording high amplitude reflections of buried walls, allowed to identify the depth of the structures. Finally, MAG results have been permitted the reconstruction of the ancient house confirming hypothesis of archaeologists (see Fig. 13.5).

Fig. 13.5
A diagram of the M A G results with encircled anomalies on two sites, along the Y in meters versus X in meters axes, between a continuous excavation zone. Data from one of the structure's walls is exhibited via 3 diagrams on the right.

MAG acquisition (a) shows several well defined anomalies that provide a good reconstruction of the structure; GPR data (b) carried out on a smaller area are characterized by reflections in correspondence of the walls; ERTs have showed an increase of electrical resistivity values where walls are present, the data are then interpolated to define a 3D electrical resistivity volume (modified from Rizzo et al. 2005)

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13.3.2 Integration of Satellite Investigation and Archaeogeophysics at the Piramide Naranjada Site in Cahuachi (Peru) and Validation with In Situ Excavation

The integration of other remote sensing techniques with archaeogeophysics offers considerable advantages for the detection of archaeological sites as showed at the site of Cahuachi (Nasca, Southern Peru) where Very High Resolution (VHR) satellite imagery, geomagnetic surveys and Ground Penetrating Radar (GPR) were applied. In order to confirm archaeological features as walls detected via satellite, geomagnetic measurements with an optical pumping magnetometer G-858 Geometrics in gradiometric configuration were made to study an area of about 3000 m2(Lasaponara et al. 2011).

The focus of MAG acquisitions was to confirm the presence of buried adobe walls individuated by satellite images. The acquired maps have detected linear anomalies associable to shallows and outcropping walls, buried structures, tombs and ceremonial offerings.

In a smaller area as showed at the top of Fig. 13.6, GPR acquisitions were made with a SIR-3000 system (GSSI Instruments) coupled to a 400 MHz antenna without survey wheel. The data were acquired in two main directions mutually perpendicular every 0.30 m in one direction while every 0.50 in the other one. The processed data showed some reflections at a depth lesser than 0.50 m due to remains of huarango (trunks and branches belonging to a wood framework) as subsequent archaeological excavations have demonstrated. Further successive excavations have unearthed a ceremonial offering of the Nasca Culture as depicted in Fig. 13.7.

Fig. 13.6
A satellite image of the Cahuachi site with labeled A 1 to A 3 sections, where A 3, with M 1 to M 4 sections along with g p r, is enlarged. Below are 2 diagrams along 0 to 60 versus 0 to 56 and 0 to 56 axes, where the second has marked points of interest.

The site was investigated with GPR (continuous line square) and MAG (dashed line square) as depicted at the top of the figure. The MAG results obtained in M2, M3 and M4 show linear anomalies due to shallows and outcropping walls (black dotted line), archaeological feature likely associable to buried structures (white arrows) or potential buried tombs and ceremonial offerings (red circles). At the left lower corner is showed the acquired maps while at the opposite corner there is the interpreted map (modified from Lasaponara et al. 2011)

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Fig. 13.7
A radargram represents an encircled area, where significant crease-like formations are observed, on the upper left of distance versus depth axes. On the right is an aerial shot of the Cahuachi site.

The radargram (a) is characterized by strong shallow reflections imputable to the presence of ceremonial objects as confirmed by the archaeological excavations that provided a direct validation of the geophysical data (b). (modified from Lasaponara et al. 2011)

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13.3.3 GPR and MAG Acquisitions to Study a Medieval Monastic Settlement in Basilicata

Archaeogeophysics is often used to support the archaeological reconstruction of disappeared architectural heritage. For this reason GPR and MAG prospections were made in Calvello (Basilicata, Southern Italy), to individuate the medieval monastic plan of San Pietro a Cellaria built between the twelfth and the thirteenth century. Geophysical surveys were supported by UAV based survey that provided to georeference and better visualize the obtained results (Leucci et al. 2015).

A Geometrics G-858 caesium vapour gradiometer was used for magnetometric prospections along parallel lines spaced 1 m apart. Data were acquired in continuous mode with a sampling interval of 0.2 s. The considered processed data were distributed in a range varying between −30 and +30 nT/m, which represents values associable to archaeologically significant anomalies. Three areas M1 (11 × 25 m), M2 (15 × 12 m) and M3 (30 × 40 m) distributed around the existing building were analysed and the results, showed in Fig. 13.8, are characterized by weak anomalies (dashed dark line) in area M3, related to the presence of a buried structure. In particular the area M1 is characterized by buried walls as confirmed by GPR surveys.

Fig. 13.8
A satellite image of San Pietro a Cellaria in Calvello with superimposed images of labeled M 1 to M 3 areas along with a scale of 0 to 30 meters at the bottom. The M 1 image exhibits 2 arrows and 4 dashed rectangles of different lengths and sizes.

Magnetometric results with indication for the three map of the main anomalies (from Leucci et al. 2015)

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A georadar Hi Mod (Ingegneria Dei Sistemi-IDS) with 200 and 600 MHz antenna was used to investigate a small are of rectangular shape of size of 12 × 32 m. Parallel radargrams were acquired very 0.50 m according only the greater direction. The comparison between the depth slices extracted by reflection amplitude volume generated with GPR and MAG data confirms the presence of well-visible structures placed in continuity with walls belonging to the existing building. In particular the anomalies indicated with letters a, b, c and d in Fig. 13.9 are present both in MAG map and in GPR results and for the shape suggest a continuation of the actually visible structures.

Fig. 13.9
A diagram exhibits a magnetic map with G P R data, along y-axis values of 0 to 25 versus x-axis values of 0 to 11 between two depth time-slices with A to D labels. Z = 63 to 80 and 94 to 111 centimeters values are labeled on the side of the depth time-slices, respectively.

Comparison between depth-slices (a–c) obtained with GPR and magnetic map M1 (b). The results show the high integration of the two techniques. In fact the anomalies recorder with GPR are clearly associable to those showed by MAG, in particular for the b that indicates a buried wall (modified from Leucci et al. 2015)

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13.4 Conclusions

Archaeogeophysics based on the integrated use of different geophysical techniques is able to address the problems of archaeological nature overcoming limitations specific to each technique. ERT and MAG surveys can support GPR acquisitions in high attenuating soils, while GPR and ERT represent considerable advantages in urban areas where magnetic surveys are not applicable. Geophysical tests realized in laboratory conditions have showed uncertainties and difficulties of GPR and ERT acquisitions in humid scenarios where physical contrasts are very low; but nevertheless some good results are obtained. The MAG surveys supported by GPR and ERT are applied with success in various archaeological study cases and the effectiveness of their comparison with also other remote sensing techniques (i.e. satellite observation) is showed and strongly recommended for archaeogeophysical issues.