Geophysical Survey and Archaeological Data at Masseria Grasso (Benevento, Italy)
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The use of geophysical methods in metrology is a significant tool within the wide research topic of landscape archaeology context. Since 2011, the Ancient Appia Landscapes Project aims to recognize dynamics, shapes and layout of the ancient settlement located along the Appia road east of Benevento, and cyclical elements and human activities that influenced the choice of landscapes. The integration of geophysical data with an archaeological infra-site analysis allowed us to investigate the area of Masseria Grasso, about 6 km from Benevento (Campania region, Italy). In this framework, an archaeogeophysical approach (Geomagnetic and Ground Penetrating Radar) was adopted for detecting anomalies potentially correlated with buried archaeological evidences. The geomagnetic results have given a wide knowledge of buried features in a large survey highlighting significant anomalies associated with the presence of buildings, roads and open spaces. These geophysical results permitted us to define the first archaeological excavations and, successively, a detailed Ground Penetrating Radar approach has been provided highlighting the rooms and paved spaces. The overlap between archaeological dataset and geophysical surveys has also allowed recognizing the path of the ancient Appia road near the city of Benevento and hypothesize the settlement organization of the investigated area, which has been identified with the ancient Nuceriola.
KeywordsArchaeogeophysics Geomagnetic GPR Ancient Appia road
This paper deals with the results of an archaeogeophysical approach based on the comparative use of Geomagnetometric (GM) and Ground Penetrating Radar (GPR) measurements with an archaeological study. Among the various geophysical techniques, the most interesting and effective are the geomagnetic and electromagnetic techniques that are able to locate and identify archaeological structures at different scale and depth with good resolution in different scenarios. The growing interest in non-invasive and fast techniques able to localize and identify the presence and geometry of archaeological unearthed structures has led to the creation of an important discipline based on combined use of archaeological and geophysical knowledge, which is called Archaeogeophysics. This specific topic is part of the more general geoarchaeology, and it is based on the use of different geophysical techniques able to detect contrasts of physical properties of the subsoil associated to archaeological buried structures. The integration of different techniques reduces the uncertainties of the interpretation and permits is to investigate archaeological structures placed in different contexts without interfering with economic and social activities. Moreover, the archaeogeophysics improves the integration between the classical archaeological observations and the geophysical results in order to identify the best image of buried structures (Rizzo et al. 2005, 2010; Piro 2009; Peregrine et al. 2012; Leucci et al. 2015).
During the last three decades, GM surveys have been often used to identify buried remains and structures providing excellent results. Among the geophysical techniques which can be employed for the archaeological research, the geomagnetic method seems to be the most suitable and the most largely exploited because of its reliability and for the aptitude to provide quick data acquisition, non-invasive investigation and high-resolution experimental data. However, its usefulness is drastically reduced in urban areas because the electromagnetic noise due mainly to the presence of objects. Generally in the preliminary phase of archaeological investigation, GM surveys are very able to delimitate archaeological areas of interest with an acceptable resolution reducing time of acquisitions without, nevertheless, giving information on the depth of the anomalies. Then, in a secondary phase, GPR measurements should be adopted to enhance the quality of the acquired data and focus on the anomalies. For this reason, the two techniques are often used in tandem (Chianese et al. 2004, 2010; Rizzo et al. 2005, 2010; Piro 2009; Capozzoli et al. 2017).
Although governed by the same fundamental equations, magnetic and electromagnetic surveys are very different. GM survey measures the variation of the magnetic field of the Earth and the effects caused by anthropogenic artifacts located in it. The physical property investigated is called magnetic susceptibility that is characteristic for each magnetizable material and depends mainly on the volume per cent content of magnetite. The induced magnetization in a rock due to the Earth’s field, F is directly proportional to magnetic susceptibility. Further, a remanent magnetization phenomenon should be considered added to the one caused by Earth’s field due to the thermoremanent magnetization acquired by magnetic grains during cooling from the Curie temperature to normal atmospheric temperature in the presence of an external field.
Actually, there are three types of magnetometer-fluxgate, proton precession and alkali vapour that can be used singly or in tandem in gradiometer array. Gradient measurements emphasize archaeological contrasts allowing the differentiation between deeply buried objects versus those that are shallower. Usually, parallel lines spaced 1 m apart for square area define a geomagnetic survey. Data are generally acquired in continuous mode and processed with the use of bandpass filters to enhance archaeological features and eliminate events, referred to as spikes, that are attributable to the presence of alignments due to recent anthropogenic activities (Piro 2009). In particular, the successful application of geomagnetic measurements to identify archaeological features is shown in particular to identify the presence and distribution of archaeological remains (Rizzo et al. 2005, 2010; Milsom and Eriksed 2011), map subsurface anthropogenic features and find buried tombs (Caggiani et al. 2012; Keay et al. 2014), detect walls and ceremonial areas also reconstructing the architectural plan of lost settlements (Cardarelli et al. 2008; Viberg et al. 2013; Masini et al. 2017).
The application of GPR for archaeological research is common, and several research papers have been published. There are many reasons for this success that made the GPR the most widely used geophysical techniques in the archaeological field. One of the most important is the high resolution of the method related to the operating frequency of the antenna. Moreover, the GPR has an acceptable depth of investigation for archaeological targets, generally ranging between 0.50 and 3.00 m depending on the conditions of the subsoil. Other GPR advantages are: the substantial indifference of the method with respect to the electromagnetic radiations that allows a wide application in urban archaeological areas where the magnetic method struggles; the high capacity of integration with other geophysical data (Goodman and Piro 2013). GPR is based on the study of the scattering phenomenon encountered by a high frequency electromagnetic signal introduced into the ground via antenna. The typical antennas for archaeogeophysical target have a range between 200 and 900 MHz, simultaneously generate and receive the EM signal while they are moved along profiles. The higher the operating frequency, the smaller is the investigated depth but the better is the obtained resolution. The dielectric permittivity influences strongly the EM velocity of propagation and the correct evaluation of these parameters is fundamental to identify the depth of the buried remains. While generally magnetic permittivity is neglected (but it must be carefully evaluated in presence of ferromagnetic minerals that can have a considerable effect on GPR wave velocity and signal attenuation), the static conductivity value assumes great importance because variations of this property generate a severe attenuation of the signal that in several cases can reduce heavily the investigation depth. This is the case of water-saturated soils (in particular with marine scenarios with salt water) or clay soils where the method is less effective (Daniels 2004; Annan 2009).
To effectively manage a GPR campaign, rectangular grids are established and profiles every half meter in at least two directions mutually perpendicular should be acquired to have a good reconstruction of remains into the soils. The acquired data, then, are processed via PC adopting filter that removes ringing noise or other disturbance sources and results often are showed using horizontal slices where the higher reflective areas are related to archaeological remains. The horizontal slices acquired in time domain are inverted in the space domain generating depth slices that are able to support archaeologists in managing the subsequent invasive excavations (Conyers 2006). Moreover, its success for archaeological prospections is shown by the increasing number of papers where the method is successfully used to detect buried structures in urban and rural areas and discover archaeological features in a great number of scenarios, such as to identify ancient settlements (Novo et al. 2008; Piro et al. 2011; Trinks et al. 2014), locate unearthed burials and ceremonial offering (Pipan et al. 2001; Rizzo et al. 2010), reconstruct the history of ancient buildings (Piro et al. 2003; Goodman and Piro 2009; Masini et al. 2017), image structures and infrastructure (Leucci et al. 2002; Ranieri et al. 2016), identify subwater structures (Capizzi et al. 2007; Simyrdanis et al. 2015; Capozzoli et al. 2017).
The outline of this paper is the following: in the second section the archaeological context will be described with a short summary of the adopted archaeological methodology. In the third section, the archaeogeophysical approach used in order to identify archaeological features will be discussed analysing and the results will be shown considering also the validation of geophysical data with the direct data obtained after the excavation activities.
2 The Archaeological Context
The Ancient Appia Landscapes Project (AAL), carried out by Dipartimento di Scienze del Patrimonio Culturale of University of Salerno, is developed in partnership with the Soprintendenza Archeologia, Belle Arti e Paesaggio delle province di Caserta e Benevento, the National Research Council (CNR-IMAA) of Tito Scalo (Italy, PZ), the University of Molise (GeoGisLab, Dipartimento Bioscienze e Territorio), the University of Sannio (Dipartimento di Scienze e Tecnologie). The research aims to study the path of Via Appia, the ancient road crossing the city of Beneventum, and to identify environmental phenomena, socioeconomic and productive activities that influenced settlement dynamics before, during and after the construction of the consular road (Fig. 1). The project is based on a multidisciplinary approach and a profitable exchange of knowledge between different topics, involving various methods and tools of investigation (study of archives, historical maps, paleobotanical analysis, surface surveys, remote sensing interpretation, geomorphological analysis) to perform knowledge of archaeological remains from a territorial dimension to an infra-site analysis (De Vita and Terribile 2014; Rossi and Santoriello 2014; Santoriello et al. 2015). This approach is based on some fundamental steps, which constitute the foundations of the modern discipline called landscape archaeology (Birks et al. 1988; Cambi 2011).
According to the achieved results, it was decided, within the project, to start an investigation on a minor territorial scale, setting up infra-site activities in some of the most significant sites. In detail, archaeological surveys in the area of Masseria Grasso, at 6 km from Benevento, has recorded an important settlement of about 7 hectares, dated, with some temporal gaps, from the middle of fourth century BC to the sixth–seventh century AD. After all, the area was already known by a significant discovery at the beginning of the twentieth century: here, A. Meomartini found remains of a paved road interpreted as a section of the ancient via Appia (Meomartini 1907), iso-oriented to the hypothetical centurial system (N 42°E) later recognized by French scholars.
From a geomorphological point of view, the Masseria Grasso site is located on a large plateau disposed between watercourses characterized by steep slopes in constant erosion. However, the archaeological deposit placed on the terraced areas suffered minor dislocations, almost exclusively due to agricultural activities. Moreover, cartographic and photointerpretation analyses have returned some iso-oriented traces pertaining to Beneventum I centuriation, suggesting a strong preservation of the Roman landscape in the actual rural fabric. Over the period 2014–2017, areas with significant anomalies in terms of archaeological evidences have been investigated by geophysical surveys in order to confirm the presence of structures.
3 Geophysical Acquisition and Data Processing
The GPR surveys were performed with a RIS MF Hi-Mod GPR System of IDS equipped with an array of two multi-frequency antennas using simultaneously 200 and 600 MHz antennas mounted on a survey cart equipped with an incremental encoder. The 200 MHz and 600 MHz data were acquired in continuous and reflection mode with a time window of 120 ns and 90 ns, respectively, samples per scan set at 512 with a resolution of 16 bits and a transmit rate of 100 kHz. With this set-up, the data were acquired on an area of about 150 m2 characterized by the presence of some obstacles such as the excavated area and some mounds of earth related to the excavations activities. For this reason, the whole area was subdivided into four areas with limited size (Fig. 2b). For investigating each area, a regular grid was adopted. In detail, the grid was based on measurements every 1 m according to two main directions mutually perpendicular. The surface of the investigated site was rough, and the presence of vegetation did not provide optimal conditions of acquisition and for this reason is impossible to use a more accurate step of acquisition with closer profiles. The GPR acquisition was supported by a topographic survey that gave the possibility to georeference the obtained data that were managed with a CAD software. The raw data have required some processing operations addressed to reduce the noise of the measurements and attenuation phenomena. In order to process the 2D-data, some classical operations were chosen according to a simple scheme with the support of the software Reflex-W based on the following steps: Editing of the data to assign the right coordinate according the defined grid of acquisition; amplitude normalization of the traces normalizing their amplitudes with the mean amplitude value measured along the complete radargram; application of the background removal filter that provide to remove the average of all background noise that influence the data; use of a gain function to amplify the measured signal based on a mean amplitude decay curve determined from all existing traces; use of bandpass frequency filter to reduce the increase in noise affecting the radargram caused by the gain function previously adopted and other source of noise; time cut to properly reduce the adopted time window until the signal was “clean” and appropriate on the basis of archaeological data; migration of the data based on the Kirchhoff algorithm with a velocity estimated quantitatively using the diffraction hyperbolas generated by potential archaeological features. Then, the processed data were interpolated to realize the 3D reconstruction of the investigated subsoil considering an interpolation distance equal to twice the used step of acquisition (i.e. 2 m). The volume was discretized according to a 3D fine grid defined by the envelope of the amplitude values calculated each 0.15 m in the two main directions. From the 3D data volume, every 0.20 m the depth slice was extracted and georeferenced in CAD to manage and facilitate the interpretation of the data.
4 Interpretation of Data and Discussion
The geomagnetic results were combined with the archaeological surveys, in order to provide useful information on the first excavation zone. The results provided an identification of several magnetic anomalies related to the presence of a significant buried archaeological record (Fig. 4). In detail, the geomagnetic map shows several iso-oriented anomalies potentially related to the presence of buried structures, which should occupy the entire northern portion of the investigated area: they probably prove the presence of relevant buildings, roads and open spaces belonging to a multi-stratified site. In the central portion of the geomagnetic map, significant geomagnetic anomalies are well depicted: they are 7 m wide and visible for about 100 m along a straight line with orientation N23°W. A similar anomaly is highlighted through the analysis of aerial photographs of 2006 shown in Fig. 5. Along this important axis, at least three orthogonal traces of smaller size (about 4 m wide) seem to define regular spaces, marked by another N23°W trace on the east side. These rectangular shaped areas show a considerable complexity probably resulting from the division of the interior spaces of some buildings. Their size (about 35 × 53 m) may correspond with a block measuring 1 × 1.5 actus (1 actus, a Roman unit of length, is about 35 m).
5 Conclusions and Future Perspectives
The authors and AAL project team (www.aalproject.eu) thank the “Scuola di Specializzazione in Beni Culturali” of the University of Salerno, the team of Laboratorio Modelli (director Salvatore Barba), Dipartimento di Ingegneria Civile, of the same University, the Centre Jean Bérard (USR 3133 CNRS - EFR) for their support for the archaeological research. Special thanks to all the students of the University of Salerno. Enzo Rizzo thanks the Director of CNR-IMAA (Dr Vincenzo Lapenna) for his support to the geophysical activities in AAL project. The authors thanks the two anonymous reviewers for their useful comments and suggestions that improved the paper.
ER, LC, GDM and FP carried out the geophysical measurements (Geomagnetic and GPR), processed and analysed the data. AS (AAL Project Leader), CBDV and DM carried out the archaeological studies, the excavations and the interpretation of the geomagnetic maps.
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