Electrical resistivity imaging (ERI) and ground

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Journal of Applied Geophysics 123 (2015) 218–226

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Electrical resistivity imaging (ERI) and ground-penetrating radar (GPR) survey at the Giribaile site (upper Guadalquivir valley; southern Spain) J. Martínez a, J. Rey b,⁎, L.M. Gutiérrez c, A. Novo d, A.J. Ortiz c, M. Alejo c, J.M. Galdón e a

Departamento de Ingeniería Mecánica y Minera, Escuela Politécnica Superior de Linares, Universidad de Jaén, Spain Departamento de Geología, Escuela Politécnica Superior de Linares, Universidad de Jaén, Spain Instituto Universitario de Investigación en Arqueología Ibérica, Campus de Las Lagunillas, Universidad de Jaén, 23071 Jaén, Spain d IDS North America Inc. Golden, CO 80401, United States e Instituto Innovación, Ciencia y Empresa, 23009 Jaén, Spain b c

a r t i c l e

i n f o

Article history: Received 20 March 2015 Received in revised form 14 October 2015 Accepted 19 October 2015 Available online 19 October 2015 Keywords: Electrical resistivity imaging Ground penetrating radar Giribaile Iberian site Ovens Carthaginian presence

a b s t r a c t The Giribaile archaeological site is one of the most important Iberian enclaves of the Alto Guadalquivir (Southern Spain). However, to date, only minimal excavation work has been performed at the site. Evaluation requires a preliminary, non-destructive general analysis to determine high-interest areas. This stage required a geophysical survey. Specifically, a 100 m2 grid was selected, where an initial campaign of nine electrical resistivity imaging (ERI) profiles was performed, where each profile was 111 m in length; these profiles were previously located using a detailed topographical survey. A total of 112 electrodes were used for each profile, spaced at 1 m apart with a Wenner-Schlumberger configuration. Secondly, 201 GPR profiles were created using a 500 MHz antenna. The 100 m long profiles were spaced 0.5 m apart and parallel to one another. The present research analyses the efficiency of each of these geophysical tools in supporting archaeological research. Using these methodologies, the position, morphology, and depth of different buried structures can be determined. 3D interpretation of the geophysical survey in 100 × 100 m grid allowed to differentiate structures square and rectangular, interesting buildings in a semicircle (interpreted as ovens) plus delineate different streets. From the geophysical survey follows the Carthaginian presence inside this ancient Iberian enclave. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Archaeological exploration is increasingly supported by geophysical surveying because these indirect, non-destructive methods allow the most appropriate locations to be selected for future excavation and documentation campaigns. Different techniques are used to determine variations in subsurface properties, among which electromagnetic and acoustic wave propagations or the electrical resistivity, and magnetic properties can be highlighted (Reynolds, 1997; Drahor, 2006; Batayneh, 2011; Zheng et al., 2013). The electrical imaging or electrical resistivity imaging (ERI) is a geoelectrical method, widely applied, to obtain 2D and 3D high-resolution images of the resistivity subsurface patterns. This has been successfully used in archaeological prospecting for a methodological approach designed to define areas of maximum interest where surface data collection and successive excavation should be planned (Leucci et al., 2007; Leopold et al., 2010). Ground penetrating radar (GPR) is a geophysical method based on the propagation, reflection and scattering of high frequency electromagnetic waves ⁎ Corresponding author at: Escuela Politécnica Superior, Campus Científico Tecnológico de Linares Jaén, Spain. E-mail addresses: [email protected] (J. Martínez), [email protected] (J. Rey), [email protected] (L.M. Gutiérrez), [email protected] (A. Novo).

http://dx.doi.org/10.1016/j.jappgeo.2015.10.013 0926-9851/© 2015 Elsevier B.V. All rights reserved.

in the subsurface. The GPR method has been successfully used in archaeological prospecting for mapping shallow subsurface objects (Porsani et al., 2010; Urban et al., 2014). Leucci and Negri (2006) used Electrical Resistivity Tomography and Ground Penetrating Radar in the ruins of Hierapolis (Lycus Valley, western Turkey). The effectiveness of the two geophysical methods in mapping the archaeological features was confirmed. At the Peinan archaeological site (Taiwan), ERT and GPR methods can provide useful information to understand the distribution of the ancient building in a Neolithic village (Tong et al., 2013). This work presents data from a study being performed at the Giribaile archaeological site, which is located near the Guadalén swamp, approximately 7 km northeast of Linares (high Guadalquivir, Fig. 1A). The Giribaile site is on an elevated plateau (Fig. 1B) on the right edge of the Guadalimar River, near its confluence with the Guadalén River. The area is a natural reserve due to its particular geological formations and its location overlooking the Iberian Central Plateau and the Guadalimar valley. These features make the site a strategic geopolitical enclave. Over the centuries different cultures settled in this site which makes it very interesting for archaeological investigations (Gutiérrez, 2011). The archaeological research project at the Giribaile site spans until 2019 and it is funded by the regional government (Junta de Andalucía). The main aim of the project is to develop strategies and methodologies in order to find a balance between archaeological prospection and

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Table 1 Characteristics of the ERI- and GPR-measured profiles. ERI profiles Profile

Electrode configuration

Orientation

Electrodes space (m)

Total length (m)

Electrodes required

Measurements

Levels

RMS

Giribaile 1 Giribaile 2 Giribaile 3 Giribaile 4 Giribaile 5 Giribaile 6 Giribaile 7 Giribaile 8 Giribaile 9

Wenner–Schlumberger Wenner–Schlumberger Wenner–Schlumberger Wenner–Schlumberger Wenner–Schlumberger Wenner–Schlumberger Wenner–Schlumberger Wenner–Schlumberger Wenner–Schlumberger

SE–NW SE–NW SE–NW SE–NW SE–NW SE–NW SE–NW SE–NW SE–NW

1 1 1 1 1 1 1 1 1

111 111 111 111 111 111 111 111 111

112 112 112 112 112 112 112 112 112

1000 1000 1258 1258 1000 1000 1000 1000 1000

7 7 9 9 7 7 7 7 7

8.19 13.08 10.79 10.70 13.18 12.06 14.85 7.27 12.42

Ground penetrating radar (GPR) Profile

Total length (m)

Orientation

Antenna (MHz)

Time windows (smp)

Velocity mm/ηs

Traces n°

Sampling frequency

Point interval (m)

1 to 201

100

SE–NW

500

84

100

3800

5220

0.026

and receiver antennas that are placed in a shielded case which is moved across the terrain surface (Fig. 2f). The penetration depth and resolution depend on the electromagnetic properties of the materials that were penetrated and on the antenna being used. Wave propagation through the subsoil is diminished as the conductivity of the terrain increases or if the frequency of the signal that is being transmitted increases. For the same profile, if antennas with higher frequencies are used, a greater resolution and lower penetration depth are achieved, and vice versa (Davis and Annan, 1989). In archaeology, this geophysical survey method has been used since Bevan and Kenyon, 1975. Gradually, its use became quite popular among archaeologists and geologists. In the last few years, new hardware developments, dense data acquisition methodologies and advanced data processing software have established three-dimensional GPR data in archaeological prospection (Basile et al., 2000; Negri and Leucci, 2006; Yalçiner et al., 2009; Porsani et al., 2010; Novo et al., 2010, 2013; Conyers, 2013; Zhao et al., 2013; Urban et al., 2014). As well, during the last two decades, several works highlighted the use of GPR for analysing facies in sediment studies (Bano et al., 2000; Neal, 2004; Bersezio et al., 2007; Rey et al., 2013b). For this project, a 500 MHz Pro-Ex RAMAC system by MALA GeoScience was used. Survey settings are detailed in Table 1. The survey area is composed of a 100 × 100 m grid that was covered with 201 parallel profiles (NW–SE orientation). In order to ensure straightness and parallelism, stakes were placed every 10 m and a string was used as guideline for each profile (Fig. 2f). 3. Results and discussion 3.1. Electrical resistivity imaging Interpreting the electrical tomography profiles was conducted based on the apparent resistivities obtained during field work (Fig. 3a and c) and were processed by the resistivity programme RES2DINV. This calculation programme is based on the method of least squares with forced smoothing, which is modified using the quasi-Newton optimization technique that is the faster method to estimate the Jacobian matrix (Loke and Barker, 1996). The inversion method builds a subsoil model using rectangular prisms, determines the resistivity values of each profile, and minimises the difference between the apparent resistivity values observed and the calculated values (Loke and Barker, 1996; Loke and Dahlin, 2002).In the nine profiles that were obtained, first, a superficial level was detected with a median thickness of 1 m, which was characterised by lower resistivities (between 80 and 250 Ω m). This set is interpreted as the superficial fill layer, which consists of fine materials (lutites or fine sands). The geophysical survey suggests that the soil thickness tends to be small. Although the thickness increases

in the SE direction, the median values do not surpass 1 m. At this level, resistivities sharply increase, surpassing 1500 Ω m (Fig. 3b and d), which is interpreted as the tertiary calcarenitic unit. As an example, Fig. 3 shows two of the nine profiles that were created, where the levels are easily observable. More specifically, the first metre of depth exhibits vertical structures characterised by increased resistivity. In all cases, these structures exhibited a different response, which is why these responses could signify a different range of anomalies. First-order anomalies (strong anomalies in Fig. 3) exhibit resistivity values that surpass 1000 Ω m and therefore mark an extremely rapid, strong change in the signal. As examples, see those indicated at 48 m and 68 m in profile 1 (Fig. 3b) However, the resistivity values of second- and third-order anomalies (moderate anomalies in Fig. 3) do not exceed 500 Ω m. As examples, see those indicated in 7 and 73–77 m in profile 1 (Fig. 3d). The different range of resistivity anomaly might be related to the thickness or the material of construction of these possible anthropic structures. Vertical structures similar to those described for profile 1 can be observed in the remaining profiles (see Fig. 3). However, given the distance between the different profiles (12.5 m), it is impossible to correlate the anomalies. However, using this technique allows us to detect the depth, dimensions and areal dispersion anomalies. This information will be very useful for designing the GPR campaign. 3.2. Ground-penetrating radar For a first analysis of the GPR profiles, 2D signal processing was performed using the MALA Ramac GroundVision and RadExplorer 1.4 software packages. Radar signal processing was applied to the radargrams that were obtained in the field in order to cancel frequency noises, overcome energy decay and therefore enhance visualisation of reflections of archaeological and geological interest. Two gain algorithms were used: Automatic gain control with a window length of 60 and a scale factor of 5000 and Time Varying Gain with a beginning sample size of 100, a linear gain of 30, and an exponential gain of 5. Different tests with horizontal and vertical filters were also performed; a bandpass filter was used applying a lower cut-off frequency at 100 MHz and upper cut-off frequency at 1000 MHz. Contrary to the ERI results, the GPR signal rapidly attenuates with depth. This low penetration is produced by the high conductivity of the upper lutitic layer. Despite this high conductivity, four main types of reflections were observed during this first analysis: a) Shallow strong reflections are present in different profiles. They have large dimensions (wider than 1 m) (Fig. 4a). When analysing these reflections over a set of consecutive profiles, the length of these structures expands to approximately 5 m.

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Fig. 3. Electrical tomography profiles 1 and 7 with their apparent and true resistivities. The position of the profiles is indicated in Fig. 1. In each profile, electrical anomalies are highlighted, which could represent the presence of vertical structures (ancient walls).

b) Moderate reflections: This type of reflection occurs more frequently than a) (Fig. 4b). They may be related to walls remains as in some areas they are currently visible at the surface.

c) “Terraced basement”: This stepped structure appears in the NW sector and is deeper in the SE direction (Fig. 4c). This morphology generates uncertainty: it could be of natural origin (an inclined fault

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Fig. 4. Details of GPR profiles 1, 90 and 75 using a 500-MHz antenna frequency. The presence of intense-moderate reflections and a terraced basement can be deduced.

dipping towards the SE) or of anthropic origin, or both simultaneously if it represents a stepped, man-made construction associated with the original slope of the terrain. d) As well, most of the radargrams show (at approximately 30 cm depth) the anthropological soil of the enclave, which is the base level of the different structures found at this depth.

In order to improve the initial data interpretation, 3D processing was also performed within GPR-Slice v7 imaging software. The following radar signal processing flow was first applied to the raw data: - Zero-time adjustment that compensates for long-term instrument drift due to temperature changes or sudden jumps between consecutive scans.

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-

DC-shift filters to remove low-frequency components. Manual gain curve was applied to compensate for energy losses. Background removal filter for eliminating horizontal band noise. Time to depth conversion was performed with a velocity of 0.1 m/ns determined from hyperbola analysis.

After, a set of 15 depth-slices (30 cm thick) were generated by first calculating the squared amplitude of each trace. Then gridding was applied by using the inverse distance algorithm with a search radius of 0.6 m to fill in areas without data. Four sections are represented in Fig. 5, from the superficial depth to 1 m of depth. Data interpretation after 3D imaging revealed square and rectangular structures, a clear street plan as well as surprising semicircular features (Fig. 5). Most of the archaeological remains are buried at shallow depths (less than 80 cm), where the geometries and amplitude of the different detected anomalies can be observed. At the north most point of the grid there is an elongated area of about 5 m wide (NE–SW orientation), free of archaeological remains, that may correspond to an old street (street I in Fig. 6). In the northwest sector (A in Fig. 6), although a bit blurry, rectangular structures are visible. The fact that they do not show up as clearly as other structures may indicate a poor state of conservation. Probably these walls were torn down before getting buried and the remains left are their foundations over the shallowest part of the calcarenite bedrock layer. At the south of the street, in contrast, the bedrock layer is deeper and walls were easily mapped (B in Fig. 6). These constructive units are generally narrow rectangles open to the south of the grid (C in Fig. 6). Sector C corresponds to a wide open squared area (25 × 25 m) where structures of sector B meet, both at the east (E in Fig. 6) and west (D in Fig. 6). E and D are 3 × 3 m squared structures. Sector C delimits with a perimeter wall in the south that opens a 3 m wide street (street II in Fig. 6). It is an elongated area, free of archaeological remains, and subparallel to which has been interpreted as street 1. The unit described could be part of a big U-shape building open to the south. Its shape and dimensions would be similar to the building documented during the 2014 excavation campaign (Fig. 1B). Actually, the long structures situated in the north seem to repeat the same type of unit with similar dimensions (10.5 × 3.5 m). This long and narrow spaces open to a patio are usually identified as warehouses (Álvarez, 1998). It is more challenging identifying the exact function of the two rows of squared rooms facing each other and sharing a common space. Sector F (west sector) may be another big building with 3 × 3 m squared rooms. South of street II, the urban plan is not well defined. However, several structures of different shape and dimensions are visible. In the southeast area, there are two elongated zones, 3 m wides and archaeological remains free have been interpreted as streets (street III and street IV in Fig. 6). There are similar elements as described for sectors F and G. But in the space between streets II ad IV, it is possible to identify other features characterised by their unique shape: Semicircular (H); Circular (I); and double wall (J). All the structures mapped with GPR in the grid are comparable, in shape and dimensions, with the ones discovered in the excavated area of the site (Fig. 1C). The most clear ones (7 units) are observed in the southeast part. The other 10 units present in the northwest are difficult to interpret, they are very shallow but expand to more than 1 m depth. This means that the bedrock was probably used as foundations for these semicircular constructions.

Fig. 5. Processing of the GPR signal through the GPR-Slice v7 programme. Four sections are represented (at the superficial depth, at a depth of 0.31 m, at a depth of 0.68 m and at a depth of 1 m), where the geometries and amplitude of the different detected anomalies can be observed.

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Fig. 6. Azimuth view using the GPR-Slice v7 imaging software (at a depth of 0.63 m). A, B, C, D, E, F, G, H, I and J: see explanation in text.

Based on what is currently known about Iberian culture and the few excavations at the site, the presence on these circular constructions is difficult to understand. However, we may consider two options: Ovens for cooking bread and roasting cereals; And ovens for ceramics and metals. In principle, rule out the latter two options since this kind of activities usually take place outside the cities in the Iberian period, besides not find mineral slag or other kinds of metallurgical products in their environment, as is usual in other cases known, well studied. Therefore, the ovens for food would be, a priori, the best hypothesis. However, the number of this type of structure and their big dimensions are still surprising. Iberian ovens are usually smaller that 3 m diameter and they are isolated and not in big groups. This is more typical of industrialised settlements of the Mediterranean area in the Classic era. The interpretation of the GPR images together with the information extracted from the excavations has resulted in a complex and peculiar archaeological scenario. It is worth to highlight the presence of a circular feature (7.5 m diameter) whose function is yet unclear for the archaeologists. (Fig. 2a and b). The presence of this structure in a big open space may be related to Carthaginian occupation in the inner city. This would explain the magnitude of the production process in the Giribaile plateau as a result of numerous settlements of Carthaginian troops across the Guadalquivir highlands. The interaction between Iberian and Punic Carthaginian cultures may explain an unexpected archaeological situation: A yet unknown economic and industrial scenario in the inner city. This would explain the huge dimensions of the structures found as well as their layout (in an angle) which is more typical of the pan de Ostia workshops than the common Iberian culture ovens. Even after excavation, it is still difficult to ensure the specific function of these structures. The current working hypothesis (still waiting for results on sediments and seeds analysis) is related to their use for agricultural product transformation or as athreshing floor. Furthermore, the anomalies detected through ERI and GPR are compared in Fig. 7. All the structures mapped with the electrical anomalies and GPR anomalies in the grid are comparable at different depths. A clear correlation can be observed between the anomalies detected by the different techniques in the first metre of depth. Three-dimensional processing using GPR-Slice v7 could better detect the urban grid. However, the signal rapidly attenuated at increasing depths; therefore, ERI techniques provide more information.

Fig. 7. Anomalies detected using ERI and GPR are compared by superimposing the electrical anomalies over the GPR anomalies at different depths (12–38 cm depth in 7A and at 49–74 cm in 7B).

4. Conclusion The present research work consists of a comparative study of two geophysical survey techniques in archaeology: electrical resistivity imaging and ground-penetrating radar. According to the resistivity survey methods, the first metre of depth exhibits vertical structures that are characterised by an increase in resistivity. The increasing value of the electric anomaly allows for differentiating different ranges. This technique allows us to detect the depth, dimensions and areal dispersion of the possible buried structures. This information will be very useful for designing the GPR campaign. In addition, 201 georadar profiles

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have been performed in a grid of a 100 m2 grid, all of them parallel to each other, spaced at 0.5 m, and 100 m in length. Because the remains are at a shallow (no more than 1 m), we used a high frequency antenna (500 MHz) allowing a good resolution to these depths. For a first analysis of the GPR profiles, 2D signal processing was performed using the MALA Ramac GroundVision and RadExplorer 1.4 software packages. In addition to 2D processing, 3D processing was used (GPRSlice v7). Different sections of the detected anomalies are shown in the mapped sections and at different depths. Different ranges and depths of the anomalies are also observed in this case. Based on the location on the map, possible buildings and the urban grid can be deduced, which will help decide the location of future excavation campaigns. A large building, U-shaped, with a central courtyard and described. In relation to this structure shows elongated and narrow spaces, which are interpreted as warehouses. On the other hand, circular structures 7.5 m diameter are mapped with GPR in the grid. The presence of these structures may be related to Carthaginian occupation in the inner city. This would explain the magnitude of the production process and storage in the Giribaile plateau as a result of numerous settlements of Carthaginian troops across the Guadalquivir highlands. Acknowledgements This study was performed within the framework of the project “Technical innovations applied to the knowledge and evaluation of Giribaile” (P11-HUM-8113) funded by the Board of Andalucía and the project, “Methods and techniques in intensive archaeological survey” (HAR2010-18422) of the National Plan of Research, Development and Innovation (I + D + I for its initials in Spanish) 2008–2011. References Álvarez, N., 1998. Producción de ánforas contestanas: el almacén de El campello (Alicante). Cypsela 12, 213–226. Apostolopoulos, G., Pavlopoulos, K., Goiran, J.P., Fouache, E., 2014. Was the Piraeus peninsula (Greece) a rocky island? Detection of pre-Holocene rocky relief with borehole data and resistivity tomography analysis. J. Archaeol. Sci. 42, 412–421. Azcárate, J.E., 1977. Mapa geológico y memoria explicativa de la hoja 905 (Linares), escala 1:50.000. Instituto Geológico y Minero de España. Bano, M., Marquis, G., Nivière, B., Maurin, J.C., Cushing, M., 2000. Investigating alluvial and tectonic features with ground penetrating radar and analyzing diffraction patterns. J. Appl. Geophys. 43, 33–41. Basile, V., Carrozzo, M.T., Negri, S., Nuzzo, L., Quarta, T., Villani, A.V., 2000. A groundpenetrating radar survey for archaeological investigations in an urban area (Lecce, Italy). J. Appl. Geophys. 44, 15–32. Batayneh, A.T., 2011. Archaeogeophysics–archaeological prospection — a mini review. J. King Saud Univ. (Science) 23, 83–89. Bersezio, R., Giudici, M., Mele, M., 2007. Combining sedimentological and geophysical data for high-resolution 3-D mapping of fluvial architectural elements in the quaternary Po plain (Italy). Sediment. Geol. 202, 230–248. Bevan, B.W., Kenyon, J., 1975. Ground-penetrating radar for historical archaeology. MASCA Newsl. 11, 2–7. Conyers, L.B., 2013. Ground-Penetrating Radar for Archaeology. AltaMira Press. Davis, J.L., Annan, A.P., 1989. Ground-penetrating radar for high-resolution mapping of soil and rock stratigraphy. Geophys. Prospect. 37, 531–551. Drahor, M.G., 2006. Integrated geophysical studies in the upper part of Sardis archaeological site, Turkey. J. Appl. Geophys. 59, 205–223. Drahor, M.G., Kurtulmus, T.O., Berge, M.A., Hartmann, M., Speidel, M.A., 2008. Magnetic imaging and electrical resistivity tomography studies in a Roman military installation found in Satala archaeological site, northeastern Anatolia, Turkey. J. Archaeol. Sci. 35, 259–271. Fontboté, J.M., 1982. Mapa geológico y memoria explicativa de la hoja 70 (Linares), escala 1:200.000. Instituto Geológico y Minero de España.

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