Ground Penetrating Radar (GPR) for identifying the archeological caves of Trinchera (Sierra de Atapuerca, Burgos, Spain)

Bermejo, L. a,f*, Ortega, A. I. a,b, Conyers, L. B. c, Benito-Calvo, A. a, Parés, J.M. a, Pérez-González, A. a, Bermúdez de Castro, J.M. a, Carbonell, E. d,e,f

(a) Centro Nacional de Investigación sobre Evolución Humana (CENIEH). Paseo Sierra de Atapuerca 3, 09002 Burgos, Spain. (b) Grupo Espeleológico Edelweiss, Excma. Diputación Provincial de Burgos, C/Paseo del Espolón s/n, 09071, Burgos, Spain.

(c) Department of Anthropology, University of Denver, 2000 E. Asbury St., Denver, CO 80208, USA (d) IPHES, Institut Català de Paleoecología Humana i Evolució Social. Unidad asociad al CSIC, C/Escorxador s/n, 43003 Tarragona, Spain.

(e) Universitat Rovira i Virgili (URV), Campus Catalunya, Avinguda de Catalunya 35, 43002 Tarragona, Spain. (f) EPPEX (Equipo de Investigación Primeros Pobladores de Extremadura).

*corresponding author: lucia.bermejo@fa.cenieh.es

INTRODUCTION

The Sierra de Atapuerca sites located in the Trinchera area are ancient caves filled with sediments that were sectioned by a XIXth Century railroad trench (figure 1). Sima del Elefante, Galería Complex and Gran Dolina sites have been studied for decades providing outstanding Early and Middle Pleistocene archaeo-palaeoanthropological remains (Bermúdez de Castro et al., 1997; Carbonell et al., 2008; Ortega et al., 2014). The understanding of the morphology of these caves and the relationship between their infillings is thus one of the research team’s priorities (Ortega et al., 2013). To that end, geophysical surveys are being applied to these sites, having the Electrical Resistivity Tomography (ERT) already revealed its ability to characterize part of this karstic landscape sub-surface geometry (Ortega et al., 2010; Bermejo et al., 2014 a and b).

This work presents the Ground Penetrating Radar (GPR) surveys that were carried out for the first time on the Trinchera area. This method offers supplementary data to the previous ERT surveys as well as provides new information about unexplored areas were ERT was not performed.

Sima del Elefante

Galería Complex

Gran Dolina

Figure 1. Trinchera area at Sierra de Atapuerca.

METHODOLOGY

GPR is a nondestructive method that uses electromagnetic radiation in the microwave band to detect the reflected signals from subsurface structures. A GPR transmitter emits electromagnetic energy into the ground and a receiving antenna records the variations in the return signal. These may be caused by a buried object or a boundary between materials that have different dielectric properties. This way, different data can be recorded at various depths (Conyers 2004).

A 300 mhz antenna (figure 2) was used to map all of Trinchera area, although only the profiles related to the known archeological sites are presented in this work (figure 3). Due to the singularity of Thinchera, where karstic features can be found less than 2 m underground, real depth was not only adjusted by hyperbola fitting but also in relation to the known karstic morphologies such as Cueva Peluda. In order to georefere the GPR profiles AUTOCAD Map 3D 2011 was used. Figure 2. GPR survey at Trinchera area.

Figure 3. Trinchera area plan with the location of GPR profiles in blue.

RESULTS

In profile A the relevant features to notice are the big amplitude reflections that match Cueva Peluda main level and the ones that are related to the limestone strata seen in Trinchera’s eastern wall (figure 5).

Something similar happens for profile B, where it is easy to see the continuity of a big fault in the subsurface. In the bottom left part of this profile there are some reflections that are consistent with changes in different sedimentological layers. These could be related to the lower level of Cueva Peluda.

Profile C.1 and C.2 show that the sediments related to Tres Simas are clearly continuing to the west. These profiles suggest that they may be sediments connecting Galeria Complex and Tres Simas (figure 6) that may reach the karst ‘s lower level, as they are more than 11 m deep. Profiles D.1 does not reflect any continuation of Gran Dolina sediments into the ground, although there has been an archaeological test proving so. This may be due to the fact that the profile went over a concrete pavement. However, there is a clear set of reflections (*) indicating the continuation into the subsurface of TR20 or TR21 sediments, or both. Profile D.2’s group of reflections (*) could be related to the continuation of the aforementioned sediments towards the west or to the ones coming from Gran Dolina, or it could be a linking point between both.

We have projected profile D.3 under Trinchera’s western wall profile to show that the northern amplitude reflections are related to the fillings that are visible from TR31 cave and that the southern ones (*) may have to do with Penal’s chimneys. (figure 8).

DISCUSSION AND CONCLUSION

GPR surveys at Trinchera area reveal a complex geology, as it can be expected from a multilevel karstic system. They have detected limestone strata, fractures and voids, as well as sediments that fill caves, some of which are consistent with ERT profiles. Moreover, GPR shows a connection of the studied caves with the lower karstic level, which can be guessed by the existing topographic, archaeological, geological and geomorphological data, but had never been confirmed before.

Although more precise work needs to be done, the results of this first GPR survey offers a better understanding of the sites’ formation processes which could be useful for archaeological excavation planning and management strategies. This way, the results have also proven the suitability of geophysical studies for archaeological sites in karstic environments.

ACKNOWLEDGMENTS

The corresponding author is beneficiary of a Fundación Atapuerca Pre-doctoral Grant. The authors wish to thank Grupo Espeleológico Edelweiss for its constant scientific and logistic support. This work has benefited from discussions with Isidoro Campaña, Enrique Darklaw and Laura Sánchez.

Figure 4. Inside of Peluda Cave. Image by M.A Martin Merino

A

B

D.1

D.2

D.3

C.1 C.2

Figure 5. Limestone strata at Trinchera.

Figure 6. Galeria Complex and Tres Simas. Image by M.A Martin Merino

Figure 7. Gran Dolina. Image by M.A Martin Merino

Figure 8. Penal. Image by M.A Martin Merino

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*

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