GPR Tomography as Support for an Archaeological Excavation in Aripuanã Indigenous Cemetery, Amazon Region, Brazil

We present here a series of Ground Penetrating Radar (GPR) survey carried out in different areas of the Dardanelos 1 archaeological site in order to generate information about subsurface anomalies associated with archaeological material that could be use in decision making within the environmental licensing process of the Dardanelos Hydroelectric Power Plant, located near of the Aripuanã city, Mato Grosso State, northwest region of Brazil. GPR surveys with 200 MHz antenna were carried out in two blocks aiming to locate archaeological resources and features. The analysis of GPR 2D and 3D results allowed detecting anomalous regions characterized by hyperbolic reflections, shallow elongated continuous targets with high amplitudes, as well as sub-horizontal reflectors. Microwave tomography allowed estimating the geometry of the GPR anomalies sources. Excavations were done by archaeologists at the locations where hyperbolic anomalies were found, revealing interesting structures related to urns in the middle of a rich in organic matter consisting of black and ceramic materials up to about 1 m deep. The first sub-horizontal reflector at approximately 1 m depth is related to the base of the black soil layer rich in organic matter and the second sub-horizontal reflector between 2 and 3 m deep suggests a lithological change or may be related to presence of the water table. The continuous elongated shallow targets observed in the depth slices are related to tree roots in the middle of the archaeological strata. The GPR results guided archaeological excavations, reduced the time and costs involved in research, and contributed to the preservation of Brazilian historical heritage. How to cite this paper: Porsani, J.L., Kipnis, R., dos Santos, V.R.N., Almeida, E.R. and Fernandes, I. (2017) GPR Tomography as Support for an Archaeological Excavation in Aripuanã Indigenous Cemetery, Amazon Region, Brazil. International Journal of Geosciences, 8, 1264-1277. https://doi.org/10.4236/ijg.2017.810073 Received: August 28, 2017 Accepted: October 28, 2017 Published: October 31, 2017 Copyright © 2017 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/


Introduction
The use of geophysical methods in archaeological investigations has increased considerably in recent decades. In particular, the Ground Penetrating Radar (GPR) method [1] [2] has contributed to non-destructive actions, guiding archaeological excavations, reducing costs and time related to research and excavations. GPR permits to obtain lateral and vertical information in high resolution about the shallow subsurface. The results can be presented by 2D profiles and 3D profiles as fence diagrams, interpolated cubes and amplitude maps in depth slices [3] [4]. Some interesting results of GPR investigations in archaeological studies can be found in the literature [5]- [11], among others.
In Brazil, the researcher Anna Roosevelt used the GPR for the first time on the Marajó Island to map archeological sites made of ceramic materials [12], followed by [13]. Ten years later, Cezar et al. [14] also employed GPR to search for ceramic materials from archaeological sites in Rio de Janeiro State. Other geophysical researches in archaeological investigations include fluvial sambaquis [15], coastal sambaquis [16], shelter on rock [17], archaeology in urban areas [18].
In this paper, GPR surveys were carried out in the framework of the environmental licensing of the Dardanelos Hydroelectric Power Plant, Mato Grosso State, northwestern of Brazil ( Figure 1). Dardanelos is located in the municipality of Aripuanã, where the Aripuanã river passes, located in the region of the legal Amazon. The hydroelectric is located on the left bank of the middle Aripuanã river, opposite of Aripuanã city. The region has been a logging and agricultural expansion front since the 1970s, and was also the nucleus of the project known as the Humboldt Science City (Aripuanã Project). The Cinta Larga and Arara indigenous people have inhabited the region since immemorial times, and are nowadays the traditional inhabitants of the region that covers the Municipality of Aripuanã and, in turn, the basin of the homonymous river and the Salto de Dardanelos [19].  studies inside the archaeological site of Dardanelos 1. The advantage of MT is that it permits to recover the geometry of the anomalies in subsurface that cause hyperbolic reflections. The main goal was to identify areas of greater archaeological sensitivity as an alternative to avoid the impact in areas of greater sensitivity, guiding archaeological excavations, reducing costs and excavation time, and assisting in protection measures. Interesting results of this research are presented in this paper, which revealed large urns, some of which might contain human remains, contributed to the preservation of the national patrimony and showed the importance of using the GPR method in archaeological studies.

Area of Study and Archaeological Context
The study area is located on the left bank of the Aripuanã river, municipality of In the contact between the Chapada de Dardanelos and the Depression, there are two great waterfalls, the Salto de Dardanelos and the Salto das Andorinhas.
To the south of these waterfalls, on the right bank, is the urban area of Aripuanã, and on the left bank the archaeological site Dardanelos 1 (Figure 1).
The Aripuanã river basin is a promising archaeological area, in the interplay of the Madeira-Tapajós rivers, considered as one of the routes of expansion of the Tupi and Proto-Tupi groups in pre-colonial times. The region is characterized as an extensive open-air ceramic site that covers a 1300 meter extension with the presence of anthropic black soil, housing the Dardanelos 1 site that features ceramics of diverse affiliation, such as Borda Incisa, Tupi and others.
Sixteen radiocarbon dates were obtained from charred samples sent to Beta Analytic (Table 1). A very ancient assay of 7700 ± 50 BP (ARIP05) and a more recent one dated to 150 ± 40 BP (ARIP02) are stratigraphic inversion and it is yet not clear if they are associated with archaeological levels. The other thirteen assays present stratigraphic coherence, are associated with archaeological levels presenting rich material culture, strongly suggesting occupation of the site between 1490 ± 40 and 4890 ± 40 years before present by indigenous societies that were manufacturing ceramic and lithic artifacts, and based on producing economy associated with Amazonian Dark Earth. Recently Zuse [28] has suggested the presence of Arawak cultural matrix peoples in the Alto Madeira region, between 3000 and 1500 years BP. In the subsequent period, that is, from 1500 years BP, occupations became denser due to the greater number of ceramic fragments observed in thicker archaeological International Journal of Geosciences layers of black earth, and with visible technological changes, whose demographic increase would have reached its Around 1000 BP, represented by Barrancoide ceramics. Also according to Zuse [28] the process of occupation of the region by sedentary, agricultural societies would have undergone profound transformations since the arrival of the holders of the Polícroma Tradition of the Amazon, called the Jatuarana Subtradition.
The Dardanelos 1 site is part of the Alto Madeira River, presenting high potential to generate information to test the models presented, since only a small portion of the archaeological site was affected by the construction of the Dardanelos Hydroelectric Power Plant.

Data Acquisition and Processing
Geophysical  The inverse problem in the microwave tomography is non-linear [30], thus it must be linearized. Although the complex dielectric permittivity is present in the equation, a quantitative analysis is not possible because of approximations done during the linearization of the inverse problem. Its linearization is done by means of the Born approximation [31], which assumes that the targets are small in terms of the wavelength and have low contrast compared to the background material. Some results of MT applied to GPR data can be found in literature, like in studies for archaeology and forensics [32] and underground features mapping [33] for instance. The tomographic images were obtained with the RadImage v1.0 program [34] and they are presented in the next section.

Results Interpretation
All 200 MHz GPR profiles from each archaeological block were analyzed together and the results are presented in the form of amplitude maps in depth slices and 2D profiles. The GPR anomalies with the greatest archaeological potential are highlighted directly in the figures of depth slices and 2D profiles. International Journal of Geosciences These anomalies were selected for further archaeological excavations aiming the confirmation of the targets. The most expressive GPR results for each archaeological block studied are presented below. Figure 3 shows the amplitude map of the GPR signal in the form of depth slice to 0.62 m for block-1. Note a well-defined rectangular-shaped anomaly (A1) with high amplitude at the center of the figure (position x = 11.3 m -13.5 m, y = 5.2 m -7.0 m). According to field observations, this rectangular-shaped anomaly is related to a previous excavation done in the studied area that had to be halted due to time constrains, and was resumed in the following year. According to Conyers [4] GPR profiles with high amplitude reflectors are more likely to be related to archaeological targets, but may also be associated with buried natural targets, for example, silicified tree roots [16], bedrock [17], among others. Note  The analysis of the anomaly A1 (Figure 3 and Figure 4) showed that it is the same target and is therefore suitable for archaeological excavations. Figure 5 shows the result of the archaeological excavation on the A1 anomaly. The excavation revealed an indigenous urn of circular shape in the middle of a soil rich in organic matter characterized by black soil.        Additionally, the second sub-horizontal reflector may be related to a lithological International Journal of Geosciences change or to the presence of water table. These interpretations are only speculative and are beyond the scope of this work.

Conclusions
The microwave tomography GPR results with 200 MHz antennas permit to detect geophysical anomalies in depth slices and reflection profiles. The anomalies were characterized by hyperbolic reflectors and sub-horizontal structures. Hyperbolic anomalies indicated areas with great potential for finding archaeological targets. Microwave tomography improved the spatial positioning and allowed estimating the geometry of GPR anomalies.
Excavations in blocks 1 and 2 revealed positive results with the location of urns related to hyperbolic anomalies. The first sub-horizontal structure at 1 m depth corresponds to the base of the black earth layer rich in organic matter and ceramic shards. Shallow continuous anomalies were related to the tree roots that are common in the Amazon region.
The results showed the efficiency of the GPR method for detecting archaeological targets at the site of Dardanelos 1. The studies guided the archaeological excavations, reduced exploration costs, and contributed to the preservation of national historical patrimony.