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Using in situ gamma ray spectrometry (GRS) exploration of buried archaeological structures: A case study from NW Spain
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Using in situ gamma ray spectrometry (GRS) exploration of buried archaeological structures: A case study from NW Spain Jorge Sanjurjo-Sánchez a,∗ , Carlos Arce Chamorro a , Carlos Alves b , Jose Carlos Sánchez-Pardo c , Rebeca Blanco-Rotea c,d , Jose Manuel Costa-García c,e a
Instituto Universitario de Xeoloxía “Isidro Parga Pondal”, Universidade da Coru˜ na A, Coru˜ na, Spain Lab2PT (FCT UID/AUR/04509/2013, FEDER COMPETE POCI-01-0145-FEDER-007528) and Earth Sciences Department, School of Sciences, University of Minho, Braga, Portugal c Landscape, Heritage and Paleoenvironment Laboratory, University of Santiago, Santiago, Spain d Unit of Archaeology, University of Minho, Braga, Portugal e School of Classics, History and Archaeology, Newcastle University, Newcastle, UK b
a r t i c l e
i n f o
Article history: Received 30 October 2017 Accepted 9 May 2018 Available online xxx Keywords: Archaeological survey Radioactive isotopes Gamma spectrometry Non-destructive survey Mapping Maps of elements ratios
a b s t r a c t Geophysical exploration methods allow the detection of archaeological features before any excavation is carried out on these sites. This is due to the contrast of properties between the buried archaeological structures and objects and the surrounding soil, sediment, or rock. Although Gamma-ray spectrometry (GRS) is [widely] used for geological exploration and mapping, it has been scarcely used in archaeology so far, despite the successful results of previous studies on the matter. In situ GRS is a non-destructive method that allows direct assessment of uranium-238 (238 U) and thorium-232 (232 Th) from daughter radionuclides of their decay chains, as well as potassium-40 (40 K), on soils and rock outcrops. The technique documents the concentration of these isotope concentrations in the topsoil by surface measurements and this enhances its potential for archaeological exploration. However, two assumptions must be made: the archaeological objects must contain a different concentration of radionuclides than the surrounding sediment or soil, and they must be buried in the terrain less than 25–30 cm deep. In this work, we present the results of the use of in situ GRS for the study of a buried structure in the archaeological site of Cidadela (Galicia, NW Spain). Firstly, we have tested in situ spot GRS measurements to detect rock-built structures buried in the sediments; secondly, we have excavated the surveyed area. The results are reliable despite the low radioactive content of the rocks used as building materials, given that the burying and sediments also have low amounts of radioactive isotopes. Although the direct use of the estimates of K, U and Th has not proved successful, the use of U/Th, Th/K and U/K ratios provided reliable results. © 2018 Elsevier Masson SAS. All rights reserved.
1. Introduction Geophysical exploration is usually performed to obtain data about the underlying substrate, being possible to build up vertical profiles or maps. Some geophysical exploration methods are also used today in archaeology, being called “archaeogeophysics” or “archaeological geophysics” [1,2]. They are very useful when trying to identify the location and layout of archaeological objects and structures, as well as to better define survey and excavation areas. Since archaeological excavation is an expensive, time consuming and destructive activity, the use geophysical techniques imply a notable saving of both resources and time.
∗ Corresponding author. E-mail address: [email protected] (J. Sanjurjo-Sánchez).
Among the geophysical methods, those most extensively used in archaeology are the seismic, electrical and electromagnetical methods, as well as the ground penetrating radar or GPR [1–4]. All of them allow the assessment of properties of the underlying soil, sediments and/or rocks, providing a cross section of soil properties up to some meters deep. These properties can be related to any buried archaeological structure and/or object, as long as the assessed physical properties (e.g. density, porosity, conductivity) are different than those of the surrounding or covering soil or sediment. Gamma-ray spectrometry (GRS) is a technique used for different geophysical purposes, including mineral exploration and geological/geochemical mapping. Portable GRS is a non-destructive method that allows a quick direct assessment of potassium-40 (40 K), and an indirect assessment, thorough isotopes resulting from the decay chain, of uranium-238 (238 U) and thorium-232 (232 Th) on rock outcrops and soils. Measurements can be performed
https://doi.org/10.1016/j.culher.2018.05.004 1296-2074/© 2018 Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: J. Sanjurjo-Sánchez, et al., Using in situ gamma ray spectrometry (GRS) exploration of buried archaeological structures: A case study from NW Spain, Journal of Cultural Heritage (2017), https://doi.org/10.1016/j.culher.2018.05.004
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Fig. 1. Map of location of the site (A), picture of GRS recording with the marked area and depth (approximate volume surveyed but not at scale) that will affect to the GRS result (B), and general view of the excavated part of the site of Cidadela (C).
hand-held, by car or by plane or helicopter being respectively referred as carborne GRS and airborne GRS [5–8]. Such assessment does not provide 238 U and 232 Th, since the contents of U and Th in the materials are assessed from the spectral emission of daughter isotopes of their decay chains. Its use in archaeological geophysics has been scarce, despite the few existing studies have provided interesting results. 1.1. Previous uses of GRS in archaeology In an early work, Aitken [1] proposed that radioactivity could be used to detect archaeological structures, due to the diverse gamma radiation emissions of different buried materials. However, he also remarked that this could be valid only for shallowly buried structures in locations with very thin sedimentary covers. He referred the earlier work of Peschel [9], who used radiation to detect a buried ditch and a wall. Two decades and half after Aitken’s suggestion, Ruffell and Wilson (1998) [10] reassessed the usefulness of the GRS as an archaeological survey tool. They set up transect profiles to get gamma-ray maps, concluding that GRS is a useful tool for the detection of buried structures or clay beds, among other features. Following this idea, Moussa [11] performed a successful study case for the detection of buried structures in Egypt, although he never matched the results of this survey by actually excavating the site. Ruffell et al. [12] applied the GRS for the detection of subsurface structures, in order to enhance the potential of this technique for the archaeological survey. They performed GRS surveys in two case studies where they know the approximate surface location of the buried structures to contrast the obtained map of K, U and Th concentrations coming from the acquired spectra. They observed that Th/K and Th/U ratios enhanced the results, allowing a better location of buried surfaces with different K, U and Th contents than the burial soil. Later, [13], using GRS in different locations, found
a significant decreasing relationship between K concentrations in soil and human activities in the past. 1.2. Problems and limitations of the use of GRS in archaeology The detection of gamma radiation and radioisotopes by GRS is limited by several factors linked with the method itself and with the studied sediments. Gamma rays are absorbed by both organic matter and water present in sediments [7]. Thus, vegetable litter accumulated on the topsoil, or the presence of a dense low vegetation coverage will reduce gamma radiation and therefore affect GRS measurements. The presence of water also alters the measurements, since it absorbs ionizing radiation. That is the reason why it is preferable to perform measurements on dry soils. Gamma-rays reach up to 700 m in air, but their penetration range is lesser through solid materials: they could penetrate rocks up to 0.5 m deep, but usually the range through sediments and soils typically is set between 0.25 m and 0.3 m [7]. Surface measurements do not just comprise the spot where the probe is located, but a bigger volume around it with a diameter in area of about ten times the distance from the probe to the measured surface (Fig. 1). Thus, geometrical effects of the surveyed surface will affect to the radioisotope estimates, being that spectrometers calibrated to perform borehole or surface measurements. Surface measurements require a flat surface, since other geometries are a cause of over- or underestimation of the radioisotopes [14]. Radioactive disequilibria can also affect the results: this is usually observed in the 238 U decay series, being the eU assessment the most affected one. Such disequilibrium can occur due to the leaching of U or radioisotope daughters, but also to radon releases from the soil. The daughter radioisotope is used to assess the U content from the 238 U chain (214Bi), a post-Rn daughter product [7]. Thus, GRS allows the assessment of the concentration of the referred radioisotopes on
Please cite this article in press as: J. Sanjurjo-Sánchez, et al., Using in situ gamma ray spectrometry (GRS) exploration of buried archaeological structures: A case study from NW Spain, Journal of Cultural Heritage (2017), https://doi.org/10.1016/j.culher.2018.05.004
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the topsoil or rock surfaces, and could be of use for archaeological survey. However, two assumptions must be considered: (i) the archaeological buried features must show differences in terms of radionuclide contents in relation with the surrounding sediment or soil; and (ii) they must be close to the surface (0–25 cm deep). Thus, it is potentially applicable for the survey of shallow structures or objects. In any case, this method does not provide cross profiles of the ground, but maps of the buried structures and objects.
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tures are not really located in depth, considering the present-day soil surface. Especially in the central part of the site, plenty of features have been located between 5 and 30 cm deep. Since the surface of the site is predominantly flat, we estimated that this was an excellent opportunity to test the application of GRS in an archaeological context. In addition, the GRS results could be matched to those coming from the excavations. 4. Materials and methods
2. The archaeological site of Cidadela 4.1. Hand-held GRS The archaeological site of Cidadela is located over a plateau between the Cabalar and Pequeno rivers, in the municipality of Sobrado dos Monxes (Galicia, NW Spain). The existence of archaeological remains in this location was known from at least the end of the 19th Century, and even some excavations were carried out there in 1934 following a pre-scientific methodology [15]. It was not until 1981 that a series of systematic archaeological excava˜ Gesto [16], who nominally tions were developed by Prof. Caamano remained as project leader until the 2000 decade. Up to date, approximately 20% of the site area has been excavated (Fig. 1). These works have revealed the existence of some phases dated to Roman times, and linked with the building and occupation of a military fort by the cohors I Celtiberorum, a mixed infantry and cavalry auxiliary unit of the Roman army [17,18]. The site was reoccupied and almost entirely reorganized after the departing of the military unit: some phases were related to the presence of a possible monastery Visigothic times, while others were interpreted as later medieval occupations [19]. However, recent interdisciplinary works carried out between 2014 and 2016 [20,21], have led to a more comprehensive chronology: Early Roman horizon (2nd–3rd c. AD), linked with the construction occupation of the mentioned fort; • Late Antique layer (4th–6th/7th c. AD) in which it is attested the reuse of some previous structures and the building of several new ones, as well as the presence of imported pottery and metallic objects of this chronology; • Early Medieval horizon (9th–11th c. AD) characterised by the presence of a church and some buildings related to it; • Extensive medieval occupation (12th to 13th c. AD), which implies the dislocation of the church outside the walls of the ancient fort and the entire reorganization of the structures within it; • A post-medieval period (14th to 21st c. AD), when the site was destined to agricultural and extractive purposes. Our survey was located in an area of the site where the presence of Early Medieval structures was expected, at a shallow (but unknown in detail) depth, due to previous survey studies (see below). 3. Aim of this study In the framework of the Marie Curie CIG project EMCHAHE (Early Medieval Churches: Archaeology, History and Heritage), a survey with ground penetrating radar (GPR) was carried out in the unexcavated area of the site of Cidadela in 2016. The goal of this survey was to identify areas with a greater presence of Early Medieval structures and, where possible, features linked with the abovementioned church for their excavation. A hand-held GRS survey was also performed before excavation works. The majority of the structures located during previous archaeological excavations belonged to wall foundations, usually about 30–60 cm height. It has also been attested that these struc-
Gamma spectra with estimates of the radioisotope contents of rocks were acquired with a portable spectrometer (carried by an operator) GF Instruments GRS-2000 with a BGO probe (Bi4 Ge3 O12 ) of 512 channels (measuring energies up to 3 MeV) that measures up to 70,000 pulses/second (in the whole channels). The device consists on a probe with 2”×2” detector (3.5 kg weight) connected to an independent battery. The instrument was calibrated by the manufacturer according to IAEA recommendations. Statistical uncertainties are estimated to be 6% for K, 30% for U, and 16% for Th for an acquisition time of 180 s. The GRS measurements were carried out in summer (2016) to ensure soil dry conditions. As a preliminary step, the low vegetation coverage was removed in the areas to be surveyed and excavated. After that, the survey area was delimited to a square of 5 × 5 m and georeferenced by using a total station Leica TCRM 1105 plus. The area was marked by using a string and a pickaxe, in order to build a network of squares with 50 cm side. The gamma emission of soil was measured by placing the GRS probe in the knots of the network, obtaining a spectrum for each knot, that is a measurement each 50 cm in 11 parallel profiles of 5 m long separated 50 cm one to other (Figs. 2 and 3). We have used a 2D coordinate system to reference these measurement points, with letters (from A to J, from west to east) and numbers (from 1 to 11, from south to north). A total of 121 spots were measured. Results were used to build up a map with isolines or isograms that mark spatial differences in the composition of radionuclides. Previously, histograms with the obtained data were observed to choose classes and isolines for these classes were later drawn by linear interpolation between points of the grid. Although the instrument is calibrated for an acquisition period of 180 s, we have performed previous tests with variable acquisition times of 60 s, 120 s, 180 s and 300 s in a given point (25 measurements) to compare results and their variability. Such test (see results) indicated that a 120 s acquisition time was enough to get confident estimates. From these spectra we can extract estimated contents of K (potassium, mass percent), eU (uranium equivalent, in mass parts per million or ppm) and eTh (thorium equivalent, also in ppm). While potassium contents are estimated from the peak of 40 K at 1.461 MeV (energy range 1.366–1.564 keV), there are several radioactive isotopes in the uranium and thorium decay series and, hence are used “equivalents” of uranium and thorium, assuming secular equilibrium. Estimations of eU and eTh are made from peaks of 214 Bi (bismuth) at the 1.764 MeV (energy range 1.57–1.959 keV) and 208 Tl (thallium) at the 2.615 MeV peak (energy range 2.42–2.81 keV), respectively. 4.2. Sampling and complementary analyses To cross-check GRS estimates, samples of the burying sediments and stones used as building materials were collected on the archaeological site to assess their nature and radioisotope content. The sediment has been identified in the geological map [22] as an alluvial one.
Please cite this article in press as: J. Sanjurjo-Sánchez, et al., Using in situ gamma ray spectrometry (GRS) exploration of buried archaeological structures: A case study from NW Spain, Journal of Cultural Heritage (2017), https://doi.org/10.1016/j.culher.2018.05.004
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Fig. 2. Results of materials analyses of rocks and sediments of the site (A), surveyed area (with excavated area in the right side) (B), map with results of K (%)(C), eTh (ppm) (D) and eU (ppm) (E) distributions. The total surveyed area corresponds to a square of 5 × 5 m. The arrows indicate the north.
Petrographic thin sections were prepared from collected rock samples to identify the rocks used as building materials under an optical petrographic microscope Nikon OPTIPHOT2-POL. Images were recorded with a Nikon DS-Fi1 camera controlled by using NIS Elements version 3.0 image software. To confirm the petrographic observations and to assess the K content of the samples, a X-Ray Fluorescence Spectrometry (XRF) was performed. Samples were grinded to fine grains < 63 m and measured in a Bruker-Nonius S4 Pioneer wavelength dispersive fluorescence spectrometer under helium purge at the University of A ˜ (Spain). Coruna To assess trace elements, namely U and Th contents, Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) was carried out in a Thermo ScientificTM Element XRTM ICP-MS, which combines a dual mode SEM with a Faraday detector Lithium metaborate fusion was used for sample preparation [23,24], and powdered samples were mixed with an equal amount of lithium tetraborate flux, placed in a carbon crucible and fused at 1000◦ C in a furnace for 30 minutes. After the melt was cooled, the resultant fusion bead was briefly grounded and dissolved in 100 mL of 4% HNO3 /2% HCl3 solution. The latter was then analysed by ICP-MS.
Table 1 Results (mean ± standard deviation) of the acquisition time test of spectra with the GRS.
5. Results
The collected rocks used as building materials in the excavated structures have been identified as serpentinised ultramafic rock (CR-1 and CR-2), and a two mica medium grained granite (CR-3). All of them are near local geologic materials as outcrops of the ultramafic rock can be found about 1 km distance, while granite outcrops are found at a distance about 5 km. XRF and ICP-MS results (see Fig. 2) provide very low K, U and Th contents for the ultramafic samples, clearly below the average values for the upper continental crust of 2.5 ppm, 10.3 ppm and 2.865% for U, Th and K, respectively
5.1. Spectra acquisition time test Before the field survey, a test to check the effect of using different acquisition times was performed. As related above the GRS is calibrated for an acquisition time of 180 s, but our tests showed that readings obtained by using measurements times of 30 s and 60 s fit the results of higher times (Table 1), being the deviation
Acquisition eTh (ppm) time (s) 30 60 120 180 300
3.68 4.04 3.67 3.53 3.63
± ± ± ± ±
0.65 1.23 0.72 0.49 0.70
Cv %
eU (ppm)
17.7 30.4 19.6 13.9 19.3
1.46 1.48 1.77 1.63 1.77
± ± ± ± ±
0.71 0.62 0.51 0.50 0.46
Cv (%)
K (%)
48.6 41.9 28.8 30.7 26.0
0.68 0.72 0.73 0.70 0.70
Cv (%) ± ± ± ± ±
0.08 11.8 0.04 5.6 0.06 8.2 0.05 7.1 0.05 7.1
Cv: coefficient of variation.
of such data significantly higher, at least for eTh and eU. However, the results for an acquisition time of 120 s are similar to those of longer acquisition periods. The K values are the only exception to this rule, but the deviation of such radioisotope is lower than those of the other ones. That is the reason why we considered this interval of 120 s as a confident acquisition period for an optimum fieldwork in terms of time and results. 5.2. Analytical results of samples
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Fig. 3. Estimated of expected ratios of each analysed material in the site (A), surveyed area (with excavated area in the right side) (B), maps of ratios estimated from results for Th/U (C), Th/K (D) and U/K (E). The arrows indicate the north.
[25]. The granite sample shows higher values, but below the average values of this rock type [26], being the U concentration above the Th, on the contrary of average granite values, being usually the Th content higher than the U content. This is because the presence of K, U and Th strongly depends on the presence of some minerals [26]. The sediment sample displays lower K and U contents than the granite (Th contents are similar in both granite and sediment). Thus, the presence of foundations with ultramafic rocks should be indicated by very low K, eU and eTh in GRS during the survey, while the very high K and U values could indicate the presence of granite. Following the suggestions of Ruffell et al. [12], we have assessed the Th/K and Th/U ratios (Fig. 2), as well as the U/K ratios to check also the potential usefulness of this ratio. The results of the sample analyses show very different ratios for the sediment and the rocks, except for the U/K ratio of the sediment and granite samples. This is because the unexpected low Th content in the granite sample. It is possible to stress that the result of the granite sample is slightly below the median of the data set collected by Sanjurjo-Sánchez and Alves [27] concerning granitic rocks (granites and granodiorites), while the ultramafic rock samples present the typically low values of this kind of rocks. Anyway, the different ratios observed indicate that GRS should be a suitable technique to explore structures built with stone in the topsoil of the site. 5.3. GRS survey The survey shows different outcomes for the three studied radioisotopes (Table 2). While the 40 K range observed in the studied area is narrow, a wider range is observed for 214 Bi and 209 Tl, being the eU and eTh oscillations higher. Thus, spatial oscillations of K cannot be considered as significant to detect buried structures
Table 2 Statistical summary of data obtained by GRS (121 measured points).
Median Mean Standard deviation Range
K (%)
eU (ppm)
eTh (ppm)
Th/U
Th/K
U/K
0.50 0.48 0.10
1.30 1.36 0.46
1.80 1.87 0.68
1.38 1.37 1.16
3.60 3.91 1.88
2.60 2.84 1.40
0.2–0.7
0.4–3.0
0.4–3.5
0.2–6.6
0.8–12
0.8–9.5
due to the narrow range measured. This effect is probably owing to the low K content observed in the sediment (1.3%). Indeed, the serpentinised ultramafic rock shows only < 0.05% values, and this is the main building material used in the archaeological structures. K shows halfway values, but probably indicate the presence of stone blocks buried in the whole studied area. In the same way, the Th and U contents of the ultramafic rock are very low and significantly higher in the sediment (Fig. 2). This implies that eU and eTh contents recorded by GRS are more useful when dealing with the detection of buried structure constructed by using ultramafic blocks. Since the eU registered by using GRS can be biased by disequilibrium in the 238 U decay chain, Th could be the most reliable element. However, an alternative tool is the use of ratios. Given the narrow K range, this element could be used to normalise the eU and eTh values. The Th/U ratio is also useful, since it is typically used to assess any possible secular disequilibrium in the 238 U decay chain [12]. Such disequilibrium is not observed in rocks [26,28], being the Th/U ratio constant when buried stones are surveyed by GRS. This way, the ratio values provide wider ranges (Table 2) and these new ratios are used to assess any drop or rise in the studied profiles. For the Th/U and Th/K ratios, low values would indicate the presence of
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ultramafic rocks, and higher values the presence of sediment. For the U/K ratio, low values would indicate the absence of rocks in the terrain down to a 0.25 m depth. The data obtained by these means were used to build a map of the distribution of K, eU and eTh in the studied area (Fig. 2). If we use this map together with the analysis of the sediment and rocks, lower values can be expected where ultramafic rock accumulations (by far the most used rock) are placed, while points with higher values should be linked with plain sediments or with the presence of granite within this 0.25 m layer. As Fig. 2 shows, there are some areas where eTh and eU values are very low but they do not completely fit. Following the procedure of Ruffell et al. [12], maps using Th/U, Th/K and U/K ratios were prepared (Fig. 3). These maps provide a sharper difference between several areas of the surveyed area. According to the chemical analyses of sediments and rocks, the areas with buried rocks (both granite and ultramafic rock) should provide higher U/K ratios (especially in the case of ultramafic rocks) and lower Th/K and Th/U ratios. There are some sectors in which the Th/U, Th/K and U/K ratios deserve a special attention. The square limited by F-6 and G-8 is characterised by high values in all ratios, as it happens with the area comprised between G-6 and K-8. This indicates a similar composition of the soil layer located in these. However, the ratios do not fit the expected values predicted from the chemical analysis of samples, at least regarding the Th/U value, which should display an opposite behaviour to the U/K ratio. The differences in the Th/K ratio are less significant. 5.4. The archaeological excavation An accumulation of small-medium-sized stone blocks (mostly ultramafic) mixed with fragmented tiles was found at the SW corner of the excavated area (UE102 in Fig. 4). Below this stratigraphic unit, an organic soil floor (UE103) covered the entire sector at 5 cm depth, followed by another layer of large to mediumsized ultramafic blocks (UE104) without any visible alignment. Once excavated UE104, a ground deposit (UE105) containing gravel (including quartzite gravel) and tile fragments was identified in the southern half of the archaeological trench, while a layer of dark brown soil material was found in the northern part (UE106). Both were lying over a deposit of stone blocks (UE107) located in the central part of the ditch at 10–15 cm deep. This structure displayed an E-W orientation, forming an enclosure, which covered almost entirely the trench (Fig. 4). I was formed by accumulation of nonsorted stones (including granite, ultramafic rocks and quartzite) without regular dip. The organic matter of the sediment found in this stratigraphic unit was dated by AMS radiocarbon, and a calibrated age interval of 895–1020 AD obtained (Sanchez-Pardo et al., in press). To the north of this deposit there was a possible collapse of stones (UE108) under EU106, formed by non-sorted ultramafic blocks. Below it, a dark brown soil deposit (UE110) containing gravel was excavated, and after it a light chestnut colour soil deposit was found (UE111). At the NE corner of the excavation, a deposit of lying ultramafic stones was observed again (UE112). Once EU107 was removed, a dark-brown soil (EU109-113) with coal remains, gravels and tile fragments was revealed, covering almost all the excavation area. At this very same level, in the central part of the trench, a new thicker accumulation of ultramafic stones was found at 20–25 cm deep (Fig. 4). This accumulation of stone blocks reveals a premeditated anthropic action, destined to form more a closure of fence (without joint mortar) than a proper wall (Fig. 4). The organic matter of the sediment found in this level was dated by AMS radiocarbon providing a calibrated age range of 905–1025 AD (Sanchez-Pardo et al., under review). However, the excavation of this trench was never concluded, so these observations still need to be confirmed.
6. Discussion After looking at the statistical results and the maps built up with the measured K, eU and eTh contents (Fig. 2), and taking into account the differences in the content of such elements between the sediment and the stone blocks, a wider range of data and clearer differences could be expected. However, this does not occur, and this situation could be due to different reasons. On the one hand, the 0.25 m upper terrain layer is composed by a mixture of materials. This includes thin layers of sediments with probable, variable radioisotope contents as well as accumulations of stones and tile fragments of different origin and radioisotope contents. On the other hand, such buried materials are disorderly arranged at different depths. Since the gamma ray emission is measured in a single spot, there is a contribution of materials placed up to a few cm deep and surface radius, with higher contribution of the shallower and closer materials. Thus, a clear correlation between the K, eU and eTh estimated by in situ GRS measurements and the underlying material is not observed. The low radioactive content of the main rock used in the structures (ultramafic rock) also hinders a better result. The use of the Th/U, Th/K and U/K ratios provided more interesting results than the direct use of the measured K, eU and eTh contents. Ruffell et al. [12] suggested that Th/K shows well-defined, but less consistent, measurements when compared to Th/U ones. The latter showed a less precise definition of the edge of the structures, but were geometrically more realistic. This is due to the wider range of values obtained for such ratios, because of the higher values of Th (in ppm) when compared with to U (also in ppm) and K ones in %. This variability could also be related with the variability of the ratio due to any disequilibrium in the U and Th decay chains that is possible in the sediment but not in unweathered rocks [28]. The analysis of the maps built up with such ratios (Fig. 3) reveals clear differences in the three ratios in the square area comprised between G-6 and K-9, being the edges more evident in the Th/K map. In that area, the values of the above mentioned ratios are significantly higher, but the differences are less obvious in the U/K and Th/U ratios. This correlation of ratios was unexpected considering the different ratios observed in the sediment and ultramafic stony material. Nevertheless, it agrees with the observations of Ruffell et al. [12], who found lower ratios in the areas where buried structures were located, since the K, eTh and eU values of such features were higher than those coming from the surrounding soil. There are two findings from the archaeological excavation that could help us to assess the GRS results. Firstly, other stones different than ultramafic were found in the excavated trench, including quartzite and granite. The Th/K and U/K ratios observed in the analysed granite sample are lower than those observed for the ultramafic ones, while the Th/U ratio is similar. These conditions could affect the in situ GRS spot measurements. Secondly, several thin sediment layers (or stratigraphic units) have been observed in the excavation that could provide the variable K, eU and eTh concentrations measured by the GRS survey. Last but not least, the thin multiple layers documented during the excavation are an unequivocal cause of fuzzy results. When comparing the excavation results with the maps of radioisotope contents and ratios, it must be remarked that stone block layers were found in the upper section (less than 5 cm deep) of the SW corner of the trench (UE102; Fig. 4), and at 5–10 cm deep in its central area (UE104; Fig. 4). Such rocks are mainly ultramafic, but they also are the nearest to the surface. Blocks coming from a collapse were also found in the northern part of the ditch at 1015 cm deep (UE108; Fig. 4). Another accumulation of ultramafic, granite and quartzite blocks (UE107; Fig. 4) was observed in the central part of the excavated area with approximate E-W orientation at 10–20 cm deep. Accumulations of non-sorted blocks are
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Fig. 4. Results obtained from excavation: (A) picture of the excavated surface; (B) UE102 (SW corner) with the accumulation of blocks and fragmented tiles and UE103 with an organic soil floor at depth ∼5 cm; (C) after UE102 and UE103 materials were removed; (D) deposit containing gravel and tile fragments of UE104 and brown soil material in the N side (UE106); (E) accumulation of blocks in the N part (UE107) and collapsing deposit of blocks (UE108) at depth ∼10–15 cm; (F) another view of UE107 and dark brown soil deposit containing gravel in the southern area (UE110); (G) Detail of the light chestnut colour soil (UE111) in the south corner; (H) ground deposit containing gravel and tile fragments in the southern half (UE 105); (I) dark-brown soil with coal remains, gravels and tile fragments (UE109–113) located below UE107; (J) final picture with UE 114 of arrangement of blocks in the excavation area at depth 25–30 cm with scale; (K) picture of the excavated ditch from the south. The arrows indicate the north.
also observed in the northern corner (UE112; Fig. 4) and again in the central part of the trench with approximate E-W orientation (UE114; Fig. 4), but the contribution of the gamma rays emitted by materials that deep (20–30 cm) was very weak or negligible to be registered. The UE107 and UE114 were the only structures (a closure or fence) identified in the archaeological excavation, which were previously identified by the GRS exploration. 7. Conclusions Although in situ GRS has not been frequently used for geophysical exploration in field archaeology, the technique has been suggested and tested with success in a still limited number of studies. This work represents a new approach to the matter that includes a crosscheck between GRS survey and archaeological excavation. The characteristics of the site were suitable for the survey, despite the low radioactive content of the analysed materials. The
flat surface and the presence of shallowly buried structures (whose building materials present a contrast in radioisotope contents with the burying sediments) were suggestive. The spot estimates of K, eU and eTh within the square grid provided a narrow range of results and, did not define clear zones related to buried archaeological features in the surveyed area. Thus, it can be concluded that the direct use of K, U and Th estimations was not successful, due to the low gamma ray emission of the buried materials. This fact hinders the usefulness of these in situ estimations to detect buried structures. However, as other previous studies showed, the use of U/Th, Th/K and U/K ratios provides better results (more useful in terms of detection of structures). The outcomes obtained here showed that the accumulations of stone blocks at different depths in the central part of the excavation trench clearly contributed to the higher Th/U, Th/K and U/K ratios observed in that zone. These results have also shown that field gamma ray spectrometry, a very expedite and simple
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technique that can be deployed by very small teams (even by just one person), has potential for the archaeological survey. Naturally, better results could be obtained in the case of more radioactive materials, as it happens with some of the granites referred by Sanjurjo-Sánchez and Alves [27]. Even if GRS was used only in a small area, our fieldwork experience reveals that this tool could be very useful if performed in more extensive surveys, helping to a better selection of areas for more detailed archaeological studies. Acknowledgements This research has been funded by project Marie Curie Career Integration Grant “EMCHAHE: Early Medieval Churches: History, Archaeology and Heritage” (Grant agreement - PCIG12-GA-2012334068) and TERPOMED project (funded by a Xunta de Galicia 2016 Excellence Projects Grants). The University Institute of Geol˜ (Spain) also received support ogy of the University of A Coruna from the Xunta de Galicia with from the programs “Consolidación y estructuración de unidades de investigación competitivas: Grupos de potencial de crecimiento” (GPC2015/024) and Redes de investigación (ED431D 2017/08). The Lab2PT – Landscapes, Heritage and Territory laboratory – AUR/04509 is supported by the Portuguese “Fundac¸ão para a Ciência e a Tecnologia” (FCT UID/AUR/04509/2013), with Portuguese funds and when applicable of the FEDER co-financing, in the aim of the new partnership agreement PT2020 and COMPETE2020 - POCI 01 0145 FEDER 007528. References [1] M.J. Aitken, Physics and Archaeology, Second ed., Clarendon Press, Oxford, 1974. [2] J. Oswin, Field Guide to Geophysics in Archaeology, Springer, Berlin, 2009. [3] J. Wynn, Archaeological prospecting: an introduction to the special issue, Geophysics 51 Zˇ 3 (1986) 533–553. [4] A.J. Clark, Seeing Beneath the Soil: prospecting method in archaeology, second ed., B.T. Batsford, London, 1996 [192 pp.]. [5] L. Rybach, G.F. Schwarz, Ground gamma radiation maps: processing of airborne, laboratory, and in situ spectrometry data, First Break 13 (1995) 97–104. [6] L. Rybach, B. Bucher, G. Schwarz, Airborne surveys of Swiss nuclear facility sites, J. Environ. Radioact. 53 (2001) 291–300. [7] I.A.E.A International Atomic Energy Agency, Guidelines for radioelement mapping using gamma ray spectrometry data, in: IAEA-TECDOC-1363, Vienna, 2003. [8] D. Sanderson, A. Cresswell, E. Scott, J. Lang, Demonstrating the European capability for airborne gamma spectrometry: results from the ECCOMAGS exercise, Radiat. Prot. Dosim. 109 (2004) 119–125. [9] G. Peschel, Radiometrische Messugen zum Nachweis verdeckter archäologisher Objecte. Ausgrabungen and Funde, Ausgrabungen and Funde 12 (1967) 287–297.
[10] A. Ruffell, J. Wilson, Nearsurface investigation of ground chemistry using radiometric measurements and spectral gamma-ray data, Archaeol. Prospect. 5 (1998) 137–142. [11] M. Moussa, Gamma-ray spectrometry: a new tool for exploring archaeological sites; a case stody form East Sinai, Egypt, J. Appl. Geophys. 48 (2001) 137–142. [12] A. Ruffell, J.M. Mckinley, C.D. Lloyd, C. Graham, Th/K and Th/U ratios from spectral gamma-ray surveys improve the mapped definition of subsurface structures, J. Environ. Eng. Geophys. 11 (1) (2006) 53–61. [13] J. Bezuidenhout, Mapping of historical human activities in the Saldanha Bay military area, J. Mil. Stud. 40 (2) (2012) 89–101. [14] P. Killeen, Gamma ray spectrometric methods in uranium exploration – Application and interpretation, Geophys. Geochem. Search Met. Ores. 31 (1979) 163–230. [15] J.M. Costa García, Las primeras intervenciones arqueológicas en A Cidadela ˜ Gallaecia 32 (2014) 109–127. (Sobrado dos Monxes A Coruna), ˜ Gesto, Excavaciones en el campamento romano de Cidadela [16] J.M. Caamano ˜ ˜ de 1981, Memoria preliminar de la campana (Sobrado dos Monxes Coruna). Noticiario Arqueológico Hispánico 18 (1984) 233–254. ˜ Gesto, C. Fernández Rodríguez, “Producción y comercialización [17] J.M. Caamano ˜ en el campamento romano de Cidadela (Sobrado dos Monxes, A Coruna)”, in: Á. Morillo Cerdán (Ed.), Arqueología militar romana en Hispania II: Producción y abastecimiento en el ámbito militar, Servicio de publicaciones de la Universidad de León, León, 2006, pp. 167–184. ˜ Gesto, I. Castro Paredes, M.J. Ínsua Linares, ˜ [18] J.M. Caamano M.C. López Pérez, M.A. Vázquez Martínez, C. Fernández Rodríguez, Evidencias materiales en el cam˜ in: V. Oliveira pamento romano de Cidadela – Sobrado dos Monxes A Coruna, Jorge (Ed.), Arqueología da Antigüedade na Península Ibérica, Actas do 3(Congresso de Arqueología Peninsular (Vila Real, 1999), VI, ADECAP, Porto, 2000, pp. 281–292. [19] J.M. Costa-García, D. Varela Gómez, A Cidadela después de Roma Introducción al estudio del yacimiento y su entorno durante el período medieval, Gallaecia 30 (2011) 181–194. [20] R. Blanco-Rotea, J.M. Costa-García, J.C. Sánchez Pardo, Análisis de la evolución constructiva de las estructuras excavadas en el yacimiento de A Cidadela ˜ y propuestas interpretativas sobre sus “fases (Sobrado dos Monxes A Coruna) tardoantiguas”, Estudos do Quaternário 12 (2015) 69–93. [21] Sánchez Pardo, J.C. (in press), “La ocupación tardoantigua del yacimiento de ˜ in: López Quiroga, J. and Martínez Cidadela (Sobrado dos Monxes, A Coruna)”, Tejera, A. (eds), In tempore sueborum. Catálogo da exposición, Ourense, Diputación de Ourense. ˜ E 1:50.000. Sobrado de los Monjes, Instituto [22] IGME, Mapa Geológico de Espana, ˜ 1981. Geológico y Minero de Espana, [23] A. Delijska, T. Blazheva, L. Petkoca, L. Dimov, Fusion with lithium borate as sample preparation for ICP and AAS analysis, Fresenius Z. Analytische Chem. 332 (1988) 362–365. [24] W. Doherty, An internal standardization procedure for the determination of yttrium and the rare earth elements in geological materials by inductively coupled plasma-mass spectrometry, Spectrochim. Acta 44B (1989) 263–280. [25] K.H. Wedepohl, Ingerson Lecture, the composition of the continental crust, Geochim. Cosmochim. Acta 59 (7) (1995) 1217–1232, http://dx.doi.org/10.1016/0016-7037(95)00038-1. [26] R.W. Boyle, Geochemical Propecting for Thorium and Uranium Deposits, Elsevier, New York, 1982 [489 pp.]. [27] J. Sanjurjo-Sánchez, C. Alves, Geologic materials and gamma radiation in the built environment, Env. Chem Letters (2017), http://dx.doi.org/10.1007/s10311-017-0643-1. [28] M. Gascoyne, Geochemistry of the actinides and their daughters, in: M. Ivanovich, R.S. Harmon (Eds.), Uranium-series Disequilibrium: applications to Earth, marine, and environmental sciences, Clarendon Press, Oxford, 1992, pp. 34–61.
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Using in situ gamma ray spectrometry (GRS) exploration of buried archaeological structures: A case study from NW Spain Jorge Sanjurjo-Sánchez a,∗ , Carlos Arce Chamorro a , Carlos Alves b , Jose Carlos Sánchez-Pardo c , Rebeca Blanco-Rotea c,d , Jose Manuel Costa-García c,e a
Instituto Universitario de Xeoloxía “Isidro Parga Pondal”, Universidade da Coru˜ na A, Coru˜ na, Spain Lab2PT (FCT UID/AUR/04509/2013, FEDER COMPETE POCI-01-0145-FEDER-007528) and Earth Sciences Department, School of Sciences, University of Minho, Braga, Portugal c Landscape, Heritage and Paleoenvironment Laboratory, University of Santiago, Santiago, Spain d Unit of Archaeology, University of Minho, Braga, Portugal e School of Classics, History and Archaeology, Newcastle University, Newcastle, UK b
a r t i c l e
i n f o
Article history: Received 30 October 2017 Accepted 9 May 2018 Available online xxx Keywords: Archaeological survey Radioactive isotopes Gamma spectrometry Non-destructive survey Mapping Maps of elements ratios
a b s t r a c t Geophysical exploration methods allow the detection of archaeological features before any excavation is carried out on these sites. This is due to the contrast of properties between the buried archaeological structures and objects and the surrounding soil, sediment, or rock. Although Gamma-ray spectrometry (GRS) is [widely] used for geological exploration and mapping, it has been scarcely used in archaeology so far, despite the successful results of previous studies on the matter. In situ GRS is a non-destructive method that allows direct assessment of uranium-238 (238 U) and thorium-232 (232 Th) from daughter radionuclides of their decay chains, as well as potassium-40 (40 K), on soils and rock outcrops. The technique documents the concentration of these isotope concentrations in the topsoil by surface measurements and this enhances its potential for archaeological exploration. However, two assumptions must be made: the archaeological objects must contain a different concentration of radionuclides than the surrounding sediment or soil, and they must be buried in the terrain less than 25–30 cm deep. In this work, we present the results of the use of in situ GRS for the study of a buried structure in the archaeological site of Cidadela (Galicia, NW Spain). Firstly, we have tested in situ spot GRS measurements to detect rock-built structures buried in the sediments; secondly, we have excavated the surveyed area. The results are reliable despite the low radioactive content of the rocks used as building materials, given that the burying and sediments also have low amounts of radioactive isotopes. Although the direct use of the estimates of K, U and Th has not proved successful, the use of U/Th, Th/K and U/K ratios provided reliable results. © 2018 Elsevier Masson SAS. All rights reserved.
1. Introduction Geophysical exploration is usually performed to obtain data about the underlying substrate, being possible to build up vertical profiles or maps. Some geophysical exploration methods are also used today in archaeology, being called “archaeogeophysics” or “archaeological geophysics” [1,2]. They are very useful when trying to identify the location and layout of archaeological objects and structures, as well as to better define survey and excavation areas. Since archaeological excavation is an expensive, time consuming and destructive activity, the use geophysical techniques imply a notable saving of both resources and time.
∗ Corresponding author. E-mail address: [email protected] (J. Sanjurjo-Sánchez).
Among the geophysical methods, those most extensively used in archaeology are the seismic, electrical and electromagnetical methods, as well as the ground penetrating radar or GPR [1–4]. All of them allow the assessment of properties of the underlying soil, sediments and/or rocks, providing a cross section of soil properties up to some meters deep. These properties can be related to any buried archaeological structure and/or object, as long as the assessed physical properties (e.g. density, porosity, conductivity) are different than those of the surrounding or covering soil or sediment. Gamma-ray spectrometry (GRS) is a technique used for different geophysical purposes, including mineral exploration and geological/geochemical mapping. Portable GRS is a non-destructive method that allows a quick direct assessment of potassium-40 (40 K), and an indirect assessment, thorough isotopes resulting from the decay chain, of uranium-238 (238 U) and thorium-232 (232 Th) on rock outcrops and soils. Measurements can be performed
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Fig. 1. Map of location of the site (A), picture of GRS recording with the marked area and depth (approximate volume surveyed but not at scale) that will affect to the GRS result (B), and general view of the excavated part of the site of Cidadela (C).
hand-held, by car or by plane or helicopter being respectively referred as carborne GRS and airborne GRS [5–8]. Such assessment does not provide 238 U and 232 Th, since the contents of U and Th in the materials are assessed from the spectral emission of daughter isotopes of their decay chains. Its use in archaeological geophysics has been scarce, despite the few existing studies have provided interesting results. 1.1. Previous uses of GRS in archaeology In an early work, Aitken [1] proposed that radioactivity could be used to detect archaeological structures, due to the diverse gamma radiation emissions of different buried materials. However, he also remarked that this could be valid only for shallowly buried structures in locations with very thin sedimentary covers. He referred the earlier work of Peschel [9], who used radiation to detect a buried ditch and a wall. Two decades and half after Aitken’s suggestion, Ruffell and Wilson (1998) [10] reassessed the usefulness of the GRS as an archaeological survey tool. They set up transect profiles to get gamma-ray maps, concluding that GRS is a useful tool for the detection of buried structures or clay beds, among other features. Following this idea, Moussa [11] performed a successful study case for the detection of buried structures in Egypt, although he never matched the results of this survey by actually excavating the site. Ruffell et al. [12] applied the GRS for the detection of subsurface structures, in order to enhance the potential of this technique for the archaeological survey. They performed GRS surveys in two case studies where they know the approximate surface location of the buried structures to contrast the obtained map of K, U and Th concentrations coming from the acquired spectra. They observed that Th/K and Th/U ratios enhanced the results, allowing a better location of buried surfaces with different K, U and Th contents than the burial soil. Later, [13], using GRS in different locations, found
a significant decreasing relationship between K concentrations in soil and human activities in the past. 1.2. Problems and limitations of the use of GRS in archaeology The detection of gamma radiation and radioisotopes by GRS is limited by several factors linked with the method itself and with the studied sediments. Gamma rays are absorbed by both organic matter and water present in sediments [7]. Thus, vegetable litter accumulated on the topsoil, or the presence of a dense low vegetation coverage will reduce gamma radiation and therefore affect GRS measurements. The presence of water also alters the measurements, since it absorbs ionizing radiation. That is the reason why it is preferable to perform measurements on dry soils. Gamma-rays reach up to 700 m in air, but their penetration range is lesser through solid materials: they could penetrate rocks up to 0.5 m deep, but usually the range through sediments and soils typically is set between 0.25 m and 0.3 m [7]. Surface measurements do not just comprise the spot where the probe is located, but a bigger volume around it with a diameter in area of about ten times the distance from the probe to the measured surface (Fig. 1). Thus, geometrical effects of the surveyed surface will affect to the radioisotope estimates, being that spectrometers calibrated to perform borehole or surface measurements. Surface measurements require a flat surface, since other geometries are a cause of over- or underestimation of the radioisotopes [14]. Radioactive disequilibria can also affect the results: this is usually observed in the 238 U decay series, being the eU assessment the most affected one. Such disequilibrium can occur due to the leaching of U or radioisotope daughters, but also to radon releases from the soil. The daughter radioisotope is used to assess the U content from the 238 U chain (214Bi), a post-Rn daughter product [7]. Thus, GRS allows the assessment of the concentration of the referred radioisotopes on
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the topsoil or rock surfaces, and could be of use for archaeological survey. However, two assumptions must be considered: (i) the archaeological buried features must show differences in terms of radionuclide contents in relation with the surrounding sediment or soil; and (ii) they must be close to the surface (0–25 cm deep). Thus, it is potentially applicable for the survey of shallow structures or objects. In any case, this method does not provide cross profiles of the ground, but maps of the buried structures and objects.
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tures are not really located in depth, considering the present-day soil surface. Especially in the central part of the site, plenty of features have been located between 5 and 30 cm deep. Since the surface of the site is predominantly flat, we estimated that this was an excellent opportunity to test the application of GRS in an archaeological context. In addition, the GRS results could be matched to those coming from the excavations. 4. Materials and methods
2. The archaeological site of Cidadela 4.1. Hand-held GRS The archaeological site of Cidadela is located over a plateau between the Cabalar and Pequeno rivers, in the municipality of Sobrado dos Monxes (Galicia, NW Spain). The existence of archaeological remains in this location was known from at least the end of the 19th Century, and even some excavations were carried out there in 1934 following a pre-scientific methodology [15]. It was not until 1981 that a series of systematic archaeological excava˜ Gesto [16], who nominally tions were developed by Prof. Caamano remained as project leader until the 2000 decade. Up to date, approximately 20% of the site area has been excavated (Fig. 1). These works have revealed the existence of some phases dated to Roman times, and linked with the building and occupation of a military fort by the cohors I Celtiberorum, a mixed infantry and cavalry auxiliary unit of the Roman army [17,18]. The site was reoccupied and almost entirely reorganized after the departing of the military unit: some phases were related to the presence of a possible monastery Visigothic times, while others were interpreted as later medieval occupations [19]. However, recent interdisciplinary works carried out between 2014 and 2016 [20,21], have led to a more comprehensive chronology: Early Roman horizon (2nd–3rd c. AD), linked with the construction occupation of the mentioned fort; • Late Antique layer (4th–6th/7th c. AD) in which it is attested the reuse of some previous structures and the building of several new ones, as well as the presence of imported pottery and metallic objects of this chronology; • Early Medieval horizon (9th–11th c. AD) characterised by the presence of a church and some buildings related to it; • Extensive medieval occupation (12th to 13th c. AD), which implies the dislocation of the church outside the walls of the ancient fort and the entire reorganization of the structures within it; • A post-medieval period (14th to 21st c. AD), when the site was destined to agricultural and extractive purposes. Our survey was located in an area of the site where the presence of Early Medieval structures was expected, at a shallow (but unknown in detail) depth, due to previous survey studies (see below). 3. Aim of this study In the framework of the Marie Curie CIG project EMCHAHE (Early Medieval Churches: Archaeology, History and Heritage), a survey with ground penetrating radar (GPR) was carried out in the unexcavated area of the site of Cidadela in 2016. The goal of this survey was to identify areas with a greater presence of Early Medieval structures and, where possible, features linked with the abovementioned church for their excavation. A hand-held GRS survey was also performed before excavation works. The majority of the structures located during previous archaeological excavations belonged to wall foundations, usually about 30–60 cm height. It has also been attested that these struc-
Gamma spectra with estimates of the radioisotope contents of rocks were acquired with a portable spectrometer (carried by an operator) GF Instruments GRS-2000 with a BGO probe (Bi4 Ge3 O12 ) of 512 channels (measuring energies up to 3 MeV) that measures up to 70,000 pulses/second (in the whole channels). The device consists on a probe with 2”×2” detector (3.5 kg weight) connected to an independent battery. The instrument was calibrated by the manufacturer according to IAEA recommendations. Statistical uncertainties are estimated to be 6% for K, 30% for U, and 16% for Th for an acquisition time of 180 s. The GRS measurements were carried out in summer (2016) to ensure soil dry conditions. As a preliminary step, the low vegetation coverage was removed in the areas to be surveyed and excavated. After that, the survey area was delimited to a square of 5 × 5 m and georeferenced by using a total station Leica TCRM 1105 plus. The area was marked by using a string and a pickaxe, in order to build a network of squares with 50 cm side. The gamma emission of soil was measured by placing the GRS probe in the knots of the network, obtaining a spectrum for each knot, that is a measurement each 50 cm in 11 parallel profiles of 5 m long separated 50 cm one to other (Figs. 2 and 3). We have used a 2D coordinate system to reference these measurement points, with letters (from A to J, from west to east) and numbers (from 1 to 11, from south to north). A total of 121 spots were measured. Results were used to build up a map with isolines or isograms that mark spatial differences in the composition of radionuclides. Previously, histograms with the obtained data were observed to choose classes and isolines for these classes were later drawn by linear interpolation between points of the grid. Although the instrument is calibrated for an acquisition period of 180 s, we have performed previous tests with variable acquisition times of 60 s, 120 s, 180 s and 300 s in a given point (25 measurements) to compare results and their variability. Such test (see results) indicated that a 120 s acquisition time was enough to get confident estimates. From these spectra we can extract estimated contents of K (potassium, mass percent), eU (uranium equivalent, in mass parts per million or ppm) and eTh (thorium equivalent, also in ppm). While potassium contents are estimated from the peak of 40 K at 1.461 MeV (energy range 1.366–1.564 keV), there are several radioactive isotopes in the uranium and thorium decay series and, hence are used “equivalents” of uranium and thorium, assuming secular equilibrium. Estimations of eU and eTh are made from peaks of 214 Bi (bismuth) at the 1.764 MeV (energy range 1.57–1.959 keV) and 208 Tl (thallium) at the 2.615 MeV peak (energy range 2.42–2.81 keV), respectively. 4.2. Sampling and complementary analyses To cross-check GRS estimates, samples of the burying sediments and stones used as building materials were collected on the archaeological site to assess their nature and radioisotope content. The sediment has been identified in the geological map [22] as an alluvial one.
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Fig. 2. Results of materials analyses of rocks and sediments of the site (A), surveyed area (with excavated area in the right side) (B), map with results of K (%)(C), eTh (ppm) (D) and eU (ppm) (E) distributions. The total surveyed area corresponds to a square of 5 × 5 m. The arrows indicate the north.
Petrographic thin sections were prepared from collected rock samples to identify the rocks used as building materials under an optical petrographic microscope Nikon OPTIPHOT2-POL. Images were recorded with a Nikon DS-Fi1 camera controlled by using NIS Elements version 3.0 image software. To confirm the petrographic observations and to assess the K content of the samples, a X-Ray Fluorescence Spectrometry (XRF) was performed. Samples were grinded to fine grains < 63 m and measured in a Bruker-Nonius S4 Pioneer wavelength dispersive fluorescence spectrometer under helium purge at the University of A ˜ (Spain). Coruna To assess trace elements, namely U and Th contents, Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) was carried out in a Thermo ScientificTM Element XRTM ICP-MS, which combines a dual mode SEM with a Faraday detector Lithium metaborate fusion was used for sample preparation [23,24], and powdered samples were mixed with an equal amount of lithium tetraborate flux, placed in a carbon crucible and fused at 1000◦ C in a furnace for 30 minutes. After the melt was cooled, the resultant fusion bead was briefly grounded and dissolved in 100 mL of 4% HNO3 /2% HCl3 solution. The latter was then analysed by ICP-MS.
Table 1 Results (mean ± standard deviation) of the acquisition time test of spectra with the GRS.
5. Results
The collected rocks used as building materials in the excavated structures have been identified as serpentinised ultramafic rock (CR-1 and CR-2), and a two mica medium grained granite (CR-3). All of them are near local geologic materials as outcrops of the ultramafic rock can be found about 1 km distance, while granite outcrops are found at a distance about 5 km. XRF and ICP-MS results (see Fig. 2) provide very low K, U and Th contents for the ultramafic samples, clearly below the average values for the upper continental crust of 2.5 ppm, 10.3 ppm and 2.865% for U, Th and K, respectively
5.1. Spectra acquisition time test Before the field survey, a test to check the effect of using different acquisition times was performed. As related above the GRS is calibrated for an acquisition time of 180 s, but our tests showed that readings obtained by using measurements times of 30 s and 60 s fit the results of higher times (Table 1), being the deviation
Acquisition eTh (ppm) time (s) 30 60 120 180 300
3.68 4.04 3.67 3.53 3.63
± ± ± ± ±
0.65 1.23 0.72 0.49 0.70
Cv %
eU (ppm)
17.7 30.4 19.6 13.9 19.3
1.46 1.48 1.77 1.63 1.77
± ± ± ± ±
0.71 0.62 0.51 0.50 0.46
Cv (%)
K (%)
48.6 41.9 28.8 30.7 26.0
0.68 0.72 0.73 0.70 0.70
Cv (%) ± ± ± ± ±
0.08 11.8 0.04 5.6 0.06 8.2 0.05 7.1 0.05 7.1
Cv: coefficient of variation.
of such data significantly higher, at least for eTh and eU. However, the results for an acquisition time of 120 s are similar to those of longer acquisition periods. The K values are the only exception to this rule, but the deviation of such radioisotope is lower than those of the other ones. That is the reason why we considered this interval of 120 s as a confident acquisition period for an optimum fieldwork in terms of time and results. 5.2. Analytical results of samples
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Fig. 3. Estimated of expected ratios of each analysed material in the site (A), surveyed area (with excavated area in the right side) (B), maps of ratios estimated from results for Th/U (C), Th/K (D) and U/K (E). The arrows indicate the north.
[25]. The granite sample shows higher values, but below the average values of this rock type [26], being the U concentration above the Th, on the contrary of average granite values, being usually the Th content higher than the U content. This is because the presence of K, U and Th strongly depends on the presence of some minerals [26]. The sediment sample displays lower K and U contents than the granite (Th contents are similar in both granite and sediment). Thus, the presence of foundations with ultramafic rocks should be indicated by very low K, eU and eTh in GRS during the survey, while the very high K and U values could indicate the presence of granite. Following the suggestions of Ruffell et al. [12], we have assessed the Th/K and Th/U ratios (Fig. 2), as well as the U/K ratios to check also the potential usefulness of this ratio. The results of the sample analyses show very different ratios for the sediment and the rocks, except for the U/K ratio of the sediment and granite samples. This is because the unexpected low Th content in the granite sample. It is possible to stress that the result of the granite sample is slightly below the median of the data set collected by Sanjurjo-Sánchez and Alves [27] concerning granitic rocks (granites and granodiorites), while the ultramafic rock samples present the typically low values of this kind of rocks. Anyway, the different ratios observed indicate that GRS should be a suitable technique to explore structures built with stone in the topsoil of the site. 5.3. GRS survey The survey shows different outcomes for the three studied radioisotopes (Table 2). While the 40 K range observed in the studied area is narrow, a wider range is observed for 214 Bi and 209 Tl, being the eU and eTh oscillations higher. Thus, spatial oscillations of K cannot be considered as significant to detect buried structures
Table 2 Statistical summary of data obtained by GRS (121 measured points).
Median Mean Standard deviation Range
K (%)
eU (ppm)
eTh (ppm)
Th/U
Th/K
U/K
0.50 0.48 0.10
1.30 1.36 0.46
1.80 1.87 0.68
1.38 1.37 1.16
3.60 3.91 1.88
2.60 2.84 1.40
0.2–0.7
0.4–3.0
0.4–3.5
0.2–6.6
0.8–12
0.8–9.5
due to the narrow range measured. This effect is probably owing to the low K content observed in the sediment (1.3%). Indeed, the serpentinised ultramafic rock shows only < 0.05% values, and this is the main building material used in the archaeological structures. K shows halfway values, but probably indicate the presence of stone blocks buried in the whole studied area. In the same way, the Th and U contents of the ultramafic rock are very low and significantly higher in the sediment (Fig. 2). This implies that eU and eTh contents recorded by GRS are more useful when dealing with the detection of buried structure constructed by using ultramafic blocks. Since the eU registered by using GRS can be biased by disequilibrium in the 238 U decay chain, Th could be the most reliable element. However, an alternative tool is the use of ratios. Given the narrow K range, this element could be used to normalise the eU and eTh values. The Th/U ratio is also useful, since it is typically used to assess any possible secular disequilibrium in the 238 U decay chain [12]. Such disequilibrium is not observed in rocks [26,28], being the Th/U ratio constant when buried stones are surveyed by GRS. This way, the ratio values provide wider ranges (Table 2) and these new ratios are used to assess any drop or rise in the studied profiles. For the Th/U and Th/K ratios, low values would indicate the presence of
Please cite this article in press as: J. Sanjurjo-Sánchez, et al., Using in situ gamma ray spectrometry (GRS) exploration of buried archaeological structures: A case study from NW Spain, Journal of Cultural Heritage (2017), https://doi.org/10.1016/j.culher.2018.05.004
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ultramafic rocks, and higher values the presence of sediment. For the U/K ratio, low values would indicate the absence of rocks in the terrain down to a 0.25 m depth. The data obtained by these means were used to build a map of the distribution of K, eU and eTh in the studied area (Fig. 2). If we use this map together with the analysis of the sediment and rocks, lower values can be expected where ultramafic rock accumulations (by far the most used rock) are placed, while points with higher values should be linked with plain sediments or with the presence of granite within this 0.25 m layer. As Fig. 2 shows, there are some areas where eTh and eU values are very low but they do not completely fit. Following the procedure of Ruffell et al. [12], maps using Th/U, Th/K and U/K ratios were prepared (Fig. 3). These maps provide a sharper difference between several areas of the surveyed area. According to the chemical analyses of sediments and rocks, the areas with buried rocks (both granite and ultramafic rock) should provide higher U/K ratios (especially in the case of ultramafic rocks) and lower Th/K and Th/U ratios. There are some sectors in which the Th/U, Th/K and U/K ratios deserve a special attention. The square limited by F-6 and G-8 is characterised by high values in all ratios, as it happens with the area comprised between G-6 and K-8. This indicates a similar composition of the soil layer located in these. However, the ratios do not fit the expected values predicted from the chemical analysis of samples, at least regarding the Th/U value, which should display an opposite behaviour to the U/K ratio. The differences in the Th/K ratio are less significant. 5.4. The archaeological excavation An accumulation of small-medium-sized stone blocks (mostly ultramafic) mixed with fragmented tiles was found at the SW corner of the excavated area (UE102 in Fig. 4). Below this stratigraphic unit, an organic soil floor (UE103) covered the entire sector at 5 cm depth, followed by another layer of large to mediumsized ultramafic blocks (UE104) without any visible alignment. Once excavated UE104, a ground deposit (UE105) containing gravel (including quartzite gravel) and tile fragments was identified in the southern half of the archaeological trench, while a layer of dark brown soil material was found in the northern part (UE106). Both were lying over a deposit of stone blocks (UE107) located in the central part of the ditch at 10–15 cm deep. This structure displayed an E-W orientation, forming an enclosure, which covered almost entirely the trench (Fig. 4). I was formed by accumulation of nonsorted stones (including granite, ultramafic rocks and quartzite) without regular dip. The organic matter of the sediment found in this stratigraphic unit was dated by AMS radiocarbon, and a calibrated age interval of 895–1020 AD obtained (Sanchez-Pardo et al., in press). To the north of this deposit there was a possible collapse of stones (UE108) under EU106, formed by non-sorted ultramafic blocks. Below it, a dark brown soil deposit (UE110) containing gravel was excavated, and after it a light chestnut colour soil deposit was found (UE111). At the NE corner of the excavation, a deposit of lying ultramafic stones was observed again (UE112). Once EU107 was removed, a dark-brown soil (EU109-113) with coal remains, gravels and tile fragments was revealed, covering almost all the excavation area. At this very same level, in the central part of the trench, a new thicker accumulation of ultramafic stones was found at 20–25 cm deep (Fig. 4). This accumulation of stone blocks reveals a premeditated anthropic action, destined to form more a closure of fence (without joint mortar) than a proper wall (Fig. 4). The organic matter of the sediment found in this level was dated by AMS radiocarbon providing a calibrated age range of 905–1025 AD (Sanchez-Pardo et al., under review). However, the excavation of this trench was never concluded, so these observations still need to be confirmed.
6. Discussion After looking at the statistical results and the maps built up with the measured K, eU and eTh contents (Fig. 2), and taking into account the differences in the content of such elements between the sediment and the stone blocks, a wider range of data and clearer differences could be expected. However, this does not occur, and this situation could be due to different reasons. On the one hand, the 0.25 m upper terrain layer is composed by a mixture of materials. This includes thin layers of sediments with probable, variable radioisotope contents as well as accumulations of stones and tile fragments of different origin and radioisotope contents. On the other hand, such buried materials are disorderly arranged at different depths. Since the gamma ray emission is measured in a single spot, there is a contribution of materials placed up to a few cm deep and surface radius, with higher contribution of the shallower and closer materials. Thus, a clear correlation between the K, eU and eTh estimated by in situ GRS measurements and the underlying material is not observed. The low radioactive content of the main rock used in the structures (ultramafic rock) also hinders a better result. The use of the Th/U, Th/K and U/K ratios provided more interesting results than the direct use of the measured K, eU and eTh contents. Ruffell et al. [12] suggested that Th/K shows well-defined, but less consistent, measurements when compared to Th/U ones. The latter showed a less precise definition of the edge of the structures, but were geometrically more realistic. This is due to the wider range of values obtained for such ratios, because of the higher values of Th (in ppm) when compared with to U (also in ppm) and K ones in %. This variability could also be related with the variability of the ratio due to any disequilibrium in the U and Th decay chains that is possible in the sediment but not in unweathered rocks [28]. The analysis of the maps built up with such ratios (Fig. 3) reveals clear differences in the three ratios in the square area comprised between G-6 and K-9, being the edges more evident in the Th/K map. In that area, the values of the above mentioned ratios are significantly higher, but the differences are less obvious in the U/K and Th/U ratios. This correlation of ratios was unexpected considering the different ratios observed in the sediment and ultramafic stony material. Nevertheless, it agrees with the observations of Ruffell et al. [12], who found lower ratios in the areas where buried structures were located, since the K, eTh and eU values of such features were higher than those coming from the surrounding soil. There are two findings from the archaeological excavation that could help us to assess the GRS results. Firstly, other stones different than ultramafic were found in the excavated trench, including quartzite and granite. The Th/K and U/K ratios observed in the analysed granite sample are lower than those observed for the ultramafic ones, while the Th/U ratio is similar. These conditions could affect the in situ GRS spot measurements. Secondly, several thin sediment layers (or stratigraphic units) have been observed in the excavation that could provide the variable K, eU and eTh concentrations measured by the GRS survey. Last but not least, the thin multiple layers documented during the excavation are an unequivocal cause of fuzzy results. When comparing the excavation results with the maps of radioisotope contents and ratios, it must be remarked that stone block layers were found in the upper section (less than 5 cm deep) of the SW corner of the trench (UE102; Fig. 4), and at 5–10 cm deep in its central area (UE104; Fig. 4). Such rocks are mainly ultramafic, but they also are the nearest to the surface. Blocks coming from a collapse were also found in the northern part of the ditch at 1015 cm deep (UE108; Fig. 4). Another accumulation of ultramafic, granite and quartzite blocks (UE107; Fig. 4) was observed in the central part of the excavated area with approximate E-W orientation at 10–20 cm deep. Accumulations of non-sorted blocks are
Please cite this article in press as: J. Sanjurjo-Sánchez, et al., Using in situ gamma ray spectrometry (GRS) exploration of buried archaeological structures: A case study from NW Spain, Journal of Cultural Heritage (2017), https://doi.org/10.1016/j.culher.2018.05.004
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Fig. 4. Results obtained from excavation: (A) picture of the excavated surface; (B) UE102 (SW corner) with the accumulation of blocks and fragmented tiles and UE103 with an organic soil floor at depth ∼5 cm; (C) after UE102 and UE103 materials were removed; (D) deposit containing gravel and tile fragments of UE104 and brown soil material in the N side (UE106); (E) accumulation of blocks in the N part (UE107) and collapsing deposit of blocks (UE108) at depth ∼10–15 cm; (F) another view of UE107 and dark brown soil deposit containing gravel in the southern area (UE110); (G) Detail of the light chestnut colour soil (UE111) in the south corner; (H) ground deposit containing gravel and tile fragments in the southern half (UE 105); (I) dark-brown soil with coal remains, gravels and tile fragments (UE109–113) located below UE107; (J) final picture with UE 114 of arrangement of blocks in the excavation area at depth 25–30 cm with scale; (K) picture of the excavated ditch from the south. The arrows indicate the north.
also observed in the northern corner (UE112; Fig. 4) and again in the central part of the trench with approximate E-W orientation (UE114; Fig. 4), but the contribution of the gamma rays emitted by materials that deep (20–30 cm) was very weak or negligible to be registered. The UE107 and UE114 were the only structures (a closure or fence) identified in the archaeological excavation, which were previously identified by the GRS exploration. 7. Conclusions Although in situ GRS has not been frequently used for geophysical exploration in field archaeology, the technique has been suggested and tested with success in a still limited number of studies. This work represents a new approach to the matter that includes a crosscheck between GRS survey and archaeological excavation. The characteristics of the site were suitable for the survey, despite the low radioactive content of the analysed materials. The
flat surface and the presence of shallowly buried structures (whose building materials present a contrast in radioisotope contents with the burying sediments) were suggestive. The spot estimates of K, eU and eTh within the square grid provided a narrow range of results and, did not define clear zones related to buried archaeological features in the surveyed area. Thus, it can be concluded that the direct use of K, U and Th estimations was not successful, due to the low gamma ray emission of the buried materials. This fact hinders the usefulness of these in situ estimations to detect buried structures. However, as other previous studies showed, the use of U/Th, Th/K and U/K ratios provides better results (more useful in terms of detection of structures). The outcomes obtained here showed that the accumulations of stone blocks at different depths in the central part of the excavation trench clearly contributed to the higher Th/U, Th/K and U/K ratios observed in that zone. These results have also shown that field gamma ray spectrometry, a very expedite and simple
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technique that can be deployed by very small teams (even by just one person), has potential for the archaeological survey. Naturally, better results could be obtained in the case of more radioactive materials, as it happens with some of the granites referred by Sanjurjo-Sánchez and Alves [27]. Even if GRS was used only in a small area, our fieldwork experience reveals that this tool could be very useful if performed in more extensive surveys, helping to a better selection of areas for more detailed archaeological studies. Acknowledgements This research has been funded by project Marie Curie Career Integration Grant “EMCHAHE: Early Medieval Churches: History, Archaeology and Heritage” (Grant agreement - PCIG12-GA-2012334068) and TERPOMED project (funded by a Xunta de Galicia 2016 Excellence Projects Grants). The University Institute of Geol˜ (Spain) also received support ogy of the University of A Coruna from the Xunta de Galicia with from the programs “Consolidación y estructuración de unidades de investigación competitivas: Grupos de potencial de crecimiento” (GPC2015/024) and Redes de investigación (ED431D 2017/08). The Lab2PT – Landscapes, Heritage and Territory laboratory – AUR/04509 is supported by the Portuguese “Fundac¸ão para a Ciência e a Tecnologia” (FCT UID/AUR/04509/2013), with Portuguese funds and when applicable of the FEDER co-financing, in the aim of the new partnership agreement PT2020 and COMPETE2020 - POCI 01 0145 FEDER 007528. References [1] M.J. Aitken, Physics and Archaeology, Second ed., Clarendon Press, Oxford, 1974. [2] J. Oswin, Field Guide to Geophysics in Archaeology, Springer, Berlin, 2009. [3] J. Wynn, Archaeological prospecting: an introduction to the special issue, Geophysics 51 Zˇ 3 (1986) 533–553. [4] A.J. Clark, Seeing Beneath the Soil: prospecting method in archaeology, second ed., B.T. Batsford, London, 1996 [192 pp.]. [5] L. Rybach, G.F. Schwarz, Ground gamma radiation maps: processing of airborne, laboratory, and in situ spectrometry data, First Break 13 (1995) 97–104. [6] L. Rybach, B. Bucher, G. Schwarz, Airborne surveys of Swiss nuclear facility sites, J. Environ. Radioact. 53 (2001) 291–300. [7] I.A.E.A International Atomic Energy Agency, Guidelines for radioelement mapping using gamma ray spectrometry data, in: IAEA-TECDOC-1363, Vienna, 2003. [8] D. Sanderson, A. Cresswell, E. Scott, J. Lang, Demonstrating the European capability for airborne gamma spectrometry: results from the ECCOMAGS exercise, Radiat. Prot. Dosim. 109 (2004) 119–125. [9] G. Peschel, Radiometrische Messugen zum Nachweis verdeckter archäologisher Objecte. Ausgrabungen and Funde, Ausgrabungen and Funde 12 (1967) 287–297.
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Please cite this article in press as: J. Sanjurjo-Sánchez, et al., Using in situ gamma ray spectrometry (GRS) exploration of buried archaeological structures: A case study from NW Spain, Journal of Cultural Heritage (2017), https://doi.org/10.1016/j.culher.2018.05.004