Archaeogeophysical-Based Approach for Inca Archaeology: Overview and one operational application
Even if, in recent decades, the use of remote sensing technologies (from satellite, aerial and ground) for archaeology is stepping into its golden age, in Southern America geophysics for preventive archaeology is more recent and less used than in Europe, Central America and Middle East. In this paper, we provide a brief overview and show the preliminary results obtained from the investigations conducted in Chachabamba (Peru). The archaeological area is located on a strategic terrace overlooking three Inca roads, which served the most important ceremonial centres (including Machu Picchu) of the Urubamba Valley also known as the Sacred Valley. In particular, Chachabamba investigations were conducted with two principal aims: (1) to give new impetus to archaeological research with targeted investigations aimed at improving and completing the site’s knowledge framework; (2) to experiment and validate an archaeogeophysical approach to be reapplied in other sites of the Urubamba valley, including Machu Picchu, having similar characteristics as those found in Chachabamba.
KeywordsArchaeo-geophysics Ground penetrating radar Geomagnetometry Chachabamba Inca archaeology
In recent decades, the use of remote sensing technologies (from satellite, aerial and ground) for archaeology is stepping into a golden age characterized by an increasing growth of both classical and emerging multidisciplinary methodologies, addressed to the study, documentation and conservation of cultural property (Kvamme 2003; Lasaponara and Masini 2008; Lasaponara et al. 2016; Cuca and Hadjimitsis 2017; Opitz and Herrmann 2018; Masini et al. 2018). The digital tools nowadays available for archaeology enable us to get extremely precise results in a non-invasive way and to speed up the work during the diverse phases of archaeological investigations ranging from survey, mapping, excavation and monitoring at diverse scales of interest, moving from small artefacts to architectural structures and landscape reconstruction. This has totally revolutionized the classical approach of studying human past activities, before mainly based on the recovery and analysis of material culture, field recognition, survey, test pits and, finally, excavation campaigns. All of these activities are time consuming, are expensive and can produce destructive impacts. Time, costs and invasive impact can be reduced or optimized through archaeological prediction tools and methods, as Earth observation technologies including remote sensing and geophysics. The use of these technologies is particularly important in the absence of historical sources, as in the case of prehistoric archaeology or for studies of civilizations that did not have writing and did not leave any documentation. This is the case of pre-Hispanic civilizations in Peru and in other countries of Southern America, from the most ancient cultures, as Chavin, Nasca and Moche, to the most recent ones including Inca. Despite this and the fact that the first pioneering geophysical application in Southern America date to 1985 (Gumaer 1985), the geophysics for the study of the Andean civilizations is still today generally not used in the operational activities of preventive archaeology.
However, it must be considered that even in Europe and North America, the use of geophysics in archaeology has strongly increased only in the last two decades thanks to the technological advances of instrumentation, availability of friendly data processing tools, improvements in data visualization, which facilitate the interpretation of the results.
In Southern America, the use of geophysics for archaeological prospection started at the end of the first decade of the 2000s. The most popular geophysical techniques used for preventive archaeological research are geomagnetometry, ground penetrating radar (GPR) and electrical resistivity tomography (ERT), selected according to the expected characteristics of both buried archaeological features and soils.
Geomagnetometry methods, based on the measuring and recording of spatial variations in the Earth’s magnetic field, have been mainly used to detect and map (1) ring ditches of the prehistoric sites in the lowlands of the Llanos de Moxos in the Bolivian Amazonia (Prümers 2006) and (2) buried earthen city walls of the ceremonial centre of Pachacamac in Peru (Lasaponara et al. 2017).
GPR, based on the analysis of radar signal reflection in the presence of subsoil discontinuities, has been mainly exploited to detect: (1) stone masonry structures, as those in andesite identified in Tiwanaku (Henderson 2004), and (2) naturally dried earth settlements, from the Formative age in the northwest of Argentina to the colonial period in Patagonia (Bonomo et al. 2010; Lascano et al. 2003). GPR has been also fruitfully employed in Peru for the characterization of looted areas in Ventarron (Lasaponara et al. 2014).
Integrated geophysical methods, including ERT, are adopted where and when the use of a single geophysical method is not suitable and effective in terms of information quality, data resolution and penetration capability. For sake of brevity, we cite the discovery in 2012 of a Nasca pyramid in the ceremonial centre of Cahuachi made by integrating GPR and ERT (Masini et al. 2016). The joint use of GPR and magnetic methods proved to be complementary in detecting several different archaeological features from (1) the buried foundation trenches of quincha (a construction system using wood and cane) walls of the colonial villages in the Zaña Valley (North Peru) (Vanvalkenburgh et al. 2015), to (2) the Nasca tombs and ritual offerings in Cahuachi (Rizzo et al. 2010).
Related to Peru, it must be considered that paradoxically geophysical investigations were mostly performed along the arid coast areas to detect raw earth material settlements, which still today represents a technical challenge due to the very low geophysical difference/contrast between the targets and the neighbouring areas. On the contrary, in the highlands of Peru, very few archaeogeophysical investigations have been performed to investigate settlements in stone, easier to be detected compared to those in earth material. In particular in the Urubamba valley, the origin region of the Incas, which includes Machu Picchu (the most famous and visited pre-Hispanic sites of Peru) geophysical prospections were only performed for the monitoring of the landslide hazard to map fractures and zones of weakness in crystalline bedrock (Best et al. 2009).
to give new impetus to archaeological research with targeted investigations aimed at improving and completing the site’s knowledge framework;
to experiment and validate an archaeogeophysical approach to be reapplied in other sites of the Urubamba valley, including Machu Picchu, having similar characteristics as those typically found in Chachabamba, i.e. typical Inca walls, with their constructive features, terracing systems and water channelling (Fig. 1).
2 Study Area
2.1 Geographical Context
The Chachabamba archaeological site is part of the Historic Sanctuary of Machu Picchu and is located over an alluvial terrace at an altitude of 2170 m, on the left side of the Vilcanota River and the right bank of the Chachabamba basin. Three Inca roads connect the site to the most important ceremonial and administrative centres in the area, such as Wiñaywayna, Condorpata, Choqesuysuy, Chaskapata, Killapata and Machu Picchu. Chachabamba covers approximately 19,000 m2 and develops in a reduced slope area that is located along the bottom of the Urubamba river valley, an alluvial surface of Quaternary origin.
2.2 Geological and Geomorphological Setting
From the geological point of view, the site is characterized by the outcrop of granitic intrusive materials that are part of the eastern Cordillera of southern Peru. In particular, intrusive permotriassic rocks and intrusive igneous rocks, part of the so-called Machul Pictu Batolite, are essentially consisting of granites and granodioritics (Carlotto et al. 2009) which develop for a very large area of the Chachabamba site, including the slopes to the north and south.
The batholitic complex of Permian/Triassic age in the Eastern Cordillera (between the High Plateau and Subandine Zones of the Peruvian Andes consists essentially of white to grey-coloured granite which are characterized by abundance of quartz, feldspar and mica, predominantly biotite. There are also local rocks as granodiorites and serpentinites.
The archaeological site of Chacabamba is built right on alluvial deposits formed by accumulation resulting from the dismantling of the Machu Picchu batholith. In particular, it is settled in an area where the tributary Chachabamba flows and has a large recharge area inside the Urubamba. In fact, in this area essentially quaternary deposits of disassembly of the batholith emerge. The latter, in its last section starting from Chachabamba, is imposed on colluvial deposits and sometimes on granites or alluvial soils as showed in the geological map from Carlotto et al. 2011.
The colluvial deposits have several metres of thickness and are located on the slopes of the Wiñaywayna hill and form a surface mantle characterized by the mixture of rock fragments of gravel granulometry up to the size of the blocks. Alluvial deposits are recognized at the mouths of the Chachabamba and Choquesuysuy rivers, with greater development at Chachabamba and are formed by large blocks of granite in a clayey and sandy clayey matrix. These deposits are very unstable, especially in the presence of heavy rains as they immediately reach saturation status.
2.3 Archaeological Context and State-of-the Art of Investigations
Chachabamba was investigated for the first time in 1941 by Wenner Gren who unearthed terraces, baths and water channels supplying the baths (Fejos 1944). These findings suggested that Chachabamba was an important religious place for the worshipping of water and fertility goddesses. The Inca had a functional and mystic connection with nature. They were able to develop innovative agricultural techniques for plant adaptation taking into account climate and microclimate conditions, as in the case of the Moray site (Plachetka 2015), and the Mother Earth (Pachamama) and water were founding elements in the Incas cult and worldview.
After the excavations that took place in 1941, other investigations were carried out in 1996–1997 only with the purpose of the restoration of the site conducted by the Instituto Nacional de Cultura. Finally, in 2016, first scientific excavations were conducted in the framework of the ‘Programa de Investigaciones Arqueológicas e Interdisciplinarias en el Santuario Histórico de Machupicchu and Centro de Estudios Andinos de la Universidad de Varsovia en el Cusco’. These investigations provided invaluable information on the use and function of the architectural spaces. In particular, the investigated area (see Fig. 2) is the typical kancha, that develops around a central open space (with buildings facing inside) enclosed by walls to form small courtyards in the junction areas (Gavazzi 2010). In Chachabamba the kancha is structured around two squares (plaza 1 and plaza 2) flanked by buildings, known as wayranas, laid out symmetrically along north–south and east–west axes. The two squares are divided by a central building. The wayranas are built with a masonry consisting of carefully laid field granite stones with finished corners and adobe binders and characterized, in some cases (at south of the central building), by niches (Fejos 1944). They are composed of two or more rectangular rooms which served to host pilgrims. In fact, Chachabamba was also a checkpoint for people on route to other sanctuaries, among which in primis, the sanctuary of Machu Picchu.
A ‘boulder shrine’ (named huaca in quechua) borders the north side of plaza 1 and overlooks the river below. It is a partially worked granite outcrop serving as an altar, smoothed off on the top, with two seats cut into it as well as a little flight of four stairs at one side of the seats (Fejos 1944). Regarding the construction materials, the survey showed that in the vast majority of cases the walls were made in granite, extracted on site.
The architectural complex of plazas and wayranas is flanked to the east and west side by two broad terraces, which were ‘plazas hundidas’ (sunken squares) characterized by the presence of ceremonial baths on the north and south side. (See map in Fig. 3.)
Chachabamba presents all the typical architectural and functional features which could be found in the ceremonial and administrative sites of Urubamba valley, as the two plazas which recall the ‘sacred-plaza’ at Machu Picchu, the boulder shrine and the baths with ceremonial significance which are similar to those in one of the plaza of Phuyu Pata Marka (Fejos 1944).
3 Materials and Methods
The results from magnetometric and GPR investigations have been jointly analysed and interpreted in a comparative way in order to maximize the information content exploiting their complementarities in detecting features of archaeological interest.
3.1 Geophysical Methods Used for the Study Area
3.1.1 Ground Penetrating Radar (GPR)
GPR is a non-invasive methodology used in geophysics to detect and characterize electromagnetic (e.m.) impedance contrasts in a medium. It is based on the analysis of the reflections of electromagnetic waves transmitted into the ground. This method provides information on subsoil from a depth of some metres up to a few tens of metres according to the frequency of the electromagnetic waves and the electrical characteristics of the potential targets and surrounding soil (permittivity and electrical conductivity). The use of different antennas and frequencies enables the imaging of subsoil at different penetration depths and with different details. The lower the frequency the greater the penetration; for this the operating frequency is always a trade-off between resolution and penetration. Moreover, the use of different antennas and frequencies enables multiple imaging at different resolutions and penetration capabilities and for different purposes and application fields ranging from civil and geotechnical engineering to geological and sedimentological studies, environmental contamination, monitoring of monuments and artifacts and archaeological research.
The use of GPR in archaeology dated back to the 1970s, and more recently (from 1990) it has been quite systematically adopted to detect and map subsurface archaeological artifacts, features and patterning, changes in material properties and voids. Indeed, its success in the archaeological prospection field is showed by the great number of researches where the method is used in order to detect buried structures in urban and rural areas and discover archaeological features in a great number of scenarios, such as to identify ancient settlements (Trinks et al. 2014), locate unearthed burials and ceremonial offering (Leucci et al. 2016; Pipan et al. 2001), reconstruct the history of ancient buildings (Goodman and Piro 2009; Masini et al. 2017), image structures and infrastructure (Malfitana et al. 2015; Florit et al. 2018), and identify sub-water structures (Qin et al. 2018; Ludeno et al. 2018).
From the technical point of view, GPR device is made up of (1) an antenna (operating in both transmitting and receiving mode), (2) a control unit, (3) battery supply and (4) a survey cart. The antenna (operating in transmitting mode) propagates e.m. wave in the subsoil and receives (operating in receiving mode) the e.m. wave back reflected by discontinuities, i.e. depositional layers and/or buried ‘objects’ in order to define a mono-dimensional description of e.m. impedance contrasts encountered by the generated signal called A-Scan. The emitted signal is repeated, according to a defined cadence, and the GPR is moved progressively along a predetermined path on the surface, so that a two-dimensional representation is obtained (‘B-scan or Radargram’). To calculate the depth of the reflections, it is necessary to determine the propagation speed of the radar waves in the investigated levels. This is mainly related to the physical–electrical characteristics of the investigated medium (in particular, it is inversely proportional to the square root of the dielectric constant ε) and is estimated or calculated through various possibilities of signal analysis or with experimental calibration tests. The data processing enables us to convert the propagation speed of the radar waves into the subsoil depth. Finally, the digital acquisition allows us to represent the acquired data in 2D-profiles that can be filtered and enhanced to detect and locate archaeological features or changes in the matrix referable to different historical phases of the site.
For the detailed characterization of the target, it is necessary to analyse the lines of GPR data which represent a sectional (profile) view of the subsurface usually acquired at 0. 50–1.0 m of distance between each other (defined according to the size of the expected target).
The use of multiple lines of data, systematically collected over the investigated area, is generally used to obtain three-dimensional representations of the scattering phenomena occurring within the subsoil. Data can be visualized as three-dimensional blocks, or as horizontal or vertical slices. In particular, horizontal slices generally indicated as ‘depth slices’ or ‘time slices’, are maps related to specific depths. In archaeological applications, time-slicing is usually adopted because the presence of horizontal patterning is an important indicator of antrophogenic/cultural activities. Nevertheless, even if horizontal sections are the most common and easy way to visualize the pattern of targets, especially for large areas; sometimes, this visualization can compromise the reliability of the information, especially for morphologically irregular surfaces and complex structures located at different depths (Masini et al. 2017).
3.1.2 Magnetic Method
In non-destructive archaeological explorations, the magnetic survey is one of the most important and widely applied geophysical techniques (Larson et al. 2003). The aim of a magnetic survey is to investigate the subsurface on the basis of the anomalies in Earth’s magnetic field resulting from the magnetic properties of the underlying rocks or buried artifacts.
The origin of magnetic anomalies within the Earth’s magnetic field can be related either to the induced or the remanent magnetism. With the term induced magnetism, it is indicated the properties according to which an unmagnetized magnetic material within the Earth’s magnetic field becomes magnetized. The induced magnetization is directly proportional to the intensity of the ambient field and to the magnetic susceptibility of the magnetized material. The remanent magnetism, on the contrary, is the magnetism that an object has also in the absence of a magnetic field. The remanent magnetism is the results of several processes (i.e. thermal or chemical), and it may differ radically from induced field (always the same direction of the inducing field).
In archaeological explorations, both the induced and the remanent magnetization are very important (Aspinall et al. 2008). The former is useful to highlight the variations in magnetic susceptibility between topsoil, subsoil and rocks; hence induced magnetization allows the detection of the possible presence of ditches, pits and other silted-up features which were excavated in ancient times and then silted or backfilled with topsoil. The remanent magnetization, on the other hand, makes it possible to sense the presence of man-made objects which were heated (pottery kiln, hearth), of volcanic or metamorphic stones or of bricks.
The range of magnetic anomalies (related both to the induced and the remanent magnetization) generated by buried archaeological features spans from a few nT (1–20 nT) for remains up to thousands of nT for fired structures (hearths, kilns, crockery, pottery) and ferrous objects (Piro et al. 2007).
For the aforementioned reasons, and being that the magnetic method is characterized by a (1) considerable cost-effectiveness and (2) extreme speed in the use compared to other survey methods (as GPR and ERT), it has been widely applied in archaeology, from the small-scale site characterization (Aitken et al. 1958) to the surveying of large-scale areas (i.e. Keay et al. 2014).
3.2 Geophysical Data Acquisition and Processing
3.2.1 GPR Data Acquisition and Processing
GPR surveys were performed with a RIS MF Hi-Mod GPR System of IDS equipped with an array of two multi-frequency antennas using simultaneously 200 and 600 MHz antennas mounted on a survey cart equipped with an incremental encoder. The 200 MHz and 600 MHz data were acquired in continuous and reflection mode with a time window of 130 ns and 160 ns, respectively, samples per scan set at 512 with a resolution of 16 bits and a transmit rate of 100 kHz.
The data processing is based on several steps addressed to (1) improve the signal-to-noise ratio and (2) enhance the discontinuities to make the interpretation easier. The data were processed using standard two-dimensional processing techniques by means of the Reflex-W software [Sandmeier 2016].
Normalization of the amplitude (performed on the mean amplitude value of the complete profile) in order to de-clip saturated traces using a polynomial interpolation procedure;
Dewow filter to leave out potential low-frequency part of the signal;
Removal of background made (1) summing all the amplitudes of reflections (recorded at the same time along a profile) and (2) dividing by the number of traces summed. In this way, the average of all background noise is subtracted from the data;
Energy decay based on a mean amplitude decay curve determined from all existing traces;
Declipping step to reduce too high amplitudes values;
Bandpass filter to reduce the noise affecting the radargram linked to the gain function previously adopted;
Kirchhoff 2D-velocity migration with a velocity estimated quantitatively using the diffraction hyperbolas generated by potential archaeological features. The adopted velocity is equal to 0.060–0.065 m ns−1 that is coherent with a scenario characterized by a high water content.
After these steps, a 3D representation was built interpolating data of the processed 2D-lines. A linear interpolation was made analysing an area equal to two times the minimum distance between the radargrams. Then the envelope of each trace of the 3D-file was calculated and showed in horizontal depth slices where the reflections with greater amplitude are highlighted with the aim to support the identification of archaeological features.
3.2.2 Magnetic Data Acquisition and Processing
The magnetometer used for the investigations carried out at Chachabamba is the Grad601-Bartington. It is a high-resolution fluxgate gradiometer, used to measure minimal variations in the magnetic field that are caused by anomalies hidden in the soil, as archaeological features, tubes, cables or unexploded ordnance. The system includes a data logger, a battery, two Grad-01-1000-l sensors mounted on a rigid carrier bar. The instrumental sensitivity is ± 1 nT (nanoTesla). Calibration is performed on-site prior to acquisition through an automated procedure which allows to correct possible misalignment in the sensors measurements.
The magnetic gradiometry acquisition technique, compared to the single-sensor measurements, has a reduced depth resolution linked to the fact that gradient field decays with the fourth power of the distance, but it allows a simplification of the measuring and analysis of the magnetic data by making them not dependent from magnetic field temporal variations, from the regional background field and from cultural noise.
The surveyed area location (13°09′47″S 72°32′44″W) imposes some considerations on the expected results of the magnetic surveys. Since, near the equator, the Earth’s field is almost horizontal, the way in which the magnetic anomalies can be detected varies according to their orientation. In particular, for a survey performed closer to the equator, the anomalies from north–south oriented object can decrease until they become invisible (Radhakrishna Murthy 1998), and therefore, features that extend in a north–south direction may not be detected. Thus considering, the integration with GPR data will be far more important for the detection of N–S-elongated features.
The data processing was based on the filtering of the rough magnetic data to obtain the best S/N ratio using the following steps: despite made using a uniform weighted window to search for and leave out outlier valued replace by the mean;
destripe to remove the striping effect between grids caused by directional effects;
pass-band filter, to remove high- or low-frequency components;
kriging interpolator with a linear variogram to highlight the main geomagnetic linear anomalies.
4 Archaeogeophysical Investigations: Results and Discussion
The following paragraphs show and discuss some of the results obtained from the GPR and Magnetic prospection (MAG in what follows). It has to be considered that: (1) the GPR slices provide information at different depths, whereas (2) the gradiometric data only give indication of the maximum possible depth of the source of magnetic anomalies. Therefore, any magnetic anomaly may have a counterpart in all, none or in some of the GPR time slices. Figure 3 shows the investigated areas. In particular, the archaeogeophysical areas have been indicated with A, B, C, D, E, F, 1 and 3. For the sake of brevity, only the results on five sectors will be described and discussed.
4.1 Sector A: The West Plaza Hundida
The MAG survey was carried out according to pre-established grids and profile spacing of 1 m. The measures involved an area of size 24 × 10 m following an acquisition direction of N–S. In the same area, the GPR surveys were carried out according to the two directions E–W and N–S with a spacing between the profiles of 0.5 m. GPR investigations were conducted using marker placed every metre.
MAG map is shown in Fig. 3a. The range of magnetic anomalies, relatively small (10 nT), and on the absence of regular or elongated patterns, make difficult the identification of buried archaeological features ‘sic et simpliciter’. The most relevant anomaly located on the west side (a1) of the magnetic map is due to the presence of a rock formation emerging from the field and also mapped in Fig. 2. Thus, anomaly a1 was generated by a reduction in the distance between the magnetic sensors and the investigated surface; this reduction occurred during the MAG data acquisition when the magnetic sensor passed over this elevated rock formation. Other magnetic anomalies observable on the west side of the magnetic map can be ascribed to the presence of archaeological remains such as walls and the channel flanking the west side of the plaza hundida, which feed the ceremonial baths. (See Fig. 3.)
Anomalies a3 and a4 seem to converge in a common point and could be associated with the presence of drainage channel. It is worth to note the presence of linear anomalies, indicated with a2, that transversely cross the area, and other anomalies indicated with a5 in the north-east corner of the plaza hundida.
Finally, chaotic and not regular reflections are due to outcropping rocks, as confirmed by trial excavations conducted by archaeologists, co-authors of this paper (Ziolkowski 2016, 2017), referred a huanca. For the Inca builder of Chachabamba, the huanca was a sacred monolithic rock with the function as altar (for additional detail about the huanca, see Nair 2015, in particular p. 42). Similar examples of huanca have been found in other Inca ceremonial centres, among which Macchi Picchu where in recent excavations conducted in 2016 a huanca has been found in a plaza hundida (Ziolkowski 2016).
Figure 4e and f shows B-scans, acquired at the frequency of 600 and 200 MHz, respectively. They image the presence of two main stratigraphic units highlighted with red and yellow arrows at different depths that likely correspond to different archaeological phases of the site. Particularly remarkable are the strong pseudo-horizontal reflectors that cross the whole radargrams, at depths approximately ranging between 0.50 and 1.00 m (visible both at 600 both 200 MHz), and 2.0 and 3.0 m (detectable only within data at 200 MHz). It is possible that the first one (see red arrows in Fig. 4e, f) is related to the presence of anthropic soil while the second one (see yellow arrows in Fig. 4f) identifies the expected bedrock.
4.2 Sector B
MAG results, as showed in Fig. 4a, have identified the possible presence of an anomaly b1 perpendicular aligned with an existing wall located at west of the map; further some disorganized anomalies are recorded near the south edge of the area (b5) that could be associated to removed material or collapsed structures. In this zone, also a linear anomaly (b4) characterized by an orientation not compatible with that of existing structures was also recorded.
GPR results have identified the presence of some aligned reflections (b1) in continuity with the near walls that confirm the MAG results. Further a perpendicular structure, indicated with b3, is also recorded at the depths of 0.90 and 1.30 m. (See Fig. 5c, d.)
The presence of not homogeneous material or collapsed structures could justify the reflective areas, indicated with b2 and b5 in the southern part of the map. Furthermore, GPR map at 1.10 m depth puts in evidence a linearly oriented reflective area consistent with magnetic anomaly b4.
GPR B-scan show three sub-horizontal layers (see Fig. 5e, f) of archaeological interest. In particular, it is possible to note the presence of a continuous sub-horizontal reflector at a depth ranging between 0.50 and 1 m. It is due to a recent earthmoving works (red arrows) which probably cover the cause of strong reflector visible at a depth of about 1.5 m and better detectable in the b-scan acquired at the frequency of 200 MHz. (See Fig. 5f.)
Finally, at a depth greater than 3 m, another reflective layer was recorded that could correspond to the geological soil. The presence of this behaviour leads us to hypothesize the presence of some terraces enclosed by walls that are the continuation of the closer ones. This hypothesis is also confirmed by the presence of the reflective alignments b1 and b2 detectable in the depth slices of Fig. 5b–d.
4.3 Sector C
Magnetometric measurements were be made according a grid with a size of 12x14 m and longitudinal acquisitions with a distance of 1 m. GPR surveys were acquired with the same scheme adopted for the magnetometric techniques but in this case the distance between the investigated lines was halved to obtain a better resolution. The presence of furrows for soil irrigation has strongly complicated the acquisition because the investigated surface was not flat and regular and this is the reason that the surveys were carried out only in parallel to the channel direction.
MAG map (Fig. 5a) shows the absence of any relevant magnetic alignment. The most significant feature is located along the Southern and the Eastern edges of the map and can be likely related to the presence of perimeter walls (anomaly xc). Another anomaly is c1 that is located inside the area where the presence of furrows for soil irrigation has been reported. For this reason, it cannot be excluded that c1 is generated by ‘modern’ man-made artefact.
The B-scan shown in Fig. 5f, acquired at the frequency of 600 MHz, identifies the presence of three main stratigraphic units highlighted in green, black and red arrows at different depths that likely correspond to different archaeological phases of the site. The top and the bottom units (green and red arrows) are mainly horizontal. The intermediate one shows a depression in correspondence of c4. The B-scan also shows the amplitude of the reflection associated to the anomaly c4 and the more chaotic and smaller ones related to anomaly c6.
4.4 Sector E
The magnetometric investigation was carried out according to a grid of 4 × 18 m with longitudinal acquisitions towards the East and a spacing of 1 m. In addition, GPR profiles were acquired only in the longitudinal direction using the same grid of 4 × 18 m as for MAG with a spacing of the acquisitions every 0.5 m and direction towards the East.
4.5 Sector 1
In this sector magnetic survey was not performed due to the presence of rivets stuck in the ground.
4.6 Sector 3: Comparing Archaeological Finds with GPR Results
In particular, it can be seen that some stones aligned to form a perpendicular orientation could explain the anomalies s31, s32 and s33 plotted in Fig. 9a, b; at the same time, the strong reflective events indicated with s34 and s35 could be associated to the presence of linear structures (walls) of which the second one only partially excavated.
The correspondence between the structures brought to light by archaeologists and the reflection of the radar is more evident by observing the radar profiles in Fig. 9f, g where the B-scans at 600 and 200 MHz are showed. As in the case of sector 1, also here it is possible observe the presence of three subparallel horizontal reflector due to the presence of different archaeological phases (identified with red, yellow and blue lines in the radargrams), among them it is to be expected that the first two are related to archaeological features while the other one is due to the geological bedrock of the site.
The comparative analysis highlights both the potentials and limits of geophysical prospections in detecting structures and artefacts of archaeological interest, especially in less regular contexts and characterized by collapsed walls as in the case of sector 3.
5 Resume and Final Remarks
The interpretation process of geophysical surveys was possible thanks to the availability of ancillary data related to previous excavations, which facilitated the identification of some archaeological targets such as walls, canals, morphological changes of terraces, stone blocks and rocks (see in particular Sects. 4.1, 4.2). In some cases, the GPR B-scans and time slices allowed the identification of different anthropogenic layers. This confirmed the hypothesis of archaeologists, also supported by trial excavations, according to which the current archaeological area of Chachabamba has not been constructed in a single phase, but, it is the result of at least two or more building phases.
Nevertheless, the results so far obtained pose still additional questions, among which are some related to the original plan of the plaza hundidas. In particular, in the western side, the results obtained in sector E provide evidence for some reflective (radar) areas characterized by the same shapes as those of the ceremonial baths, still well preserved. (See sector E in Fig. 10 bottom). Therefore, it is reasonable to argue that, in the past, the western plaza hundida was different from how it appears today both in shape and dimensions. Moreover, observing the plan, it seems clear enough that the oblique buildings B1 and B2 (in Fig. 10 bottom) were constructed after the kancha, that is the architectural complex around plaza 1 and plaza 2. This suggests that in the original plan the western plaza hundida was symmetrically laid (as the eastern plaza hundida), with respect to the North–South axis. Subsequently, the western plaza hundida (see red rectangular block in Fig. 10 bottom) was moved and rotated to make room for buildings B1 and B2. (See Fig. 10).
Additionally, the results from the integration of MAG and GPR show anomalies referable to: (1) structures (see red circle in Fig. 10 bottom) preceding plaza 2, (2) some canals in the western plaza hundida; (3) buried walls in continuity with emerging structures (see small red rectangular box in sector B, Fig. 10 bottom).
In conclusion, the geophysical results we obtained shed new light on the Chachabamba archaeological area, where there are rich buried remains that still wait to be excavated and numerous pieces of its puzzling history to be revealed. Moreover, Chachabamba presents all the typical architectural and functional features that are also present in the ceremonial and administrative sites of Urubamba valley. Therefore, the successful results we obtained in Chachabamba encourage the same methodological approach to be adopted for other sites in the sacred valley which also includes Machu Picchu (Fig. 10).
The research was funded by the National Science Centre of Poland (Grant OPUS nr UMO-2015/19/B/HS3/03557). We acknowledge also the support and funding from Italian of CNR and Italian Ministry of Foreign Affairs.
N. M. conceived and directed archaeogeophysical investigations, wrote Sects. 1 and 5, coordinated Sects. 3 and 4, contributed to Sect. 4 for archaeogeophysical interpretation; L.C. acquired and processed GPR data and wrote Sect. 3.2.1 and contributed to Sect. 4 with particular reference to GPR results discussion; G.R. acquired and processed MAG data, wrote Sects. 3.2.2, contributed to Sect. 4 with particular reference to GPR results discussion. M. S. contributed to GPR and MAG data acquisition, wrote Sect. 2.2; D.S. contributed to Sect. 2, with particular reference to state-of-the art of investigations, and the archaeological interpretation in Sect. 4; F.A., J.B., M.Z. contributed to Sect. 2 with particular reference to state-of-the art of investigations, R.L. revised the whole paper, wrote with N.M. Sect. 1, contributed to Sects. 4 and 5.
- Aitken MJ, Webster G, Rees A (1958) Magnetic prospecting. Antiquity 32:270–271Google Scholar
- Aspinall A, Gaffney C, Schmidt A (2008) Magnetometry for archaeologists. Geophysical methods for archaeology. Altamira Press, Lanham, pp 189–201Google Scholar
- Carlotto V, Cardenas J, Fidel L (2009) La Geologia, evolucion geomorfologica y geodinamica externa de la ciudad Inca de Machupicchu, Cusco-Perù. Rev Asoc Geol Argent 65(4):725–747Google Scholar
- Carlotto V, Cardenas J, y Fidel L (2011) La Geología en la conservación de Machupicchu. Boletin N° 1 serie I Patrimonio y Geoturismo, p 305Google Scholar
- Fejos C (1944) Archeological explorations in the Cordillera Vilcabamba Southeastern Peru. Paul Fejos, New YorkGoogle Scholar
- Gavazzi A (2010) Arquitectura Andina. Formas e historia de los espacios sagrados. Apu Graph Ediciones, LimaGoogle Scholar
- Goodman D, Piro S (2009) Ground penetrating radar (GPR) surveys at Aiali (Grosseto). In: Campana S, Piro S (eds) Seeing the un-seen. Geophysics and landscape archaeology. Taylor and Francis, London, pp 297–302. ISBN 978-0-415-44721-8Google Scholar
- Gumaer DR (1985) Preliminary report of geophysical surveying at the Nanchoc Cemetery Site. MS on file, Department of Anthropology, University of Kentucky, LexingtonGoogle Scholar
- Henderson KK (2004) Ground-penetrating radar at Tiwanaku, Bolivia. Unpublished MA thesis, Department of Anthropology, University of DenverGoogle Scholar
- Larson DO, Lipo CP, Ambos EL (2003) Application of advanced geophysical methods and engineering principles in an emerging scientific archaeology. First Break 21:51–62Google Scholar
- Lasaponara R, Masini N (2008) Advances in remote sensing for archaeology and cultural heritage management. In: Proceedings of international EARSeL workshop “advances in remote sensing for archaeology and culturale heritage management”, Rome 30 September–4 October, 2008, Aracne, Roma, 2008. ISBN: 978-88-548-2030-2Google Scholar
- Malfitana D, Leucci G, Fragalà G, Masini N, Scardozzi G, Cacciaguerra G, Santagati C, Shehi E (2015) The potential of integrated GPR survey and aerial photographic analysis of historic urban areas: a case study and digital reconstruction of a Late Roman villa in Durrës (Albania). J Archaeol Sci 4:276–284. ISSN 2352-4103CrossRefGoogle Scholar
- Masini N, Rizzo E, Capozzoli L, Leucci G, Pecci A, Romano G, Sileo M, Lasaponara R (2016) Remote sensing and geophysics for the study of the human past in the Nasca drainage. In: Lasaponara R, Masini N, Orefici G (eds) The ancient Nasca world new insights from science and archaeology. Springer International Publishing, Berlin, pp 469–527. https://doi.org/10.1007/978-3-319-47052-8_20 CrossRefGoogle Scholar
- Masini N, Capozzoli L, Chen P, Chen F, Romano G, Lu P, Tang P, Sileo M, Ge Q, Lasaponara R (2017) Towards an operational use of geophysics for archaeology in Henan (China): archaeogeophysical investigations, approach and results in Kaifeng. Remote Sens 9(8):809. https://doi.org/10.3390/rs9080809 CrossRefGoogle Scholar
- Nair S (2015) At home with Sapa Inca: architecture, space and legacy at Chinchero. University of Texas Press, Austin. ISBN 978-1-4773-0250-7Google Scholar
- Oviedo M, Concha R, Astete I, Cárdenas J, Flores T, Carlotto V (2011) Geologia y geodinamica del camino sagrado en Machupicchu. Boletin N° 1 serie I Patrimonio y Geoturismo, 238–244Google Scholar
- Piro S, Sambuelli L, Godio A, Taormina R (2007) Beyond image analysis in processing archaeomagnetic geophysical data: case studies of chamber tombs with dromos. Near Surf Geophys 5(6):405–414Google Scholar
- Plachetka UC (2015) The significance of Andean terraces for indigenous knowledge on agro-ecology: the Moray puzzle. Bonn, Uwe Plachetka, p 13Google Scholar
- Prümers H (2006) Improntas de esteras en ceramica prehispanica del sitio Bella Vista (Depto. Beni, Bolivia) Actas III Jornadas Internacionales sobre Textiles Precolombinos. In: Solanilla Demestre V (ed) Departament d’Art de la Universitat Autonoma de Barcelona—Institut Catala Iberoamericana, Barcelona, pp 207–212Google Scholar
- Radhakrishna Murthy IV (1998) Gravity and magnetic interpretation in exploration, geophysics. Memoir 40 of the geological society of India (Bangalore)Google Scholar
- Sandmeier KJ (2016) ReflexW Version 8.1. Program for processing of seismic, acoustic or electromagnetic reflection, refraction and transmission data. Karlsruhe, Software Manual, p 628Google Scholar
- Ziolkowski M (2016). Informe anual 2016—Programa de Investigaciones Arqueológicas e Interdisciplinarias en el Santuario Histórico de MachupicchuGoogle Scholar
- Ziolkowski M (2017). Informe final 2014–2017—Programa de Investigaciones Arqueológicas e Interdisciplinarias en el Santuario Histórico de MachupicchuGoogle Scholar