Near Surface Geophysics, 2010, 8, 433-439
doi:10.3997/1873-0604.2010030
Archaeo-geophysical methods in the Templo del Escalonado, Cahuachi, Nasca (Peru) Enzo Rizzo1*, Nicola Masini2, Rosa Lasaponara1 and Giuseppe Orefici3 CNR-IMAA, Institute of Methodologies for the Environmental Analysis, C.da S.Loja, 85050 Tito Scalo (PZ), Italy CNR-IBAM, Institute of Archaeological and Architectural Heritage, C.da S.Loja, 85050 Tito Scalo (PZ), Italy 3 Centro de Estudios Arqueológicos Precolombinos (CEAP), Nasca, Peru 1 2
Received December 2009, revision accepted July 2010 Abstract The application of geophysical prospection methods to detect earthen remains is one of the major challenging issues to be addressed in archaeological investigations. The subtle geophysical contrast between earthen buried remains and the surroundings makes the detection of archaeological features very difficult. In this paper, we address this challenge using a multi-technique approach. The integration of different geophysical methods has been used to identify archaeological remains in Cahuach, Peru, which is the largest adobe ceremonial centre in the world. The investigations herein presented are focused on two different geophysical campaigns carried out in 2008 in an area of the Templo del Escalonado, which is highly representative of the whole archaeological site. It is a desert environment where the archaeological features are covered by sand and alluvial material. The geophysical prospection, required by the archaeologists to guide excavation planning, was performed using both ground-penetrating radar (GPR) and geomagnetics with a gradiometer system. The first allowed the detection of significant anomalies, the latter confirmed the presence of these anomalies and also provided additional features not visible from GPR. Trial excavations were carried out in correspondence of some anomalies. The archaeologists unearthed a ceremonial offering in correspondence of an anomaly detected using both GPR and geomagnetic methods. Moreover, an altar and precious archaeological materials were discovered in the area characterized by a magnetic anomaly. From an archaeological perspective, these findings were very significant, because they enabled us to cast new light on the Templo del Escalonado. From a geophysical perspective, our results pointed out the high potentiality of magnetic and GPR techniques to detect, investigate and document adobe archaeological remains in a desert environmental setting. Introduction The detection of buried adobe (earthen) structures by using noninvasive techniques, such as geophysical prospection, is a complex and crucial challenge. It is complex because of the subtle physical contrast between earthen remains and the surrounding subsoil. It is crucial because of the long and widespread use of earthen materials by several civilizations mainly in arid and semi-arid lands. In the coastal regions of Peru, the adobe architecture is well preserved due to the hyper-arid climate, as in the case of the Mochica remains on the northern coast, or the Nasca buildings and monuments on the southern coast (Moseley 2001). Our efforts to address this challenge are presented in this paper and discussed for an area selected from within the ceremonial centre of Cahuachi (Orefici and Drusini 2003; Orefici 2010), *
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© 2010 European Association of Geoscientists & Engineers
which was made by the Nasca, one of the most important and evolved pre-Colombian civilizations (Silverman 1993). The archaeological structures to be unearthed are walls, platforms and terraces in adobe. The excavations carried out during the last 25 years have shown a complex stratigraphy composed of overlaid structures, refilled with earth, vegetable material and offerings. All these materials are covered by alluvial deposits. In this context, the detection of buried remains is very critical due to i) the complex archaeological stratigraphy and ii) the low geophysical contrast between adobe structures and alluvial subsoil. To cope with these issues a multi-method geophysical approach was adopted during two different field trips (April 2008 and November 2008) carried out in an area located near the Templo del Escalonado (see Figs 1 and 2). In order to detect buried structures, two different techniques were used: i) geomagnetic with a gradiometric system and ii) ground-penetrating radar (GPR). 433
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FIGURE 1 The sectors of Cahuachi (A and B) investigated by the archaeologists. Letters a1, a2, a3 and a4 indicate respectively Gran Piramide, Grande Templo, Piramide Naranjada and Templo del Escalonado.
FIGURE 2 Gran Piramide in the Cahuachi archaeological site. a) The black circle is the investigated area near the Templo del Escalonado. b) The GPR instruments used in the field trip of April 2008 and c) geomagnetic survey by G858 magnetic instruments during the field trip of November 2008.
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© 2010 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2010, 8, 433-439
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The choice to acquire magnetic and electromagnetic data was due to the widespread use of these methods in archaeology (Sambuelli et al. 1999; Chavez et al. 2001; Piro et al. 2003; Chianese et al. 2004; Rizzo et al. 2005; Bavusi et al. 2009; Hartsch et al. 2009 and reference therein). Nevertheless, their use to highlight buried adobe archaeological features is quite new and still an open issue. Cahuachi Archaeological context The investigated site was selected from within the Nasca ceremonial centre of Cahuachi, situated at an elevation of 365 m asl in the drainage basin of the Rio Grande in Southern Peru. This territory has been populated since the Formative Period (Initial Period, 1800–800 BC; Paracas culture, 800–200 BC). In the Early Intermediate Period (200 BC–600 AD) the region flourished under the Nasca culture and the ceremonial centre of Cahuachi was founded and developed. The archaeological evidence, characterized by around forty tells (earthen mounds), spreads out on a large desolate area sited on the south bank of the Nasca river. Since 1984 archaeological investigations have been in progress by an Italian-Peruvian mission directed by G. Orefici, who focused on two sectors, named A (0,16 km2) and B (0,10 km2) (Fig. 1). The results of archaeological investigations allowed the identification of five historical building phases (400–100 BC; 100 BC–100 AD; 100–350 AD.; 350–400 AD; 400–450 AD), which reflect the functional and cultural evolution of the site (Orefici 2010). At the beginning (400–100 BC), it was a sanctuary. From the 2nd century BC it became a ceremonial centre and finally (100–400 AD) a theocratic capital up to its abandonment as a consequence of an earthquake and a sequence of mudslides that submerged the monumental structures (400–450 AD). This was probably due to a mega-Niño event with disastrous proportions that struck the west coast of South America every 500 years (Orefici 2010). Each historical phase was characterized by enlargement and remodelling of pre-existent temples and platforms by filling the walls with earth, vegetable material and offerings (ceramics, textiles) according to specific rituals. The excavations have only been concentrated on sector A, considered to be the core of the entire ceremonial settlement (Orefici and Drusini 2003). So far, about half of sector A has been brought to light and most of the monuments excavated have been restored, among them the Gran Piramide (see a1, in Fig. 1), Pirámide Naranjada (see a3, in Fig. 1) and part of the Templo del Escalonado (see a4, in Fig. 1). Finally, trial excavations have been performed on the so-called Grande Templo (see a2, in Fig. 1). Although important findings have been recorded, we still know very little about the history of Cahuachi and new information can be captured from remote sensing methods. The results obtained from satellite data (Masini et al. 2009) in some areas of the ceremonial centre encouraged us to pursue our investigations by also using archaeo-geophysical techniques.
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In this paper, we focus on an area near to the Templo del Escalonado (Fig. 2a). This temple is characterized by precious incised friezes on the walls, belonging to a transitional period between the architectural phases II and III of Cahuachi. It was discovered during archaeological investigations carried out between 1984–1988 (Orefici 2010). Since 2001 the investigations were enlarged to a flat rectangular area, close to the northern side of the Templo del Escalonado. Trial excavations unearthed some walls. Unfortunately, the archaeological records available until now do not allow us to understand the spatial and functional relationships between this area and the Templo del Escalonado. This made it necessary to conduct additional investigations by using geophysical methods. Archeo-geophysical Methods: GPR and Geomagnetic prospecting Geophysical prospections were carried out in two different field trips using: i) a ground-penetrating radar (GPR) in a three dimensional survey (April 2008) and ii) a geomagnetic survey with a gradiometer configuration (November 2008). Figure 2(a) shows the location of the investigated areas known as Templo del Escalonado square. Figure 3 shows: i) the area investigated by the geomagnetic system (contoured by the black line) and ii) the GPR survey (dot line). The GPR is an electromagnetic (EM) method used for several kinds of applications and with different investigation depths. The archaeological targets are generally investigated by using medium frequency antennas. The EM-wave frequency along with the
FIGURE 3 Templo del Escalonado square. The black line was investigated by geomagnetic survey and the dot one by GPR.
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electrical characteristics of the subsoil (permittivity and electrical conductivity) determine the depth of investigation. The GPR radiates short EM pulses into the ground and detects the signals reflected from subsurface structures. The reflected signal is generated in the presence of a dielectric contrast between potential targets and surrounding soil. This is a crucial topic in the investigated area because the adobe (made from sand, clay, fibrous material and water) structures have characteristics similar to the alluvial soil. In order to investigate the Templo el Escalonado square, the GPR measurements were performed by using the Subsurface Interface Radar (SIR) 3000 manufactured by GSSI: SIR 3000. It consists of a digital control unit with a keypad, VGA video screen, connector panel and is powered by a 12-V DC battery. The system is connected by fibre-optic cables at a monostatic type antenna (400 MHz) manufactured by Geophysical Survey Systems. The survey was acquired without a ‘wheel accessory’ (see Fig. 2b). To reduce uncertainties on the antenna position, a reference metre rule was located along each profile and marked at each metre. An interval band-pass filter of 100–800 MHz was used to reduce electronic, antenna-to-ground coupling noise as well as other low- and high-frequency noise. The ReflexW software was used to process the data. The high quality of the traces only required standard analysis techniques for data processing and for reducing background noise, linked to trace editing, normalization, acquisition gain removal, zero time correction and a background removal filter. High-frequency noise was attenuated by means of a 2D average filter. An interactive velocity adoption, based on EM reflection waves, was used to estimate the average EM-wave velocity of the geological material that covers the archaeological deposits. The diffraction hyperbolas in the data provided an estimated value of about 0.15 m/ns. The GPR survey area of around 235 m2 (Fig. 3) was investigated by 26 profiles 18 m long and with a line separation distance of 0.5 m. The two-way time acquisition range was 40 ns (a)
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but after the processing the range of useful signals was restricted to 20 ns, which corresponds to around 1.5 m depth. In order to better interpret potential buried features, a 3D analysis was planned using a developed GPR time slice, which required very closely spaced profiles (Goodman et al. 1995; Conyers and Goodman 1997). Once the signal processes were applied to the radargram, a time slice analysis was defined. In this analysis, the recorded amplitudes of the reflections across the entire site are compared at different times to generate amplitude time slice maps. Figure 4(a,b) shows two GPR time slices where it is possible to detect two different kinds of potential archaeological features with circular (Fig. 4a) and linear (Fig. 4b) shapes. The time slice at around 0.15 m (Fig. 4a) shows two reflected zones, A and B1, with circular shape at high amplitude (located respectively at X = 16 m, Y = 10 m and at X = 7 m, Y = 8 m). Looking at the section across the main reflections, it is possible to observe several reflection hyperbolas due to some buried objects. In the time slice at around 0.3 m (Fig. 4b) several linear reflections are visible (see black arrows) In the same area (Fig. 5a) geomagnetic mapping has been carried out with a gradiometric configuration. The investigated area was enlarged compared to that of the GPR survey. The geomagnetic technique is considered the most suitable geophysical tool for archaeological research because of its reliability and for its aptitude to provide a fast magnetic image with high resolution data. The measurements were performed using an optical pumping magnetometer G-858 (by Geometrics) in gradiometric configuration, with two magnetic probes set in a vertical direction at a distance of around 1 m from each other. Such a configuration allowed the automatic removal of the diurnal variations of the natural magnetic field. Before defining the acquisition modalities, it was necessary to set up the orientation of the two magnetic sensors of the Cesium Magnetometer. Such an orientation depends on the survey direction and site location in the world. CSAZ software (by Geometrics) provides information about the FIGURE 4 a) A time slice (0.15 m) obtained by several 2D parallel radargrams. The black circles indicate the reflection with high amplitude associated to buried features with circular shape. b) Time slice at 0.30 m. The black arrows indicate the probable top of buried adobe clay structures. The colours are sensitive to the amplitude of the signal.
© 2010 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2010, 8, 433-439
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FIGURE 5 a) Geomagnetic map with a shaded relief image. The white circles and squares indicate previous looting pits. The black circles indicate the magnetic anomalies, two of them (indicated with A and B) visible also by GPR time slices. The dotted black line is the area investigated by GPR. The coordinates are in metres. b) Image of the CSAZ software (by Geometrics) that indicates the survey direction to have the best measured geomagnetic signal (green area).
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Earth’s magnetic field parameters including total field, inclination and declination anywhere in the world, using the IGRF (International Geo-Magnetic Reference Field). After entering latitude and longitude of the archaeological site and indicating the survey direction, the software provides the orientated caesium sensor to have the maximum signal and best performance (Fig. 5b). Therefore, the instrument was set with a tilt angle of 45° and the survey was defined along a parallel profile in N-S direction. The magnetic map was obtained by using a regular grid 20 m × 20 m and 21 parallel profiles were carried out with a line spacing of 0.5 m and a sampling rate of 10 Hz. The rough magnetic data were filtered to obtain the best signal-to-noise (S/N) ratio using a MagMap software (by Geometrics) that provides pass-band filter, spikes elimination and de-stripe. The data were further processed using a kriging interpolation and visualized as a shaded relief image (Fig. 5a). This visualization highlights the main geomagnetic anomalies. Figure 5(a) shows the final geomagnetic map, where several anomalies are visible. Black circles indicate the main magnetic anomalies and white circles and squares put in evidence pits excavated by looters. The gradiometric map shows two anomalies (A and B2), which could be correlated with ones localized on the GPR time slice (A and B1 in Fig. 4a). In particular, anomaly A corresponds to the one in the GPR time slice at 0.15 m; whereas anomaly B1 and B2 are distant from each other, around one metre. This suggests that there is a correspondence between these anomalies, therefore they could be referred to as the same archaeological features. Moreover, two other circular geomagnetic anomalies, C and D, are detected at X = 1.5 m and Y = 18.5 m and at X = 7 m and Y = 2.5 m, respectively. Finally, a large anomaly with a regular shape, indicated as E in Fig. 5(a), is between X = 4 m and 10 m and Y = 4 m and 10 m. As a whole, in the area investigated by using both of the two techniques (Figs 4a and 5a), geomagnetic and GPR (at 0.15 m
depth) detected the same anomalies (A and B1 in Fig. 4a, A and B2 in Fig. 5a). The geomagnetic map shows a further anomaly (E in Fig. 5) not detected by GPR. The enlargement of geomagnetic prospection allows us to identify another two anomalies, indicated as C and D in Fig. 5(a). Moreover, the linear reflections observed from GPR at 0.30 m depth (Fig. 4b) do not correspond with any anomaly in the geomagnetic map (Fig. 5a), due to the small magnetic contrast between soil deposit and wall remains. Archaeological findings On August 2009, a trial excavation was carried out in correspondence of anomaly A detected using both GPR and geomagnetic maps. A ceremonial offering was unearthed. It was characterized by the presence of coal and remains of a ritual fire (Fig. 6) made by fluvial stones with a lens shape. Inside, several coals are well defined and covered by leaves of Pacae. From an archaeological perspective, this ceremonial offering was very significant, because the hearth was located below a floor dated back to the end of phase IV (known as phase IVc) and, therefore, archaeologically associated to a platform built after the earthquake and the mudslides described in the section ‘Cahuachi archaeological context’. Phase IV (350–400 AD) of Cahuachi was characterized by several offerings and sacrifices, as consequence of a crisis determined by the above said devastating natural disasters that determined profound and quick changes. The discovery of this ritual fire and its archaeological implications oriented the archaeologists to conduct further analysis also in correspondence of the other anomalies indicated in Figs 4(a) and 5(a). After a month, archaeologists started excavating again in this area. They focused on the zone characterized by geomagnetic anomaly E. The excavation unearthed a ceremonial altar (Fig. 7) dated back to phase IV and composed of two large platforms symmetric with respect to a rectilinear groove. The latter was
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FIGURE 6 Detail of the trial excavation carried out in correspondence of anomaly A, detected by GPR and geomagnetic surveys (see also Figs 4–5). The excavation unearthed a deposit composed of rubble and coal, which are remains of a fire related to a ceremonial offering.
FIGURE 7 Detail of the excavation performed in correspondence of geomagnetic anomaly E.
axial to a well, characterized by a mouth composed of two eccentric circles. In the well, archaeologists found four gold bars arranged according to a square shape whose centre is characterized by the presence of a shell. Additional offers, such as necklaces and animals (bird and cuy) sacrificed to the divinities were also found. Final Remarks In this paper, we focused on some investigations performed in 2008 using an archaeo-geophysics approach to detect archaeological features in the Templo del Escalonado located in the
adobe ceremonial centre of Cahuach, Nasca, southern Peru. Geophysical investigations using both GPR and magnetic measures were carried out and analysed in order to assess their capability in detecting buried archaeological adobe structures. Such analyses provided the detection of significant anomalies and a correspondence was clearly visible between GPR and geomagnetic results. In detail, the GPR put in evidence two high amplitude zones (A and B1 in Fig. 4a) in agreement with two geomagnetic anomalies (indicated with letters A and B2 in Fig. 5a). The geomagnetic survey allowed the identification of an additional significant anomaly (D in Fig. 5a) that was not detected by using GPR. The geophysical results were the guidelines of the 2009 archaeological investigation plan near the Templo del Escalonado. In detail, trial excavations were carried out in correspondence of anomalies A and D (shown in Fig. 5a) where archaeologists unearthed a ceremonial offering and a ceremonial altar, respectively. From an archaeological perspective, these findings were very significant, because they enabled us to better understand the building phase of the investigated site; thus casting new light on the history and function of the Templo del Escalonado. From a geophysical perspective, our results pointed out the high potentiality of magnetic and GPR techniques to detect, investigate and document adobe archaeological remains in a desert environmental setting. References Bavusi M., Giocoli A., Rizzo E. and Lapenna V. 2009. Geophysical characterisation of Carlo’s V Castle (Crotone, Italy). Journal of Applied Geophysics 67, 386–401. doi:10.1016/j.jappgeo.2008.09.002 Chavez R.E., Camara M.E., Tejero A., Barba L. and Manzanilla L. 2001. Site characterization by geophysical methods in the archaeological zone of Teotihuacan, Mexico. Journal of Archaeological Science 28, 1265–1276. doi:10.1006/jasc.2000.0627 Chianese D., D’Emilio M., Di Salvia S., Lapenna V., Ragosta M. and Rizzo E. 2004. Magnetic mapping, ground penetrating radar surveys and magnetic susceptibility measurements for the study of the archaeological site of Serra di Vaglio (Southern Italy). Journal of Archaeological Science 31, 633–643. doi:10.1016/j.jas.2003.10.011 Conyers L.B. and Goodman D. 1997. Ground Penetrating Radar: An Introduction for Archaeologists. Alta Mira Press/Sage Publications.
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Goodman D., Nishimura Y. and Rogers J.D. 1995. GPR time slices in archaeological prospection. Archaeological Prospection 2, 85–89. Hartsch K., Weller A., Rosas S. and Reppchen G. 2009. The Nasca and Palpa geoglyphs: geophysical and geochemical data. Naturwissenschaften 96, 1213–1220. doi:10.1007/s00114-009-0587-9 Masini N., Lasaponara N. and Orefici G. 2009. Addressing the challenge of detecting archaeological adobe structures in Southern Peru using QuickBird imagery. Journal of Cultural Heritage 10S, e3–e9. doi:10.1016/j.culher.2009.10.005 Masini N., Nuzzo L. and Rizzo E. 2007. Investigations for the study and the restoration of the Rose Window of Troia Cathedral (Southern Italy). Near Surface Geophysics 5, 287–300. doi:10.3997/18730604.2007010 Moseley M.E. 2001. The Incas and Their Ancestors. Thames & Hudson. Orefici G. 2010. Cahuachi, el centro ceremonial en adobe más grande del mundo. In: Nasca. El Desierto de los Dioses de Cahuachi, pp. 36–59. Graph Ediciones, Lima (in Spanish). Orefici G. and Drusini A. 2003. Nasca: Hipótesis y Evidencias de su Desarrollo Cultural. Centro Italiano Studi e Ricerche Archeologiche Precolombiane, Brescia (in Spanish).
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Piro S., Goodman D. and Nishimura Y. 2003. The study and characterization of Emperor Traiano’s Villa (Altopiani di Arcinazzo-Roma) using high-resolution integrated geophysical surveys. Archaeological Prospection 10, 1–25. doi:10.1002/arp.203 Rizzo E., Chianese D. and Lapenna V. 2005. Integration of magnetometric, GPR and geoelectric measurements applied to the archaeological site of Viggiano (Southern Italy, Agri Valley-Basilicata). Near Surface Geophysics 3, 13–19. Sambuelli L., Socco L.V. and Brecciaroli L. 1999. Acquisition and processing of electric, magnetic and GPR data on a Roman site (Victimulae, Salussola, Biella). Journal of Applied Geophysics 41, 189–204. doi:10.1016/S0926-9851(98)00042-1 Sánchez Borjas A. 2010. Estaquería: Sobreviviendo a la extinción. In: Nasca. El Desierto de los Dioses de Cahuachi, pp. 60–71. Graph Ediciones, Lima (in Spanish). Sandmeier K.J. 2002. REFLEXW, Version 2.5. Windows 9x/2000/ NT-program for the processing of seismic, acoustic or electromagnetic reflection, refraction and transmission data: Instruction manual. Silverman H. 1993. Cahuachi in the Ancient Nasca World. University of Iowa Press.
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