A Special ERT-3D Array Carried Out to Investigate the Subsoil of the Pyramid El Castillo, Chichen Itza, Mexico

Abstract

It is not always possible to apply traditional 3D electrical methods to investigate the subsoil beneath ancient structures in archaeological sites. Special three dimensional electric resistivity tomography (ERT-3D) arrays have been designed to ‘illuminate’ the subsoil beneath the structure under study. The square array was designed to surround the target to acquire reliable geophysical information from the subsurface. This ERT-3D array is applied to investigate the subsoil of the pyramid of El Castillo, located in the archaeological site of Chichen Itza, in the southern lowlands of Mexico, in the Yucatan Peninsula. This building is the emblematic structure of this well-known site and elected as one of the man-made world seven wonders. Previous GPR studies provided evidences of a buried man-made tunnel excavated within the limestone rocks near this ancient structure. Now, and ERT-3D study was carried out by employing 96 flat-surface electrodes, which surrounded the edifice forming a square geometry. A total of 8,500 apparent resistivity observations were measured, with a total depth of investigation of 25 m. A low resistivity body was found beneath the pyramid, which can be associated to a sinkhole filled with sweet water.

Full text

26230. A Special ERT-3D Array Carried Out to Investigate the Subsoil of the Pyramid El Castillo,
Chichen Itza, Mexico
R.E. Chavez* (Mexico National Autonomous University), A. Tejero (Fac. Ingenieria UNAM), G.
Cifuentes (Instituto de Geofisica UNAM), D.L. Argote (INAH Mexico) & J.E. Hernandez (Instituto de
Geofisica UNAM)
Main objectives
A special ERT-3D array was designed to investigate the subsoil beneath the pyramid of El Castillo, and
to provide new evidences of the existence of a cavity beneath the pyramid.
New aspects covered
Non-invasively special ERT-3D arrays have been designed to characterize the subsoil of the main
Pyramid of El Castillo.
Summary
It is not always possible to apply traditional 3D electrical methods to investigate the subsoil beneath
ancient structures in archaeological sites. Special three dimensional electric resistivity tomography
(ERT-3D) arrays have been designed to ‘illuminate’ the subsoil beneath the structure under study. The
square array was designed to surround the target to acquire reliable geophysical information from the
subsurface. This ERT-3D array is applied to investigate the subsoil of the pyramid of El Castillo, located
in the archaeological site of Chichen Itza, in the southern lowlands of Mexico, in the Yucatan Peninsula.
This building is the emblematic structure of this well-known site and elected as one of the man-made
world seven wonders. Previous GPR studies provided evidences of a buried man-made tunnel excavated
within the limestone rocks near this ancient structure. Now, and ERT-3D study was carried out by
employing 96 flat-surface electrodes, which surrounded the edifice forming a square geometry. A total of
8,500 apparent resistivity observations were measured, with a total depth of investigation of 25 m. A low
resistivity body was found beneath the pyramid, which can be associated to a sinkhole filled with sweet
water.
Topic(s)
1. New Technologies in Applied Geophysics
2. Non-destructive Tests and Prospection for Cultural Heritage
3. Not selected
4.
Presentationtype
Prefer Oral but accept Poster

Introduction
Chichen Itza is one of the major pre-Hispanic cities, established in an area of 20 km2 in the southern lowlands of
Mexico, within the Yucatan Peninsula (Figure 1, inset). A series of sinkholes and cavities carved in the limestone
rocks at depth, provided of potable water to settlers. Some of these geological features are found within the ancient
city limits. Chichen Itza reached its main period of development during 900-1100 ad; and the last constructive
phase of the Pyramid of El Castillo occurred was between 900-1000 ad (Baudez, 2004). This temple is located
towards the northern portion of the archaeological site, and it is considered the emblematic pyramid of this
well-known site within the ancient Mayaland civilization elected as one of the seven man-made world wonders
(Figure 1, image).
Figure 1. The emblematic Pyramid of El Castillo built in the ancient city of Chichen Itza is located towards the
southeastern lowlands of Mexico, to the north of the Peninsula of Yucatan.
A pyramid named the Osario (or Great Priest Tomb) was built as a smaller version of the pyramid of El Castillo
(Fernandez de Souza, 1999). This is located towards the southwest of the main edifice El Castillo, about 500 m
apart, near the Cenote Xtoloc (to the east). It is interesting to point out that besides the architectonical similarities
between both structures, archaeologists found at the top of the Osario pyramid, a vertical shaft down into the
structure (about 9 m height), which connected to a ‘natural’ tunnel ending in a 12 m in diameter cave (sinkhole).
This feature was explored by Thompson and Thompson (1938) at the end of the XIX century, unveiling seven
tombs. For this reason, specialists have thought that similar conditions could exist under the main pyramid of El
Castillo (Schmidt, 2011).
Ground penetrating radar (GPR) studies carried out within the pyramid’s main plaza during the 90’s (Desmond et
al., 1993) provided evidence of a buried wall (originally interpreted as a tunnel). Such structure was built within
the limestone rock and hidden under infill materials that cover the platform of the main plaza. The interpreted
tunnel or wall seemed to disappear under the northern façade of the Pyramid of El Castillo (Sauck et al., 1998).
This evidence suggested the possibility that a cavity might exist under the pre-Hispanic structure.
In this investigation, a special methodology was designed to study the subsoil of the pyramid, already tested in a
small archaeological site located in Central Mexico (Argote, et al., 2013). The square array consisted of a series of
electrodes surrounding the structure. An automatic sequence of observations was programmed that allowed
acquiring apparent resistivity data from the subsoil (Tejero et al., 2015).
ERT-3D Method (The Square array)
Sometimes, spatial conditions of the surveyed area do not allow deployment of conventional three dimensional
electric resistivity tomography (ERT-3D) geometries. Such constraints are imposed by natural or anthropogenic
‘obstacles’, which are often the targets of the proposed survey. Alternative arrays have to be developed to solve
this problem. Argote et al. (2013) and Tejero et al. (2015) suggested novel geometries to characterize the subsoil
beneath archaeological structures.
In this work, the ‘square’ geometry was employed (Figure 2). The apparent resistivity data was obtained by a
combination of well known electric arrays (‘L’-array, ‘Corner’-array, Equatorial (Eq), and minimum coupling
(MC), described in detail by Argote et al. (2013) and Tejero et al. (2015). These non-conventional geometries
attempted to cover the subsurface with apparent resistivity observations underneath the ancient pyramid.
ERT-3D data was obtained utilizing a Syscal-Pro Resistivimeter (IRIS Instruments, France) with 48 channels
extended to 96. Display of apparent resistivities at depth were computed with the software Electre-Pro (Iris
Instruments, 2010), which estimates the attribution point of each apparent resistivity measured at depth. The
inversion process was carried out with the commercial software EarthImager 3D (AGI, 2010).

Figure 2. Different geometries employed to cover the subsurface of the pyramid. 96 flat electrodes were used with
a total of 8,500 apparent resistivity observations.
Traditional electrodes (copper rods) could not be employed within the site, because recent archaeological
excavations carried out near El Castillo discovered evidence of ancient Mayan floors and structures, which could
be damaged by the rods. Therefore, a geophysical study was carried out employing 96 flat-surface copper
electrodes (Athanasiou et al., 2007), which surrounded the edifice forming a square geometry. A total of 8,500
apparent resistivity observations were measured, with an estimated maximum depth of investigation of about 22 m
(Figure 2).
Geophysical Results
The inverted model displayed in Figure 3A was obtained after 7 iterations with a RMS = 11.2%. Even though this
value seemed to be high, a reasonable smooth resistivity distribution at depth was achieved. Low resistivity values
(<130 Ohm-m) showed evidence of a sweet water saturated body. Such feature can be observed beneath the
Pyramid of El Castillo and extends from near the surface (about 3 m deep) to a depth of 20 m, approximately.
Intermediate resistivity values (~300 Ohm-m) could be associated with limestone rocks, predominant in the
Yucatan peninsula. High resistivity values (>1000 Ohm-m) might correspond to materials employed in the
different constructive periods of the pyramid. It is interesting to point out that such high resistive bodies were
mainly located beneath the northern main façade of this ancient edifice.
It was possible to visualize specific regions within the resistivity spectrum. The geometry associated to the highly
saturated materials was observed much clearly, when low resistive values were only selected (Figure 3B). This
structure extended beneath the pyramid from the surface to more than 20 m depth. However, the shallow
resolution was poor towards the central portion of the ‘square’-array, due to the lack of resistivity observations,
forcing the model to come up to the surface in the inversion process. Therefore, it can be concluded that a blind
zone of 3 m deep exists from the top (Tejero et al., 2015), as estimated for the position of the shallow measured
data at depth in Figure 2.
Figure 3. Working resistivity cube obtained after the inversion process is depicted (A). Low resistivity values have
been selected (B) to better display the highly saturated body buried beneath the pyramid.
The contour image (Figure 4A) may help to better visualize the geometry of the low resistivity body. Green values

(~300 Ohm-m) might correspond to a drier feature (limestone) surrounding the low resistivity structure,
suggesting the geometry of a possible buried sinkhole (or karst), filled with water.
Finally, high resistivity values (>1000 Ohm-m) are displayed in Figure 4B. These could be associated to
consolidated materials (A), probably employed in the construction (or leveling, B, C and D) of the pyramid. The
discontinuous line shown (red arrow) defines an interesting alignment from the central portion of the array
towards its northern end. This feature was already discussed by Desmond et al. (1993) as a possible trace of a
buried tunnel or wall.
A B
Figure 4. The 3D contour image (A) depicts the limits of the highly saturated material beneath the pyramid, which
can be associated to a sinkhole (or karst) filled with water. The high resistivity model (B) displays infill materials
(A) or previous constructive periods (B, C and D). Discontinuous red arrow and square in both diagrams depict
the assumed wall or a possible tunnel mentioned by Desmond et al. (1993).
Conclusions
The ‘square’ geometry was able to characterize the subsoil beneath the Pyramid of El Castillo. Unfortunately poor
shallow resolution was obtained when using the ERT-3D array in the central portion due to the lack of data points.
However, the results provided useful information on the distribution of resistivity beneath the study area at
intermediate depths.
From the archaeological point of view, the results obtained depicted evidence of the existence of a sinkhole (karst)
beneath the pyramid, extending to a depth of more than 20 m. The low resistivity values obtained suggest that this
buried structure may be partially filled with sweet water. High resistivity values could be associated to the leveling
done during the different constructive periods. A resistive alignment was found, which coincides with the GPR
evidence described by Sauck et al. (1998).
Acknowledgments
We thank the authorities of INAH for providing the corresponding permits to work in the archaeological zone of
Chichen Itza. Arch. M.A. Santos (Director of the archaeological site) helped us in the field work logistics. We
must thank our Faculty of Engineering UNAM students for their enthusiastic help during the field work. This
investigation was financed by DGAPA-UNAM IN103614 grant.
References
Argote-Espino, D., Tejero-Andrade, A., Cifuentes-Nava, G., Iriarte, L., Farías, S., Chávez, RE. and López, F.
[2013] 3D electrical prospection in the archaeological site El Pahñu, Hidalgo State, Central Mexico. Journal of
Archaeological Science, 40, 1213-1223.
Athanasiou, E.N., Tsourlos, P.I., Vargemezis, G.N., Papazachos C.B. and Tsokas G.N. [2007] Non-destructive DC
resistivity surveying using flat-base electrodes. Near Surface Geophysics, 5, 263-272.
Baudez, C.F. [2004] Una Historia de la Religión de los Antiguos Mayas. CEMCA-NAM-CCCAC, México.
Desmond, L., Sauck, W., Callaghan, J.M., Muehlhausen, J. and Zschomler K. [1993] Report I: A geophysical
survey of Great Plaza and Great Ball Court at Chichen Itza, Yucatán México. INAH-Report.
Fernández de Souza, L. [1999] Un contexto funerario del Osario de Chichen Itza. Temas Antropológicos, 2,

264-279.
Sauck W., Desmond L.G. and Chavez R.E. [1998] Preliminary GPR results from four Maya sites, Yucatan
Mexico. In Proceedings Seventh International Conference on Ground Pernetrating Radar, GPR’98, University of
Kansas, I, ISBN: 0-9396352-16-7, 101-113.
Schmidt, P. [2011] Los oficiantes de la pirámide del Osario en Chichen Itzá. In B. Arroyo, L. Paiz, A. Linares and
A. Arroyave (eds.), XXIV Simposio de Investigaciones Arqueológicas en Guatemala 2010. Museo Nacional de
Arqueología y Etnología, Guatemala, 1163-1179.
Thompson, E. and Thompson, E. [1938] The High Priest Grave. Field Museum, Chicago, 412.
Tejero-Andrade A., Cifuentes G., Chávez R.E., López-González, A. and Delgado-Solorzano, C. [2015] “L” and
“Corner” arrays for 3D electrical resistivity tomography: An alternative for urban zones. Near Surface
Geophysics, 13, in press.