Bibliographic reference

Desmond, Lawrence G. and William A. Sauck
            1996    Entering the Maya Underworld: A ground Penetrating Radar Survey at Chichen Itza,
                         Yucatan, Mexico.   In, Innovation et Technologie au Service de Patrimoine de l'Humanite,
                        Actes du colloque organise par Admitech en collaboration avec  l'Unesco, Paris,
                        24 Juin 1996, pp. 23-30.
 

Abstract

A ground penetrating radar survey of the Great Plaza, including the Great Ball Court, at the World Heritage Site of Chichen Itza (Map) in Yucatan, Mexico was made in 1993. The Great Plaza, an architectural feature 250 meters by 400 meters, was built by the ancient Maya to provide a vast open, level and elevated space to enhance the religious center of the city with its monumental buildings and pyramids.

The use of ground penetrating radar, previously untried at Chichen Itza, was considered a new approach for exploration of the site with its Maya underworld of sacred passages and caverns into and through the limestone bedrock, for the measurement of the depth and extent of the plaza, and for the discovery of structures and cultural objects buried within the plaza.

The Great Plaza was surveyed systematically using a GSSI Subsurface Interface Radar System-10, and a monostatic 100 MHz antenna with transects spaced by 5 and 10 meters for a linear total of more than 6 kilometers. The 144 Megabytes of data gathered during the survey was post-processed using RADAN III^TM software.

A number of discrete blocks of stone or similar structures were located within the fill of the Plaza, a sacbe (causeway) that pre-dates the Plaza was recorded, the topography of the bedrock was mapped, and a cultural feature that may be an opening into a cavern leading to beneath the Castillo Pyramid was found to penetrate the bedrock a short distance to the east of the pyramid.
 

The World Heritage Site of Chichen Itza
 

While archaeological verification is lacking for the exact date of the rise of Chichen Itza, the largest Maya city in Yucatan, most Mayanists would place its beginnings as early as AD 200. Continuous occupation until its fall around AD 1200 resulted in the construction of a vast urban complex approximately four kilometers square with hundreds of stone faced civic and religious buildings, and plazas connected by roads and causeways. Virtually all the buildings at the site incorporate bas-reliefs of historic persons or religious motifs and sculptural elements while a few buildings have polychrome murals of yet to be fully understood historical events.

The city has a number of precincts, but we have focused the work of this project in the center of the city, an area of very complex and large architecture. Archaeologists who worked at Chichen Itza during the first half of this century proposed that this part of the city was built by a conquering people called Toltecs who came from the highland Mexican city of Tula, and took control from the Maya. Recent work by archaeologists and other researchers proposes that a cosmopolitan and innovative Maya remained in control of Chichen Itza, and developed their own variation in architectural style.

An important feature of this central precinct of Chichen Itza is the Great Plaza. It measures approximately 250 meters north to south and 400 meters east to west, and incorporates a variety of buildings for both religious and secular purposes. Dominating the city and the plaza area is the Castillo Pyramid (the Maya called it the Pyramid of Kukulkan to honor one of their most important gods), to the east of the pyramid is the Temple of the Warriors, to the west is the Great Ball Court (the largest in Mesoamerica), to the north are a number of ritual platforms and the Sacred Cenote of Sacrifice. Clearly, this is the religious center of the city.

Until the 1970s archaeological work at Chichen Itza focused primarily on architectural restoration. Sylvanus Morley directed the Carnegie Institution of Washington's Chichen Itza project in the 1920s and 1930s and carried out extensive restoration. From 1940 through the 1960s restoration was on a less massive scale, and buildings were chosen by Mexico's National Institute of Anthropology and History (INAH) as single projects for conservation and restoration.

During the past 20 years a number of researchers working at Chichen Itza have focused on scientific questions about the civilization of the ancient Maya. In the late 1980s and 1990s important contributions have been made by a number of researchers, and recently Dr. Peter Schmidt, director of the Chichen Itza Project for INAH, along with archaeologists Agustin Peña and Rafael Cobos, have carried out extensive archeological projects at the site. During the past few years, Chichen Itza's hieroglyphic texts and iconography have become the focus of epigraphers and art historians, and historians of religions have explored the religious dimensions of Chichen Itza and the Chichen Itza-Tula problem.

In the 1970s, the technique of using GPR for archaeological exploration was relatively new, but the work at Giza by Lambert Dolphin and Roger Vickers, geophysicists with the Radio Physics Laboratory at SRI International, in detecting features and chambers using GPR, seemed like an excellent model for what might be carried out at Chichen Itza (Dolphin et al 1977). In 1977, Desmond contacted Dolphin and Vickers to discuss a strategy for GPR exploration at Chichen Itza.

Americanists who worked in Mesoamerica had early-on discovered that caves were an integral part of ancient Mesoamerican religious practices, and were often associated with religious architecture. The most well-known example of the link between caves and architecture is the Pyramid of the Sun at the archaeological site of Teotihuacan near Mexico City. The Pyramid of the Sun is built directly over a cave.

The religious belief of the Maya in the realm of Xibalba, the underworld, points to utilization of caves. The Castillo Pyramid, with its nine terraces ascending to a temple on the top, reflects their belief in the levels of heaven which are cosmologically linked to the levels of Xibalba. Caverns allow an actual descent of the body and mind from an upper world into the underworld.

In a passage from the Popul Vuh, the creation myth of the Maya, the sinister Lords of Xibalba, who reside in the underworld, are attracted to the sound of a ballgame being played in the world above. The Maya ballgame which was played in the Great Ball Court, the largest in Mesoamerica, may have also caused resonations of religious belief in the caverns of the underworld under Chichen Itza's Great Plaza.

It seems essential for the Maya at Chichen Itza to access the inverse of the visible cosmology, and descend into the hidden realm of the Lords of Xibalba.

Caves used for ritual and burial are known at Chichen Itza and in the immediate vicinity. In the 1950s, a camouflaged ritual chamber was discovered in the often visited tourist cave of Balankanche, about a kilometer from central Chichen Itza (Andrews 1970). Under the High Priest's Grave (a small replica of the Castillo Pyramid), about 200 meters from the pyramid, a Maya priest was discovered to have been buried in a cave (Thompson 1938). A complicated and narrow cave with an opening in the side wall of the Xtoloc Cenote, also about 200 meters from the pyramid and ball court, has recently been explored (Veni 1992).

In 1985, limestone bedrock samples from Chichen Itza were analyzed by Dolphin and found to be "quite low in losses even when wet..." He continued, "We are greatly encouraged to see that ground penetrating radar appears so ideally suited for probing into structures and into the ground at Chichen Itza" (Dolphin 1986).

In 1991, Dolphin's analysis was sent for review to geophysicist William Sauck. Sauck had considerable experience applying geophysical remote sensing techniques in a tropical archaeological environment, and concurred with Dolphin that GPR might have an excellent chance of success at Chichen Itza. Discussions with Sauck began in 1991 for sub-surface exploration at Chichen Itza, and a GPR project proposal was made to INAH for fieldwork with approval granted for a project to be carried out in February 1993.
 

Techniques and Methods
 

The geological environment at Chichen Itza is Tertiary limestone with a thickness of 200-400 meters. Breaks, or sinkholes in the limestone bedrock called cenotes dot the great plain of Yucatan, and some, such as the Sacred Cenote at Chichen Itza, were places of religious worship. But, most cenotes provided a supply of water in the otherwise barren land, and at Chichen Itza the water table is about 20 meters below the surface.

In general, the limestone surface of Yucatan is not flat, but is broken to such an extent that the Maya built the Great Plaza at Chichen Itza to improve the urban setting.

Our plan was to study the performance of GPR at Chichen Itza by carrying out an actual project which would measure the topography of the bedrock under the Great Plaza to learn the plaza's depth, extent, and construction, and to probe deep enough into the bedrock to locate caverns.

The radar equipment selected to carry out these tasks was a Subsurface Interface Radar System-10 made by Geophysical Survey Systems, Incorporated. This included a central processing unit and software, a color monitor and two antennas. The unit was mounted in the back of a pickup truck and run on a twelve volt battery. An antenna was connected to the central processing unit with a fifteen meter cable, and pulled behind the moving truck by archaeologists along previously laid out transect lines.

The project datum was set at the base of the northeast corner of the Castillo Pyramid, and a transect grid was laid out with ten and five meter intervals using fiberglass measuring tapes (Figure 1). Ten meter transects were used for general surveying while five meter intervals were used in areas of special interest. Geographical control of the radar data was maintained by marking each ten meters of a transect with a small flag. As a marker flag was passed, the archaeologist electronically marked the data by closing a switch which allowed the location of the radar data to be later interpreted from an electronic imprint on the top edge of the data display.

The length and range of scan was set after some field tests to 200 nanoseconds and the 100 MHz antenna used. Parameters for recording were: 16 bit precision, 16 or 24 scans per second with 512 samples per scan. Horizontal smoothing from a three scan moving average was the only data filtering used in the field.

While the radar equipment was being driven over the plaza, Sauck monitored the real time results on the video display in the back of the truck, and noted any areas of interest. At the end of a transect, data was often reviewed to determine if an intermediate transect of five meters might provide additional data. The importance of the cultural anomaly detected to the east of the Castillo Pyramid became immediately apparent, and caused additional transect lines to be set at five meter intervals for a more thorough survey.

Post-field processing of the survey data was done by downloading the field tapes to a personal computer running RADAN III^TM software. A number of procedures were carried out to enhance the data and improve interpretation including removal of stationary antenna data from the beginning and end of each transect, the horizontal scale of the records was made constant, the amplitude of some records was increased, horizontal bar artifacts were removed using a high pass filter, a vertical low pass filter was used to enhance the bedrock reflector, high frequency residuals were smoothed out using a three point low-pass filter, and descriptive information was edited into the file header.
 

Results and Conclusions
 

Some plaza reflections were chaotic due to the heterogeneous nature of the fill which is made up of both small and large pieces of limestone, gravel, and soil with air spaces between the limestone blocks. But, at many locations the planar character of the different plaza levels did line up in the record to indicate previously laid down floors. We know from excavations in the 1930s and 1980s that the plaza was built over a long period of time with a number of completed floors layered one above the other (Morris et al 1931).

A contour map was developed by posting depths or radar times to the bedrock reflector (Figure 2). A verified depth calibration is not yet available, but a velocity function of about 23 nanoseconds to the meter is suggested by the measurement of two hyperbolic reflections at the base of fill. The presumed bedrock surface is very likely the reflective soil layer, described by Morris, et al. (1931), resting on the bedrock. We think that the interface between the rubble fill of the plaza and massive limestone would not provide a good enough contrast to be noticed in the radar echo, so a soil layer would act as a reflector.

A buried sacbe (causeway) was detected beginning near the southeast corner of the Platform of Venus and trending across the plaza to the southwest, and then passing to the west of the Castillo Pyramid. This sacbe would predate the construction of the plaza.

The north/south survey lines in the Great Ball Court showed a shallow depth of plaza, less than a meter to bedrock, near center court with depth increasing to the north to a maximum of 3.5 meters, then shallow again at about 40 meters north of mid-court. Also, noted about 10 meters from the west wall of the north end zone was a near surface indication of active cavity formation at great depth. It took the form of a depression at the surface with a loss of amplitude of the bedrock reflection with steeply dipping, centrally convergent reflections below this zone similar to GPR patterns indicative of vertical piping in limestone noted by Benson and Yuhr (1992).

An anomalous cultural feature (Figure 2)cut into the bedrock about 20 meters east of the Castillo Pyramid was noted from downward and inwardly curving patterns in the radar profile (actually diffractions)--typical of an excavated trench or widened natural fissure which may be connected to a deeper cavern system (Figure 3). It parallels the pyramid and is about 10 meters east-to-west and 30 meters north-to-south. Removal of the soil from the bedrock, and excavation into the bedrock by the Maya may be the cause of the disruption of the reflected radar pulse. Such an excavation by the Maya would have been carried out prior to building of the plaza unless it was dug through three meters of fill to reach the bedrock. But, four east/west survey lines across the feature show a change from a double pulse reflector to at least a triple pulse and thicker reflectors on the east side of the feature. This layering effect has caused us to hypothesize that the soil excavated from the feature was piled on the east side.

The next radar survey should employ a bistatic antenna to avoid loss of pulse at shallow levels, and a low-frequency antenna with better depth range should be used for exploration of cavern formation in the area of the Castillo Pyramid and Great Ball Court.

A complete survey of the Great Plaza, including the Great Ball Court, should be carried out to provide a three dimensional CAD model of the plaza which would include a contour map of the bedrock, and the location of cultural anomalies within the fill. A test excavation should be made to calibrate radar depths, and to examine the buried cultural anomaly east of the Castillo Pyramid.

And finally, once the surveys are complete and the data processed, a symposium on geophysical methods should be held at Chichen Itza to present the results of the survey to researchers, and to develop a strategy for excavation and future exploration at the site.
 

Acknowledgements
 

This project is the culmination of ten years of effort by a great number of people. The authors remember with great respect the late Don Feliciano Salazar L., one of the great men of Chichen Itza, and thank the following persons: Alfredo Barrera Rubio, David Bieling, James M. Callaghan, Lambert Dolphin, Fernando Barbachano, Jr., Arthur Dunkleman, Jose de Lima, Ernie Marc, John Muehlhausen, Roger Vickers, Thomas Yetter, Kristen Zschomler; and the following institutions: California Academy of Sciences, Institute for Minnesota Archaeology, Institute for Water Sciences of Western Michigan University, Mesoamerican Archive and Research Project at Princeton University, Mexicana Airlines, and the National Institute of Anthropology and History of Mexico.
 

References Cited
 

Andrews, E. Wyllys IV
            1970    Balankanche, Throne of the Tiger Priest. Publication 32.  New Orleans: Middle American
                        Research Institute.

Benson, Richard C. and Lynn Yuhr
            1992    A summary of methods for locating and mapping fractures and cavities with emphasis on
                        geophysical methods.  In, Proceedings of the symposium on the application of geophysics
                        to engineering and environmental problems, pp. 471-485.  Golden: Society of Engineering
                        and Mineral Exploration.

Dolphin, Lambert, Ali Helmi Moussa, Gamal Mokhtar, et al.
            1977    Applications of modern sensing techniques to Egyptology.  Menlo Park: SRI International.

Dolphin, Lambert
            1986    Report to L. Desmond on the electrical properties of limestone bedrock samples from
                        Chichen Itza, Yucatan, Mexico.  Desmond research files.

Morris, Earl H., Jean Charlot, and Ann Axtell Morris
            1931    The Temple of the Warriors. Publication 406.  Washington: Carnegie Institution.

Thompson, Edward H.
            1938    The high priest's grave, Chichen Itza, Yucatan, Mexico.  Field Museum of Natural History
                        Anthropological Series Vol. 27, No. 1.

Veni, George
            1992    Personal communication concerning exploration of Xtoloc cave and preliminary map.
                         Desmond research files.