INTRODUCTION

The 1997 Yucatan Ground Penetrating Radar Project was an outgrowth of a ground penetrating radar (GPR) project carried out at Chichen Itza in 1993 by Desmond and Sauck (Sauck, Desmond, et al, 1994, and Desmond, Sauck, et al 1996). In 1993, the Great Plaza, east and north of the Castillo Pyramid, was surveyed using GPR to detect cultural and natural features in the sub-surface, to develop a topographic map of the bedrock under the fill of the Plaza, and to survey the sub-surface of the Great Ball Court, and under Sacbe 1 from the Platform of Venus to the Sacred Cenote.

Data from the 1993 project not only confirmed our long held hypothesis that GPR technology was greatly suited to the Chichen Itza environment, but added to our understanding of the extent and construction of the Great Plaza. We also recorded the type of topography underlying the Plaza, identified cultural features buried within the fill of the Plaza, and located caverns or fissures in the bedrock.

The 1997 Yucatan GPR project was a collaborative project involving archaeologists and geophysicists from National Institute of Anthropology and History of Mexico (INAH), the National Autonomous University of Mexico (UNAM), the California Academy of Sciences, Mesoamerican Archive at Princeton University, University of Zurich, Institute for Minnesota Archaeology, Mesoamerican Research Foundation, and Western Michigan University. The project sought to provide answers to Sauck and Desmond's questions about caverns and buried cultural features at Chichen Itza, Balankanche, Izamal and Dzibilchaltun, and to answer questions posed by Mexican archaeologists carrying out research at those same sites.

In 1993, at Chichen Itza Desmond and Sauck had detected a cavern-like feature east of the Castillo Pyramid that had attributes of both a cultural and natural feature. To better interpret the feature, we planned to use recently developed GPR time slice, 3-D, imaging which required an intense GPR survey of that area with very closely spaced transects. Our colleague Juerg Leckebusch at the University of Zurich was unable to participate in the 1997 fieldwork, but planned to provide time slice images developed on computers of the Swiss Federal Institute of Technology from our field data.

The well mapped Cave of Balankanche was considered ideal for development of an understanding of how caves with known dimensions are detected and displayed by the radar. Interpretation of the signature of an object or void detected by the radar requires experience, and can be particularly difficult in a new research environment. We planned to improve knowledge of radar cavern signatures by a GPR survey at Balankanche.

INAH archaeologists Dr. Peter Schmidt at Chichen Itza, Luis Millet at Izamal, and Ruben Maldonado at Dzibilchaltun had requested we survey selected areas at those sites using GPR. Dr. Schmidt asked that we survey the sub-surface in the vicinity of the High Priest's Grave to detect earlier structures, and additional caverns that might be associated with the known cave under the pyramid. Archaeologist Luis Millet has been directing the conservation of the Kinich Kak Moo Pyramid at Izamal, and requested an evaluation of the use of GPR for mapping cultural features within the interior of the pyramid. Such information, he felt, would be very helpful in the on-going conservation of the pyramid. Maestro Ruben Maldonado asked that a survey be made of the interior of Structure 44, and the sub-surface of the plaza to the north of that building. A survey of the building might give indications of the sequence of construction and locate voids or vaults. And, voids or caverns might be detected under the plaza which is within 100 meters of the Cenote Xlacah.

PROJECT REPORT

A total of 380 Mb. of Ground Penetrating Radar data was collected from approximately 10 kilometers of transects at the archaeological sites of Chichen Itza, Balankanche, Izamal and Dzibilchaltun in Yucatan, Mexico during the period of fieldwork from February 2 through 13, 1997.

The equipment used was a Geophysical Survey Systems, Inc., Subsurface Interface Radar (GSSI SIR-10A+) with 60 MHz and 300 MHz bistatic antennas (Photo 1).  Because of the expense and difficulty of transportation, the two 60 MHz antennas were fabricated in Merida by William Sauck.   (Photos 2, 3, 4, 5)

Chichen Itza

Data collected: 195 Mb. Survey transects: 5.6 Kilometers

Map of Chichen Itza showing GPR survey areas
GPR profile of anomaly east of the Castillo Pyramid

At Chichen Itza, surveys for time slice analysis, and 3-D modeling were carried out east of the Castillo Pyramid, and adjacent to the west facade of the Temple of the Warriors (areas A and B on Chichen Itza project map)  (Photo 6).   GPR Profile 1 illustrates the cultural anomaly detected by transect line 00 North which runs east from the northeast corner of the Castillo Pyramid (Photos 7, 8, 9).  Additional surveys were made in the Great Ball Court (Photo10), Great Plaza from the Temple of the Warriors to the Great Ball Court (Photos 11 and 12), vicinity of the Platform of Venus, Sacbe 1, and the vicinity of the High Priest's Grave (Photos. 13, 14, and 15), Observatory (Photo 16), Monjas and Akab Dzib (Photo 16a).

Time slice and 3-D modeling will be used to help interpret a possible cultural feature east of the Castillo Pyramid in the bedrock, as well as the possible remains of a structure buried under the Plaza fill close to the west edge of the Temple of the Warriors.

East-west transects north of the Castillo Pyramid, 250 meters in length, were made from the Temple of the Warriors to the Great Ball Court using a 60 MHz bistatic antenna and penetrated 20 meters.

The surface layer, primarily the dark reddish residual soil, has a moderate to high conductivity in the immediate vicinity of the Platform of Venus, High Priest Grave, Observatory, and Monjas building, and allowed only superficial penetration of the radar signal. The survey data collected from those locations has very limited usefulness for archaeological interpretation.

The bedrock within 10 meters of the Akab Dzib is less conductive than in the area from the High Priest's Grave to the Monjas building, and allowed some limited penetration of the radar. No caves or subsurface cultural features were noted in the survey of the west and south sides of the building.

The fill that covers the bedrock in Great Ball Court allowed for good penetration, but electrical cables limit the usefulness of radar prospection because of "noise" they generate which is displayed in the graphical representation made by the radar of the sub-surface. Electrical cables are also located in the Great Plaza to the north and west of the Castillo Pyramid which interfere with radar prospection.

Balankanche Cave

Data collected: 96 Mb. Survey transects: 1.3 Kilometers.

Map of Balankanche Cave showing GPR survey
Profile of segment of GPR transect at Balankanche Cave

The large and well mapped cave of Balankanche was considered ideal for testing the capability of GPR in an environment where the depth from the surface, and size of a cavern is known (see Balankanche project map). The access road crosses over and parallels the cavern in a number of locations, and so GPR transects were made down the center of the road. GPR Profile 2 illustrates the results of a transect in the center of the road in the vicinity of the project datum. Transects were also made around the periphery of the parking lot which is built over part of the cave. In addition to detecting known caves, the survey also detected caves or large cavities in areas where caves are not indicated on the Balankanche map. GPR data are within 10% of map measurements of relative cave size, location, and depth below surface, thus confirming the accuracy of GPR surveys of caverns in this environment. However, we must caution that we cannot as yet easily distinguish small, cm-scale, sub-horizontal openings from true cavern-scale openings.

At point "A" on GPR Profile 2, at about 1.5 to 2 meters below the surface and between the left edge of the profile and the double mark vent pipe reference, is a hump or hyperbola which is the top part of a cavern. At "B" which is a few meters closer to the vent pipe, and about 4 meters below the surface is another hyperbola indicating the top of a cavern. The strong sub-horizontal reflectors about mid-way, and 2/3 of the way down the profile, is very likely limestone bed boundaries with reflection caused by thin shale lamination or else by thin or discontinuous solution cavities which often begin along bed or horizontal formation boundaries, or along vertical fractures.

Izamal: Pyramid of Kinich Kak Moo
(Photos 17, 18, 19 and 20)

Data collected: 27 Mb. Survey transects: 1.1 Kilometers.

Plan of the Kinich Kak Moo Pyramid at Izamal showing GPR survey transects
Profile of segment of GPR transect on Kinich Kak Moo Pyramid

The streets immediately adjacent to the pyramid on west, north and east sides were surveyed using single GPR transects running the full length of each street (see sketch plan of the Pyramid of Kinich Kak Moo)(Photo 21). No cultural features were noted nor was there any evidence of pyramid construction in an area 10-15 meters away from the pyramid base.

The first terrace on the south side of the pyramid at the top of the main stairway was surveyed. Radar penetrated to the bedrock, and some representations may have been reflections from large stones at the interface of the structure and bedrock (Photo 22).

Two north-south transects were made on the west side of the pyramid's large platform (Photo 23). With the 60 MHz antenna the radar penetrated through 17 meters of pyramid fill, and 5 meters into the bedrock. A number of strong reflectors which indicate cultural features were detected. The 300 MHz antenna penetrated 12 meters into the structure (GPR Profile 3), and also showed numerous reflections indicating that the platform fill is not homogeneous. This could be the result of intermittent construction to raise the platform level. The feature at "A" might be large slabs of rock, a compacted floor or a sizeable area of fine grained soil.

Two east-west lines, 10 meters south of the small pyramid at the north end of the platform, and one east-west transect 4 meters south of the small pyramid, both made with the 60 MHz antenna, also penetrated the entire pyramid and into the bedrock (Photos 24, 25, and 26).  This structure is ideally suited for a detailed radar study using time slice and 3-D imaging techniques because the fill is apparently low in conductivity, and does not cause severe radar wave attenuation like at locations where there are thick occurrences of soil. The limestone blocks and the vegetation on the top of the platform will make systematic surveys difficult, and some clearing is recommended.

Dzibilchaltun

Data collected: 62 Mb. Survey transects: 2 Kilometers.

Map of Dzibilchaltun showing GPR survey areas
Profile of segment of GPR transect of Sacbe 1

Structure 44 (Photos 27, and 28) was surveyed with transects running the full length of the top of the building from east to west (Photos 29, and 30).  The data did indicate some penetration to variable depths along the crest of the structure, but no large voids or objects were detected. The limited penetration indicates that the fill used in the building probably has a greater soil content, or the soil is more conductive, than that used within the Kinich Kak Moo pyramid.

A grid patterned survey of the plaza, adjacent to and north of Structure 44, located a number of cavities of undetermined height within 20 meters of the structure (see Dzibilchaltun project map) (Photo 31).

A GPR survey transect down the center of Sacbe 1 between Structure 12 and Structure 36 also located cavities under the Sacbe. A cavity was detected adjacent to Structure 12 on its west side, and is illustrated by GPR Profile 4.

CONCLUSIONS

Time slice analysis of areas A and B at Chichen Itza will provide imaging of what we believe are cultural features buried under the fill of the Great Plaza. While we can be reasonably certain from our GPR profiles that the feature detected adjacent to the stairway of the Temple of the Warriors (area B) is the remains of a structure, we will need 3-D imaging (Photos 32 and 33) or time slices (Photos 34 and 35) of the deep excavation into the bedrock parallel to the east side of the Castillo Pyramid (area A) for a better understanding of that feature (GPR Profile 1).

In the area of the High Priest's Grave, and in the vicinity of the plaza west of the Observatory and north of the Monjas our GPR signal was almost completely blocked. There is a strong possibility that additional caverns are associated with the known cave under the High Priest's Grave pyramid, and a GPR survey of that area might detect them. Additional GPR testing is required to find the cause of the signal attenuation phenomenon, and to determine a method to improve GPR penetration.

The success of our GPR survey and equipment tests at Balankanche Cave would indicate that additional surveys would be useful in locating additional caverns (GPR Profile 2). Most of the area in the vicinity of the cave is not cleared of vegetation and the ground is very broken, so we would suggest that testing be carried out at discrete locations rather than with a moving antenna.

The Pyramid of Kinich Kak Moo at Izamal is ideally suited for GPR prospection, and mapping. During our survey of the pyramid we detected features within the pyramid which are illustrated by GPR Profile 3 (300 MHz antenna), and the 60 MHZ antenna penetrated the complete depth of the structure and a few meters into the bedrock. A 3-D image of the interior of the pyramid, and into the bedrock, could be developed using current GPR technology. Such a representation would be helpful in understanding the chronological sequence of the building and construction methods, and in detecting earlier phases and cultural features now buried within the pyramid.

At Dzibilchaltun possible caverns were detected in the primary survey area to the east of the Cenote Xlacah, and under Sacbe 1 a number of cavities of unknown size were detected. As mentioned previously, a cavity was located near Structure 12 (GPR Profile 4), and may continue under it. The area appears to be rich in cavern development, and additional GPR surveys covering more area would be useful in locating and mapping the extent of cavern systems. Determination of cavern depth from the surface, and their size would be helpful to archaeologists in developing a strategy for excavation.

PROJECT PERSONNEL

Photo 36.
Photo 37.

Sunset on the Temple of the Warriors at Chichen Itza.

REFERENCES

Sauck, William A, and Lawrence G. Desmond, James M. Callaghan, John Muehlhausen and Kristen Zschomler.
        1994 "A Reconnaissance GPR Investigation at Chichen Itza, Yucatan, Mexico." In, Ronald S. Bell and
                C. Melvin  Lepper, Eds., Proceedings of the Symposium on the Application of Geophysics to
                Engineering and Environmental Problems, Environmental and Engineering Geophysical Society,
                Vol. 2, pp. 869-81, Boston.

Desmond, Lawrence G., and William A. Sauck, James M. Callaghan, John Muehlhausen and Kristen Zschomler.
        1996 "A geophysical survey of the Great Plaza and Great Ball Court at Chichen Itza, Yucatan, Mexico." In,
                Proceedings of the Eight Palenque Roundtable Conference, pp. 271-280.  San Francico:
                Pre-Columbian Art Research Institute.

BIBLIOGRAPHIC REFERENCE FOR THIS PAPER

Sauck, William A, Lawrence G. Desmond and Rene E. Chavez

1998  Preliminary GPR results from four Maya sites, Yucatan, Mexico.”  In, Proceedings Seventh
    International Conference on Ground-Penetrating Radar, GPR '98, 2 volumes, University of
    Kansas, Lawrence, Kansas, May 1998, Vol. I, pp. 101-113. Lawrence: University of Kansas,
    Radar Systems and Remote Sensing Laboratory, ISBN 0-936352-16-7.

Funding and substantial in-kind support was provided from the following persons and institutions:
The Institute for Minnesota Archaeology, Minneapolis, Minnesota.
The Mesoamerican Research Foundation, Puebla, Mexico.
Western Michigan University, Kalamazoo, Michigan.
The National Institute for Anthropology and History (INAH), Mexico.
The National Autonomous University of Mexico:
Institute of Anthropological Investigations
Institute of Geophysics
Swiss Federal Institute of Technology, Zurich, Switzerland
Alexander and Suzanne Rush, San Rafael, California.
Mark Callaghan, Merida, Yucatan, Mexico.

PROJECT PERSONNEL

Lawrence G. Desmond, Ph.D. Co-Principal Investigator, Project Archaeology. Mesoamerican Archive and Research Project, Princeton University, and Department of Anthropology, California Academy of Sciences.
William A. Sauck, Ph.D. Co-Principal Investigator, Project Geophysics. Institute for Water Sciences, Western Michigan University.
Linda Manzanilla, Ph.D. Archaeological Advisor. Institute of Anthropological Investigations, National Autonomous University of Mexico.
Juerg Leckebusch, MASc. Assistant, Geophysics. University of Zurich and Swiss Federal Institute of Technology, Zurich, Switzerland.
Dr. Peter Schmidt, Co-Principal Investigator, Archaeology of Chichen Itza, INAH, Centro INAH Yucatan.
Arqlgo. Luis Millet, Co-Principal Investigator, Archaeology of Izamal. INAH, Centro INAH Yucatan.
Maestro Ruben Maldonado, Co-Principal Investigator, Archaeology of Dzibilchaltun. INAH, Centro INAH Yucatan.
Rene E. Chavez, Ph.D. Geophysics Advisor. Institute of Geophysics, National Autonomous University of Mexico.
David Maki. Archaeological Assistant. Institute for Minnesota Archaeology, Minneapolis, Minnesota.
Tim Tucker. Archaeological Assistant. Foundation for Mesoamerican Research, Puebla, Mexico.