Introduction

On February 10 and 11, 1997, Ground Penetrating Radar (GPR) profiles were made at Dzibilchaltun (Map 1) by pulling antennae along the top of Structure 44, and near the base of Structure 44 adjacent to its north side (Map 2). The results of this GPR survey are presented in this report.

A Geophysical Survey Systems, Inc. (GSSI) Model SIR-10 GPR system was used with paired 300 MHz antennae, and paired center frequency 60 MHz antennae built in Merida by Sauck. Additional details about the GPR system electronics and hardware are provided in an earlier report (Desmond and Sauck 1997:1). During the survey, the authors were accompanied by archaeologist Maestro Ruben Maldonado C. (Photo 1), director of research at Dzibilchaltun for the National Institute of Anthropology and History (INAH), and Dra. Linda Manzanilla N., archaeological consultant to the project, of the Instituto Investigaciones Antropologicas of the National Autonomous University of Mexico (UNAM) . A Nissan pick-up truck (Photo 2) was provided by INAH for antenna towing and logistics. We would also like to thank the many people at INAH, including archaeologist Alfredo Barrera R., director of Centro INAH Yucatan, for their important assistance and continuing support of the project.

Note, the scale at the bottom of Map2 is for measurement of radar transects made along Line 00N. Transects made along the top of Structure 44 use the same scale of measurement, but begin at the west-end of the line shown on Structure 44 designated with 0 meters at its west-end and 110 meters at the east-end.

GPR Survey of Structure 44

Structure 44 was profiled from west-to-east, and east-to-west along a narrow "lane" on the top of the building (Map 2). The full length of the accessible "lane" was 110 meters, and the starting point for the transects was the west-end.  (Photos 3 and 4)

Data from the transects made along the top of Structure 44 have been reprocessed, and printed out as two data files, one above the other (Plate 1, A and B). The radar data files from the survey of Structure 44 are noted as DZ-3RCP (Plate 1B, 300 MHZ, 160 nsec.), and DZ-4RCP (Plate 1A, 300 MHZ 160 nsec.). While slightly different equipment settings were used in each direction of survey, the 300 MHz antenna was used in both directions.

If fill material within the structure is relatively dry, the radar scan length of 160 nanoseconds will produce a penetration of the radar signal to about 8 meters. From 0-43 meters (west-to-east) the depth of penetration was 8 meters. This segment of the total survey transect showed one zone of weak signal (amplitude fade) at 3 to 6 meters along the profile. The weak signal was probably caused by somewhat more conductive shallow fill different from the rest of the fill in this area.

The first 43 meters also showed undulating reflectors, with the strongest reflections at depths of 3 to 5 meters which may be near the depth of the original land surface. These reflections may be caused by clay or soil deposited at the interface between structural rubble fill and the limestone bedrock at ground level. Without residual surface clay or soil at the base of the building (possibly removed prior to construction) the reflections noted on the profile would have appeared less prominent because there would be less radar contrast between the limestone bedrock and limestone rubble fill used in building construction.

From 43 meters to the end of the profile at 110 meters (east end), the profile is very much more subdued because the amplitude of reflection is lower (except from 57 to 62 meters). The penetration appears to be only about half way down the scan length, or 4 meters. This implies conductive fill material was deposited near the top of the structure. Below 4 meters much of the profile shows a repetitive ringing which is usually caused by the top of an even more conductive layer of residual soil at that depth. From 57 to 62 meters, the profile resembles the first 43 meters of the transect with stronger returns and deeper penetration.

Structure 44 GPR survey conclusions

Our GPR profiles indicate that there is something different in the construction method or kind of materials used in the construction (or reconstruction) of the building near the original ground level from 0 to 43 meters and also from 57 to 62 meters. The first 43 meters are quite different from the eastern part of the structure from 44 to 110 meters (except for 57 to 62 meters). While our radar profiles cannot provide detailed information on differences in construction methods and types of materials used in construction of Structure 44, our data does suggest archaeological investigation would be worthwhile to determine answers to those questions.

GPR survey adjacent to the north side of Structure 44

The radar profile on the Plaza level just to the north of Structure 44 is transect Line 00N (Map 2). Line 00N was measured with four different GPR transects. East-to-west transects were 110 meters long, and were shorter than the west-to-east transects because space was need to turn around the truck used to tow the antenna. Transects made from west-to-east were 120 meters long. All east-to-west profiles have been electronically reversed so they can be easily compared with the others, and horizontal scales which can vary when the antenna is towed have been rectified to 25 scans per meter allowing for more accurate mapping of the antenna location.

The first two transects, Plate 2A (DZ-1RBP) and Plate 2B (DZ-00NBP), were made with the low-frequency 60 MHz antennae with scan lengths of 400 (Plate 2B) and 560 (Plate 2A) nanoseconds which provides maximum depths of about 20 and 28 meters (assuming very low water content in the pore spaces of the limestone). These depths were surveyed in order to obtain background geological information. While the profiles provide data from much deeper than the depths normally of interest in archaeology, the authors believe this data is valuable for future GPR surveys, and for an understanding of the geological context of this part of the site.

Plate 2A (60 MHz, 560 nsec.) shows a strong reflection along its entire length at a depth of about 20 meters which is probably a geological boundary between two major limestone units. Strong reflections are also noted in the upper 8 meters along the entire length. The boundary indicated at 20 meters depth shows undulations of the limestone as small as 1 to 2 meters measured horizontally. Sub-horizontal internal reflections from within the limestone units indicate the bedrock lies nearly flat, and penetration was excellent reaching the bottom of the record along the entire profile

Plate 2B (60 MHz, 400 nsec.) shows a strong reflection at 20 meters, as well as more detail in the upper 10 meters. Both Plates 2A and 2B show additional intense reflections between 2 meters West and 30 meters East on the transect. Some of these reflections may be due to incipient cavern development, although solution cavities only a few centimeters thick may be the cause of these strong reflections.

Plate 3 compares a profile (Plate 3A, DZ-2RBP) made by the 60 MHz antennae (400 nanosecond scan length) with a profile (Plate 3B, DZ-00NCP) using the 300 MHz antennae (160 nanosecond scan length). The lower frequency antennae (60 MHz) is designed for deep penetration while the higher frequency antennae (300 MHz) is used for more near-surface detail.

In order to assess the quality of our profiles using the 60 MHz antennae set at 400 nsec., transects were made west-to-east, and then repeated from east-to-west along the 00N Line. Plates 3A and 2B compare the results of those transects, and confirm accurate profiles with strong reflections between 2 meters West and 30 meters East.

As expected, the high frequency antennae, Plate 3B (300 MHz, 160 nsec.), shows more detail of the uppermost 8 meters than the lower frequency antennae, Plate 2A (60 MHz, 560 nsec.). In addition to the strong reflection zone shown in Plate 2B and 3A between 2 meters West and 30 meters East, Plate 3B shows individual features (indicated by strong local reflections) at 41 meters West, 37 meters West, 20 meters West, 9 meters West, 0 East/West, 8 meters East, 15 meters East, 16 meters East (deep), and in a number of other locations to the east.

Line 00N survey conclusions

Some of the reflectors mentioned above in Plate 3B are arched which indicate the possibility of subsurface cavities at 41 meters West (1.6meters depth), 37 meters West (3.4 meters depth), 15 meters East (1.7 meters depth), and 16 meters East (5 meters depth). While subsurface cavities are indicated at these locations they may be too small to be entered or used in some way by humans because the GPR cannot measure the height of a cavern until the bottom reflector is separated from the top reflector by approximately 1 meter. The GPR reflections noted along Line 00N could be due to air or water filled openings, or thin or discontinuous lenses of clay (shale) between limestone beds.

Between 65 meters West and 20 meters West the surface layer appears different. Horizontal continuous layers exist between the surface to as much as 1.4 meters in depth, and may indicate different layers of plaza fill. The layering is generally flat-lying, but there are several segments with appreciable easterly slopes between 44 meters and 38 meters West (Plate 3b). While we cannot be certain, the original bedrock in this area might have had a depression which was filled by the Maya to provide a flat surface for construction of Structure 44.

Should more detail data about the uppermost 1.5 meters along Line 00N be needed, another survey using a higher frequency, such as 500 MHz antennae, would be recommended. Of the existing surveys, lines profiled with the 300 MHz antennae using 160 nsec. scan lengths appear to be optimum for examining the subsurface between 1 and 8 meters. The 60 MHz antennae were useful for examination of the geological structures from 2 to 30 meters in depth.

References: Current and related GPR projects in Yucatan, Mexico by Sauck and Desmond

Desmond, Lawrence G., and William A. Sauck
                1997     Yucatan Ground Penetrating Radar Project 1997: Chichen Itza. Additional GPR surveys
                             at Balankanche, Izamal, and Dzibilchaltun. Report to the Consejo de Arqueologia, National
                             Institute of Anthropology and History, Mexico.

Desmond, Lawrence G., and William A. Sauck
                1996    A geophysical survey of the Great Plaza and Great Ball Court at Chichen Itza,
                            Yucatan,  Mexico. IIn, Proceedings of the Eighth Palenque Roundtable Conference,
                            Pre-Columbian Art Research Institute, San Francisco.

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.

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.

Sauck, William A., 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.

Attachments

Plate 1 (A and B), Plate 2 (A and B), Plate 3 (A and B).  [Not available]