A Reconnaissance Gpr Investigation At Chichén Itzá, Yucatan, Mexico

A Ground Penetrating Radar survey was done over selected areas of the Great Plaza and<br>the Ball Court at the Mayan archaeological site of Chichen Itz& Yucatan, Mexico. The purpose<br>of the work was to test the GPR technology for the first time at this site; to map the bedrock<br>surface beneath the extensive artificial fill which was used to elevate the Plaza; to search for<br>buried structures; and if depth penetration allowed, to search for caverns within the limestone<br>bedrock. Geologically, the site is underlain by Tertiary limestone which forms a smooth plain<br>having approximately 5 meters of local relief, broken by occasional sinkholes. Water table was<br>at about 20 - 25 meters depth. The GPR study was done partly because physical properties<br>measurements on limestone samples done 9 years ago by Dolphin and Vickers of SRI showed<br>very low attenuation of radio waves. A GSSI SIR-10 with bistatic 500 MHz and monostatic 100<br>MHz antennae was used, with most work done at the lower frequency. Penetration was not as<br>great as was anticipated because the plaza fill includes appreciable amounts of residual clay soil<br>mixed with the limestone blocks and cobbles. Hence, cavern features were not within the range<br>of the instrument. However, penetration was sufficient to map relief of the “bedrock” surface,<br>to reflection times as great as 125 nsec. A number of bedrock knolls were found below the NE<br>part of the Plaza. At shallower levels, sub-horizontal reflections are probably derived from<br>earlier plaza levels. Internal structures within the fill give an indication of the direction in which<br>the plaza filling took place. Occasional hyperbolic reflectors at and above the bedrock surface<br>hint at large discrete blocks or similar structures. These await excavation by our colleagues at<br>INAH, to whom we are also grateful for on-site support during the field work. The success of<br>this test has led to a recommendation to cover the entire site in detail with GPR.