Shallow GPR and seismic surveying in a carbonate environment: Belize, Central America

Julie A. Aitken and Robert R. Stewart


The use of non-invasive geophysical techniques to assist archaeologists in focusing excavation activities has been steadily increasing world-wide. Over the last several years, ground penetrating radar (GPR) and near surface seismic surveys have been conducted at the ancient Maya site of Maax Na in Belize, Central America. Accurate spatial and topographic measurements were also recorded using a Leica TC805L total station survey.

Velocity determination based on the hyperbolic fitting of curves to point diffractors has resulted in a wide range of velocity measurements. These differences have been attributed to climatic conditions during surveying, ranging from wet conditions in 2002 and 2004, to dry weather in 2003. Measured velocities in 2002 and 2004 ranged from 0.072 - 0.106 m/ns. Conversely, velocities ranging from 0.122 - 0.140 m/ns were acquired in 2003. The maximum depth of penetration was approximately 2-3 m.

Processing of the GPR was achieved using Reflexw software. The flow consisted of a dewow filter and gain, a smoothing operator, a bandpass filter, a diffraction stack migration and a bulk shift. Processing challenges were encountered due to calibration problems, and skipped traces.

The GPR method provides coherent and interpretable images of the subsurface of the plaza with good signal penetration. To date, the GPR lines have highlighted a number of interesting features, one of which was excavated in 2004. No buried features were unearthed, but confirmation of the noticeable structure present on the feature will be investigated based on the archaeologist's description of the excavated pit.

A brief comparison between identical lines extracted from the GPR 3-D survey and the 3C-3D micro-seismic survey serves to illustrate the potential of combining these two methods to resolve and image deeper into the subsurface. The resolution of GPR records in the near surface is superior to that of the seismic in the depth range of 0.7 -1.75 m but the depth of penetration of the seismic is greater than the GPR. Ultimately, a combination of the two would be advantageous to archaeologists in their quest to understand the past.

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