GTS Technologies -- Geophysical

A geomorphological investigation had discovered a set of naturally infilled sinkholes that were suspected of containing ancient Amerindian and colonial period artifacts. The archaeological team wished to excavate the sinkholes, but was concerned with the stability of them. GTS was retained to perform a ground penetrating radar (GPR) and electrical resistivity geophysical survey over the area to determine the extent of sinkhole activity, the subsurface stratigraphy of the sinkholes and whether any large voids might still exist that could be a hazard to the archaeologists working in the excavation.

GTS ran 39 parallel GPR profiles in a grid over a 75 by 110-foot area using its Mala RAMAC X3M Ground Penetrating Radar with a 100 MHz antenna. These data were analyzed as conventional two-dimensional cross-sections and were also combined to create quasi-three dimensional map volumes (see below) that could be sectioned in multiple directions to reveal the shape and structure of the sinkholes.

In addition to the GPR, three AGI SuperSting electrical resistivity cross-sectional profiles were collected across the area as a backup to the GPR data and to determine whether there were any high resistivity zones indicating open void development. The resistivity technique also "looked deeper" into the subsurface than the GPR.

Results showed a series of merged sinkholes that had subsided an average of ten to sixteen feet before stabilizing and then infilled with natural materials. For most of the area no large voids or open sinkhole throats were found. This indicated that the sinkholes had been stable for a long period and were safe to excavate to the buried ancient surface layers. The results also showed that the sinkholes were larger than had been identified by the geomorphological investigation. The combination of GPR and resistivity proved more reliable in this investigation and minimized the cost and risk of unintentional damage to the archeological value.

Because of the geophysical investigation, GTS recommended only seven confirmatory boreholes over areas of particular interest as defined by the geophysics thus reducing the project cost significantly. Small voids were encountered during the boring program at depths well below the planned excavation and were not considered dangerous to people or equipment.

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