Archaeo-Physics, LLC - geophysical survey and subsurface mapping

Case Study

Silver Bluff (38AK7)

18th Century Trading Post and Fort, South Carolina

Silver Bluff was the site of a trading post operated by George Galphin for many years prior to the American Revolution. It was occupied and fortified by the British (who renamed it Fort Dreadnought) before being destroyed by American Forces in 1781. Excavated portions of the site have revealed numerous wall trenches, pits, postholes, and other archaeological features. Geophysical survey results from unexcavated portions of the site show numerous anomalies consistent with achaeological features that have been found elsewhere on the site. Large scale patterning reveals much of the layout of the site that would be unknown without extensive excavation, which would be both costly and destructive.

Ground Penetrating Radar
Sandy soils with very high electrical resistance were ideal for propagation of the radar signal. This data plot represents the statistical variance of signal reflected from estimated depths of 30-80cm. Although tree roots created significant "clutter" in portions of the site, extensive rectilinear patterning caused by architectural features is visible over much of the survey area. Discrete "point" anomalies may have a variety of sources, but many are consistent with pit and postmold features found in previously excavated portions of the site.

Resistance
Although fine detail associated with individual features is not obviously visible in the resistance data, large scale site patterning is readily apparent. Resistance anomalies may be due to the distribution of organic components and disturbed soils as well as architectural and other features. Although many are faint, discrete resistance anomalies that correlate with anomaies in the GPR dataset are likey to be of interest.

Magnetic Field Gradient
Magnetic survey results show a distribution of high-amplitude (in the colored ranges of the scale) anomalies that is typical of historic sites. Many of these anomalies are bipolar (having both positive and negative components). This type of anomaly is generally caused by historic or modern artifacts of metal or brick. Excavators have found that a high percentage of the metal recovered from the site was associated with the 18th century occupation. Substantial amounts of brick are also present, and are thought to be associated with George Galphin's residence. Comparison of magnetic survey results with other datasets may identify metal and brick components of features. Although the survey results are dominated by this "scatter" of metal and brick, fainter rectilinear anomalies and diffuse magnetic highs may represent architectural and organically enriched features.

These surveys were performed by Archaeo-Physics as part of "New Approaches to the Use and Integration of Multi-Sensor Remote Sensing for Historic Resource Identification and Evaluation" a study sponsored by SERDP (Strategic Environmental Research and Development Program).

 

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