When considering subsurface archaeological resources, it is often helpful (and sometimes necessary), to use equipment that allows the archaeologist to see below the surface without digging. When historic trolley or railroad tracks are below existing pavement, they can be accurately located and mapped using ground-penetrating radar. When hearths or the burned remains of living structures left behind by American Indians are expected at an archaeological site, magnetic gradiometry and resistivity methods allow the archaeologist to conduct wide-area subsurface investigation without destruction. When cemeteries are located within or in close proximity to project areas, all three geophysical methods are useful in determining their exact boundaries and ensuring their safety.
For these and many other purposes, the GDOT Archaeology Unit operates a GSSI SIR-3000 ground-penetrating radar, a Bartington Grad601 fluxgate gradiometer, and a Geoscan RM-15 Resistance meter. As professional archaeologists, our goal is to employ these methods whenever possible, both in our own projects and in projects being conducted by consultants working under contract with GDOT.
Method Descriptions
Ground-Penetrating Radar
Ground-penetrating radar is an active geophysical method that transmits electromagnetic energy waves into the ground and measures reflections off of interfaces between different subsurface properties. Since archaeological features typically have different physical or chemical compositions than their surrounding soil matrix, they will cause an energy wave reflection. Oftentimes this geophysical method is preferred as it collects data in three dimensions, allowing an archaeologist to measure both the vertical and horizontal extent of an archaeological feature.
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Magnetic Gradiometry
Magnetic gradiometry is a passive geophysical method that measures and maps how the earth’s magnetic field interacts with subsurface magnetic fields. Past activities related to archaeological sites, particularly burning or concentrations of top soil, leave behind magnetic traces that are identifiable using this method. Archaeological feature types that are often distinguishable in magnetic gradiometry data are ditches, hearths, storage pits, and structural foundations. One major benefit of this method is relatively fast survey speed compared with other geophysical methods.
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Electrical Resistivity
Electrical resistivity is an active geophysical method that introduces an electrical current into the ground and assesses how resistant the soils are to allowing the current to pass though. As with the other two methods, it is dependent upon soil conditions and the types of buried archaeological resources. Electrical resistance varies between types of soil and archaeological features. For example, a grave that retains moisture will be less resistive to an electrical current than a stone foundation wall.
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