Geophysical methods have been incorporated into the environmental approval process as a means of identifying, delineating, and evaluating archaeological sites.
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.
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.
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.
The Department first became interested in geophysical applications for archaeology in 2001, as the staff archaeologists became more aware of the potential benefits through professional articles and conference presentations. Between April 2001 and July 2002, the Department spent approximately $20,000 on contracts for ground-penetrating radar (GPR) investigations on a variety of archaeological sites including cemeteries, trolley tracks, and prehistoric village sites. Following these contracted projects, the Department realized it could have purchased its own GPR unit for the price of the combined contracts. This lead to the purchase of a GSSI SIR-3000 GPR unit with 400MHz antennae in December 2003.
Since 2003, the Department has has seen both a proven cost benefit from owning its own GPR unit and an increase in collaborative efforts with other local, state, and federal agencies to aid in archaeological site management. Based upon these benefits, the Department decided to invest in two more geophysical methods. In 2006, a Bartington Grad601 magnetic gradiometer was added to the geophysical toolkit followed by a Geoscan RM15 resistance meter in 2007.