• Bricks, fire hearths- Magnetic response, high electrical resistivity
• Compacted earth floors, paths, roads - Relatively high electrical resistivity
• Filled post holes - Relatively low electrical resistivity
• Buried stone walls - Possible magnetic response, high electrical resistivity
• Subsurface void - High resistivity, low density
• Metal - Magnetic response (if ferrous), low resistivity, strong dielectric contrast
• Soil layering - Variable electrical and dielectrical properties
• Graves - Good GPR reflections,sometimes electrical or magnetic
  contrasts
• Depth to bedrock - Acoustic wave velocity contrast

Geophysical techniques that are commonly applied to archaeology include:

DC Resistivity – This method is used to map electrical variations in the subsurface. Traditional archaeological application of this method has been to map lateral changes in resistance at shallow depths, but advances in computer processing capabilities have allowed mapping the variations of electrical properties as two- or three-dimensional profiles.

Electromagnetic (EM) methods – EM methods allow the rapid measurement of variations of soil electrical properties, as well as identification of metallic objects. EM techniques are commonly applied to industrial sites to locate underground tanks or pipes, but EM methods can also be very effective in mapping variations in shallow soil properties.

Ground Penetrating Radar (GPR) – GPR offers the highest resolution of any geophysical method when soil conditions are favorable. Typical applications that we have conducted with this technique include mapping of graves, middens (refuse piles), buried foundations, and industrial targets such as buried waste, tanks, pipelines and other utilities.

Magnetics – We routinely conduct total field and gradiometer surveys for mapping the distribution of buried materials. The sensitivity of this method allows for its application to mapping of subtle changes in soil conditions at archaeological sites.

Often the best results are obtained when more than one geophysical technique is employed. For example, a combination of magnetics and DC resistivity is often effective as a means to map the limits of cultural resources in advance of extensive digging. Targets associated with metal are generally best delineated with an EM method combined with magnetics. Soil layering is frequently best delineated with GPR with DC resistivity.

The decision whether to use a specific geophysical technique or combination of techniques in an archaeologic exploration depends on the cost-effectiveness as compared to other technologies. We have found that geophysics is usually cost-effective if there is reason to believe that a significant cultural resource is present. If the cultural resource has a significant physical contrast with the surrounding ground, it is usually beneficial to use geophysics as a tool for planning physical excavation.


EXAMPLE PROJECTS

Geophysical Characterization of Prehistoric Site (PDF-1749K)

D’Appolonia was retained to apply geophysical techniques to identify the optimum locations to perform archaeological digs. Three geophysical techniques were employed at the site: magnetic gradiometry, DC resistance and DC resistivity.



Geophysical Characterization of Historic Maryland Chapel (PDF-915K)

D’Appolonia was retained by the St. Mary’s City Historical Commission to perform geophysical studies of burial sites in the vicinity of a chapel constructed in 1667. The geophysical techniques employed at the site included electromagnetics, ground penetrating radar, and DC resistivity.



Geophysical Survey at Historic Cemetery (PDF-1058K)

D’Appolonia used electromagnetics, magnetic gradiometry, and ground penetrating radar to identify grave sites and building locations associated with the 1792 Legion Ville camp site.