Geophysical surveys used to save time, money on archaeological site selections  

Mar. 30, 2007

By Dr. Michael Hargrave
U.S. Army Engineer Research
and Development Center -
Construction Engineering Research Laboratory

Geophysical surveys provide useful images of subsurface archaeological features such as houses, pits and graves, without excavation. Not all sites are good candidates for this type of survey and some techniques work well, while others may not, given local conditions.

To help installations avoid spending time and money on geophysical surveys that are not likely to succeed, the U.S. Army Corps of Engineers has published guidance for selecting sites and survey methods. Public Works Technical Bulletin (PWTB) 200-4-42, "Selecting Archaeological Sites for Geophysical Survey," is available at http://www.wbdg.org/ccb/browse_cat.php?o=31&c=215.

The National Historic Preservation Act of 1966 requires federal agencies to take into account the effect of proposed activities on any district, site, building, structure or object that is included in or eligible for inclusion in the National Register of Historic Places (NRHP). 

Compliance typically requires the agency to identify historic properties within an area that may be impacted by an activity and to evaluate those properties' eligibility for nomination to the NRHP.  In the case of archaeological sites, this evaluation often includes excavations designed to define a site's boundaries and to assess its integrity and historical and cultural significance relative to one or more historic contexts.

Evaluations of a site's NRHP eligibility based on hand excavation are highly invasive, expensive and - because only a tiny portion of each site is excavated - potentially unreliable.  In the eastern United States, for example, many prehistoric sites that have been plowed have no intact cultural stratum, but the preserved lower portions of pit features may contain scientifically important deposits.  A site assessment program based on a grid of shovel tests and a small number of hand-excavated test units can easily fail to discover any of the pits.  In many cases, such a failure could lead to an incorrect recommendation that the site is not eligible for the NRHP.

Geophysical techniques can be used to search for subsurface features across a large portion of a site.  Excavation units can then be targeted directly on possible features, thereby improving the likelihood of detecting intact, culturally and historically significant archaeological deposits.  This targeted excavation can reduce the volume of excavation required to evaluate a site's NRHP status and may reduce costs associated with fieldwork, analysis and curation.

The PWTB provides guidance for choosing good sites and appropriate equipment to conduct geophysical surveys. It is intended for cultural resources managers, land managers and archaeologists with limited experience using geophysical techniques.