Application of geophysics to North American prehistoric sites

William J. Johnson and Donald W. Johnson, members of the US-based Archaeology and Geophysics Consortium, explain how North America has some catching up with Europe to do in applying geophysical methods in archaeological investigations. Archaeological geophysics, sometime called remote sensing by archaeologists, is the measurement of geophysical properties at the ground surface to create images of the subsurface that can be interpreted by a geophysicist and archaeologist working together to identify subsurface features of cultural origin. As such, geophysics is a tool to infer the presence of cultural features before excavations actually take place. Geophysical measurements also allow for obtaining a continuity of subsurface information that compliments the detailed point information from shovel tests or unit excavations. The geophysics can then provide context to the excavations made by the archaeologists. Geophysical measurements are not routinely obtained at North American prehistoric sites. This situation contrasts with European practice, where geophysical studies are a routine part of archaeological investigations. A contributing factor for this situation is the subtlety of North American targets. There is a perception by many archaeologists that because prehistoric peoples in North America did not routinely leave massive stone foundations, there is little to image with geophysics. Another factor is that professional geophysicists have not been involved with the majority of the available case histories. Even the case histories published at the North American Database of Archaeological Geophysics (NADAG) at the University of Arkansas (http:// www.cast.uark.edu/nadag/) are not of uniform quality. Of the approximately 80 results of geophysical surveys presented for prehistoric sites by NADAG, more than three quarters do not demonstrate representative results and/or appropriate procedures. Nevertheless, within this group there are also some excellent case histories. Especially worthy of note are recent studies conducted at the Hollywood Mounds site in Tunica County, Mississippi by the University of Mississippi (Johnson et al., 2000); the Mit-tutta-hang-kush Village (Fort Clark State Historic Site, 32ME2), ND presented by Kvamme (2001); and the Double Ditch Indian Village State Park (32BL8) presented by Kvamme (2002). Many of the results of the geophysical surveying appear to indicate promising results, but have not been ground-truthed, such as the work at the Greenbriar site in Arkansas (Johnson et al., 1999). The work by the University of Arkansas at the Toltec Mounds (Lockhart, 2001) shows interesting results that could also benefit from additional ground truthing.