Archaeo-Physics, LLC - geophysical survey and subsurface mapping

Case study

The Mehaffey Site

Archaeo-Physics, LLC was contracted by the Cultural Resource Group of the Louis Berger Group, Inc. to conduct a geophysical investigation at 13CA33, a Late Archaic Period archaeological site located in Cass County, Iowa. The investigations were sponsored by the City of Atlantic, Iowa, and were conducted in advance of new runway construction at the Atlantic Municipal Airport.  The site is located on a cultivated high bench over the East Nishnabotna River.  Soils at the site consist of a Marshall silty clay loam.  The parent material of these soils is loess.  Previous archaeological investigations at the site had identified burned rock features approximately one meter in diameter and buried 25 to 50 cm below surface. 

The geophysical investigation consisted of a magnetic field gradient survey over 7600 square meters of the site, followed by a soil magnetic susceptibility analysis of samples collected from the vertical profiles of four excavated features and two non-cultural control profiles. Magnetic anomalies identified as possible archaeological features were tested by targeted hand excavation of 97 square meters, followed by mechanical stripping of an additional 1,650 square meters at the site.

The project results are presented using an interactive data analysis tool (see below). The results of the magnetic survey can be viewed at several different contrast levels by selecting the appropriate button from the image contrast menu. Archaeological testing results can be overlaid onto the magnetic imagery by selecting the appropriate button from the interpretations menu. The results of the soil susceptibility analysis can be viewed by selecting the button labeled location of soil susceptibility profiles and then using the mouse to select individual profiles from the resulting map.

Adjusting the contrast of the magnetic imagery allows the viewer to understand the relative signal strength of anomalies, as well as view very subtle low contrast patterning. The low contrast imagery is dominated by very intense magnetic fields associated with numerous metal pinflags inadvertently left at the site during previous archaeological investigations, while the more subtle archaeological signal response is visible in the higher contrast imagery. By viewing several different displays of these data it is possible to discern the archaeological signal response from the non-archaeological signal clutter.

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Summary and Conclusions

The geophysical investigation successfully located numerous buried archaeological features prior to the excavation of the Mehaffey site.  Locating and mapping archaeological features before heavy equipment is involved reduces the chances these features may be damaged during data recovery.  The high-resolution maps produced from the magnetic data allowed the excavation team to focus their limited hand excavation budget on excavating features rather than searching for features.

The thorough and accurate mapping of the archaeological features identified during hand excavation and mechanical stripping of the site has allowed an unusually detailed post excavation analysis.  This analysis has increased our understanding of the relationship between buried archaeological features and the magnetic signal these features create.  With caution, these finding can be extrapolated to portions of the site that were not archaeologically tested.

The soil magnetic susceptibility analysis provided estimates of the susceptibility contrast between archaeological features and the surrounding natural soils.  This contrast is responsible for inducing the magnetic fields that are detected during magnetic surveys.  Additional variables that influenced the signal strength of magnetic anomalies included: the amount of burned rock within the features; and the estimated volume of the features (see Table 1 below.)

The susceptibility analysis also provided evidence of mass transport of A-horizon soils.  Soil profiles in the vicinity of Feature 20 and Control Profile 2 (in the northern portion of the survey area) possessed enhanced susceptibility values at a greater depth below surface than the profiles in the southern portion of the site.  This accumulation of highly magnetic A horizon material in the upper soil horizons is presumably due to the movement and deposition of topsoil as colluvium.  These data have helped us better understand site formation processes and preservation at the Mehaffey site.

The primary source of uncertainty in the magnetic survey imagery was signal noise created from iron pinflags.  This signal clutter obscured some of the more subtle archaeological signal, although it was generally possible to identify the archaeological signal even when it was partially obscured.   Signal clutter from iron pinflags is nearly ubiquitous on archaeological sites that have been the subject of several archaeological studies.  The use of non-metallic pinflags during future archaeological investigations is strongly encouraged.

A more comprehensive summary of the archaeological results from the Mehaffey site investigation can be viewed by clicking here.

 

Table 1: Factors affecting observed anomaly strength

factors affecting magnetic anomaly strength

 

magnetic survey in progress at the Mehaffey Site

magnetic survey at the Mehaffey site

References

Phase III Archaeological Data Recovery at Site 13CA33: The Mehaffey Site, Cass County, Iowa (2006). The Louis Berger Group, Inc. 950 50th Street, Marion, Iowa 52302. Iowa State Historic Preservation Office Review and Compliance No. 940315183.

 

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