RESISTIVITY AND CONDUCTIVITY SURVEYS AT A MIDDLE MISSISSIPPIAN ARCHAEOLOGICAL SITE, PEORIA, ILLINOIS, USA

An alluvial fan built by a small tributary to the Illinois River, just outside Peoria, Illinois is believed to be the site of a Middle Mississippian (ca. 1,000 to 700 years old) settlement (Conrad, 1994). In May of 2000, we conducted conductivity and resistivity surveys of a portion of the site. The purpose of the study was to determine if evidence of anthropological features could be recognized by their conductive and resistive properties. Using a Soiltest Model ER-2 Earth Resistivity Meter and a Geonics EM-38 conductivity meter, a rectangular section 27 meters by 55 meters, was surveyed. Resistivity measurements were made at four-meter increments along eight east-west transects using a Wenner array with a two-meter spacing. Conductivity measurements were made in both N-S and E-W directions at each resistivity measurement point. To identify anomalies both spatially and with depth, conductivity measurements were also taken in both the vertical and horizontal dipole mode. The conductivity measurements were converted into inductive resistivity, in order to compare them with the resistivity measurements. Surface and contour maps of the measurements were constructed to interpret the data. The results of the survey indicate locations that contained higher values of resistivity, by a 15-20 ohm-meters difference, which may be due to anthropological remains. Results also show similarities between the inductive resistivity computed from the vertical dipole conductivity and the direct resistivity measurements. On a return visit in the fall of 2000 to the site, which had been excavated over the past summer, the presence of abundant artifacts was confirmed in the northwest corner of the surveyed area. These results suggest that a quick and easy conductivity survey is just as effective as a labor-intensive resistivity survey with regard to locating future excavation plots at this site.