North American Database of Archaeological Geophysics
North American Database of Archaeological Geophysics
Abstract/Summary:
Project Name: Fort de Chartres, IL;
Reference: Bevan, B.W. (1983). Electromagnetics for Mapping Buried Earth Features. Journal of Field Archaeology 10(1):47-54. Reproduced from Journal of Field Archaeology with the permission of the Trustees of Boston University. All rights reserved.
Electromagnetic surveys are fast method of locating and mapping some large earth features such as leveled mounds and refilled ditches. They can be a substitute for resistivity surveys and are particularly suitable where the surface soil is dry, hard, or rocky, or where the vegetation is moderately dense. As in all geophysical techniques, it is necessary that the features to be located are sufficiently different from the surrounding terrain. Results from three sites will illustrate the capabilities of electromagnetic surveys.
Fort de Chartres is an 18th century French fort built next to the Mississippi River in llliniois. An excavation trench along a curtain wall of this fort revealed the location of a fortification ditch; this fortification had been long ago filled in with rock rubble and earth. An electromagnetic survey was carried out adjacent to the excavation. A band of high conductivity probably indicates the extension of the fortification ditch parallel to the curtain wall of the fort and about 7 m from it. Electrical tests indicate that this band is caused by high conductivity material in the soil directly below the bottom of the refilled ditch, possibly resulting from chemicals that have seeped through the bottom of it. The refill material within the ditch is actually low in conductivity, as is to he expected, since much of this material is probably stone. The soil around the ditch has a high content of clay.