North American Database of Archaeological Geophysics
North American Database of Archaeological Geophysics
Abstract/Summary:
Project Name: Effigy Mounds, IA (1);
Reference: Bevan, B. (1999). Some Geophysical Experiments at Effigy Mounds. Geosight, Weems, Virginia. Submitted to the Midwest Archeological Center, U.S. National Park Service, Lincoln, Nebraska.
This report summarizes several geophysical surveys conducted on Mound 20 and Mound 52 (Little Bear) in Effigy Mounds National Monument, Iowa. These surveys took place before and after the National Park Service sponsored workshop: "Recent Advances in Archaeological Prospection Techniques," held at the monument from May 10-14, 1999. Geophysical techniques used in this study include ground-penetrating radar, self-potential, induced polarization, electrical resistivity pseudosections, and seismic refraction.
There were three purposes for these geophysical tests. The first purpose was that of re-examining a geophysical find from a survey done in 1981. In that year, a ground-penetrating radar survey detected a flat feature within the head of the Little Bear effigy mound. The feature is an oval about 1 by 2 m in size and 0.5 m underground. These measurements were made again in 1999 to see how time had changed them.
The second purpose of these tests was to examine the planar feature within the head of the Little Bear mound with other geophysical instruments. The feature was originally detected with a ground-penetrating radar; during this experiment, several other instruments made measurements across this feature. These measurements add further information about the physical properties of the feature and also provide a comparison of geophysical instruments in their detection of a known feature.
The third purpose of this experiment was to try some new geophysical procedures. While magnetic and resistivity mapping are known from wide experience to give valuable information at many archaeological sites, they will not necessarily be the best techniques at a specific site. There are many other geophysical instruments and procedures that should be applied to a wide variety of archaeological sites in order to understand the advantages and limitations of these lesser used instruments and procedures.
In 1999, detailed resistivity measurements found low electrical resistivity near the planar feature, but not coincident with it. There was an induced polarization anomaly at the feature, but it was too broad to have the same origin as the planar feature. Measurements of self-potential did find an anomaly that was located at the radar feature; while the anomaly was rather weak, it was repeatable.
These experiments have provided more information about the Little Bear mound, and they have also tested new instruments and procedures for geophysical surveys. It was a surprise that the planar feature within the head of the Little Bear effigy could readily be detected by a radar survey, but not by detailed resistivity measurements. This finding suggests two possible characteristics of the feature: It could be caused by a thin soil layer between two different types of soil; or it could, be the result of a single soil interface which abruptly changes from shallow sandy soil to deeper clayey soil (this transition must be gradual or thick in the nearby soil).
In addition to the geophysical surveys conducted at Little Bear Mound, seismic measurements were made on linear Mound 20, to the north. These showed that there was a thicker layer of low velocity soil within the mound, as compared to adjacent to it. An experiment was also made with seismic Rayleigh waves on this mound; this showed a contrast between the soil of the mound as compared to the nearby soil. Within the mound, the soil is rather lumpy and complex; distant from the mound, the soil strata were simpler.