Archaeo-Physics, LLC - geophysical survey and subsurface mapping

Case study

Kansas Monument site (14RP1)

During the summer of 2007 a geophysical investigation of the Kansas Monument site (14RP1) was conducted by Archaeo-Physics, LLC for the Kansas State Historical Society. 14RP1 is a Pawnee village associated with the Kitkehahki band of Pawnee dating to the late 18th century, with a possible second occupation during the early 19th century. The village is located on the Republican River in north central Kansas USA. Portions of the village were never impacted by agricultural plowing, making this the only remaining Kitkehahki village containing intact archaeological resources (one other Pawnee village associated with the Skiri band in Nebraska also contains intact elements). The Kansas Monument site is owned by the State of Kansas, and is home to the Pawnee Indian Village Museum.

The site has been the subject of two previous archaeological investigations, conducted in 1949 and again between 1965 and 1968. These investigations focused almost exclusively on earth lodges, providing extremely detailed data concerning their size, structure, and contents. With this in mind, our investigation was designed to investigate portions of the site that remained relatively unknown, namely the plaza areas between earth lodges and the southern fringe of the site that contained no known features and was suspected to have been impacted by agricultural plowing.

The investigation consisted of a magnetic field gradient survey over most of the preserved portions of the site, covering approximately 1.4 hectares, and a smaller high-resolution electrical resistance survey over 0.45 hectares.

The electrical resistance survey was conducted using an experimental probe array that provided unusually detailed images of buried archaeological resources. This experimental configuration is described in the electrical resistance survey section below.

At least one (and possibly several) anomalies that appear to represent previously unknown earthlodges were identified in portions of the site that had been plowed. To date only one of these has been tested by soil coring. Coring targeted the central hearth feature of this new earthlodge. Burned soil, charcoal and ash were identified in the core at a depth of 29 centimeters below surface, providing strong evidence of an intact central hearth feature. This hearth and associated earthlodge will be tested during the upcoming 2008 KATP field school.

Magnetic field gradient survey

The interactive tool below allows the viewer to examine some of the magnetic anomalies of interest at 14RP1, while the button in the lower right corner of the graphic provides an introduction to the electrical resistance survey results. Please note: Internet Explorer users will need to activate ActiveX Controls before viewing this graphic. The image represents an area measuring 150 by 110 meters.

The magnetic field gradient survey data were collected at relatively “standard” North American sample densities, with a sample collected every 12.5 centimeters along transects spaced at 0.5 meters, resulting in an overall data sample density of 16 samples per square meter.

Initial viewing of low contrast magnetic imagery from 14RP1 shows that the signal response was dominated by modern disturbance. A concrete sidewalk meandering through the museum grounds contained rebar; a wrought iron fence surrounds the property, and iron stays were used to support guy wires for an American flag planted at the site in 1906. Another form of signal clutter is more archaeologically significant. Numerous small but relatively strong dipoles probably represent European trade items such such iron knives, ax heads, hoes, and gun parts.

More subtle archaeological signal is visible as the image contrast is increased. This includes: weak positive signal created by susceptibility contrasts associated with earth lodges; a weak positive anomaly created by a palisade wall (two 1949 excavation trenches bisecting this palisade are also visible); and relatively strong magnetic signal created by fire hearths within the lodges and storage/refuse pits in plaza areas between lodges.

Electrical resistance survey

The electrical resistance investigation utilized a custom built square and crossed twin electrode configuration. Four multiplexed resistivity reading were collected per measurement station and four measurement stations were collected per square meter, resulting in an overall data sample density of 16 resistance measurements per square meter. These measurements included two 50 cm orthogonal square array readings (square alpha and beta) and two twin probe readings across both 70.7 cm diagonals of the frame. A comparison of raw data from the four electrical resistance arrays reveals variation in the signal response with changes in array geometry.

The interactive tool below allows the viewer to examine the electrical resistance data after different processing and image display methods have been applied. Significant anomalies can be highlighted, and detailed visual comparisons with the magnetic survey results can be made. The image represents an area measuring 75 by 60 meters.

Square array surveys have the advantage of providing greater sensitivity to small, low contrast targets – a great benefit in many North American archaeological applications. This advantage is offset by the limited depth of investigation inherent to square array configurations, and a directional bias in the resistance signal. To overcome the depth of penetration limitation, two 70.7 cm twin probe readings were also recorded at each measurement station. The depth of investigation of twin probe arrays is roughly proportional to the distance between the mobile probe spacing. Although the sensitivity profile is somewhat less than for the square array, especially for shallow low contrast targets, the twin probe array has the ability to detect buried archaeological features to a greater depth. Directional bias issues were addressed by collecting two orthogonal square alpha and beta readings and combining these data to create a final composite square array image.

The results of the resistance survey were fascinating, both from an archaeological perspective and to those who are interested in high-resolution electrical resistance survey design.

Archaeological Interpretations

The resistance survey mapped buried phenomena in both plaza areas investigated. Plaza Area 1 and 2 contained anomalies with size and geometry characteristics suggestive of storage/refuse pits, foot trails, and perhaps ancillary structures.

The southern fringe of the site appears to contain at least one previously unknown house feature and several additional anomalies - perhaps buried storage pits. Plow scars are plainly visible in both the resistance and magnetic survey results. It seems likely that plow damage has impacted the archaeological record, and this damage is responsible for the relatively poor definition of anomalies in the southern portion of the site.

Resistance Array Comparison

Differences between the square array and twin probe data sets were pronounced in some areas, and the two array configurations were found to complement each other very well. Not surprisingly, the best overall resistance imagery was produced by merging all four resistance data sets. The square array configuration successfully detected features not often thought of as realistic targets for resistance survey (i.e. the central fire hearths show up as resistance lows near the center of the earthlodges), while the twin probe array detected features that appear to be buried more deeply (i.e. probable storage pits) and were not clearly visible in the square array imagery. Directional sensitivity can be observed when comparing both the square alpha and beta and two twin probe data sets.

Future Research

The 2008 KATP field school will test the posited "new earthlodge" between May 31 and June 15, 2008. Be sure to check back in the fall of 2008 to learn what the KATP excavations reveal!

Additional geophysical survey at the Kansas Monument site would undoubtably lead to more exciting archaeological discoveries. Approximately 1/2 of the village is located in agricultural fields adjacent to the museum grounds. Although this half of the site is under active cultivation, the discovery of intact archaeological resources in previously plowed portions of the museum grounds has provided us with hope that buried resources may well exist. We hope to evaluate these areas via additional geophysical survey. Additionally, the very successful high-resolution electrical resistance survey (described above) covered only about 30% of the museum grounds. We are currently seeking funds to expand this coverage over 100% of the museum property.

If you are interested in providing a tax deductable contribution to further research at the Kansas Monument site, please click here for more information.

resistance survey at Pawnee Village

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