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Geophysical Research at Strawtown
Although we have used geophysical
methods in several research projects, the IPFW Archaeological Survey's
geophysical reseach program is focused in Strawtown, Indiana. Overall, the sites at Strawtown
are excellent subjects for geophysical research, experimentation, and education. More information
about ongoing research at Strawtown can be found here.
The Strawtown Koteewi Park contains at
least 134 archaeological sites spanning nearly the entire range of human
occupation of central Indiana, from Native American camp sites dating
around 8,000 BC to an early 1800s homestead site. Archaeological excavations
conducted by the IPFW-AS at the park have been focused on reconstructing
the daily lives of the Native Americans who inhabited the area from about
AD 1100 to 1450. These people were Indiana’s first maize farmers, who planted
their cornfields in the rich alluvial soils along the White River and hunted deer and small game in the nearby forests.
The Strawtown Enclosure (12-H-883)
The Strawtown Enclosure is a Native
American earthen enclosure nearly 100m in diameter. Excavations at
the enclosure have revealed an intensively occupied village site with multiple
occupations. Geophysical investigations at
the Strawtown enclosure began during the REU in 2005. REU students collected
resistivity and ground penetrating radar data from two different areas
of the site to try to gather information about community plan and to attempt
to understand how these methods could be used to generate data about various
kinds of archaeological features.
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Excavations within the
Strawtown Enclosure in 2005 were limited to a 2m x 2m unit excavated to investigate
a large, oval anomaly that was interpreted as a possible structure. This
anomaly was visible in both resistivity and ground penetrating radar data
collected from a north-south swathe across the enclosure. Excavations
revealed that this anomaly was caused by a natural irregularity in the
culturally sterile sediments beneath the A horizon.
Contrasts between cultural and non-cultural sediments at the Strawtown
Enclosure are not as high as at the nearby Castor Farm site. Magnetic
survey is presently impossible, as the enclosure was covered by a junkyard
until just a few years ago. The high clay content in the sediment,
as well as the many trees, complicate survey by ground penetrating radar.
In all, the enclosure offers several challenges to geophysical research.
The presence of large features, such as storage features over 2m
deep and the earthen ditch/embankment, however, suggest that geophysical
methods may be very useful for mapping the site in the future if these
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The Castor Farm Site (12-H-3)
The enclosure is only one of several large village sites within the
park that date to the Late Prehistoric period. The Castor Farm
site (12-H-3), located in the bottoms below the enclosure, is a large
village site that appears to date slighly earlier than the enclosure.
Geophysical investigations and three
seasons of excavation at the Castor Farm site have focused on answering
several questions about the kinds and arrangements of cultural features
within the site.
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A variety of features has been identified
on the Castor Farm site, including pits, hearths, earth ovens, a stockade
wall, post structures, and at least two rectangular, semi-subterranean
structures (visible in the data to the right). Excavation of half
of one of these structures was the focus of the 2004 field school and
the 2005 Archaeology Month excavations.
These structures were detected using
magnetometry, resistivity, and ground penetrating radar. The interplay
of the natural and cultural stratigraphy at the site often produces high
contrasts between archaeological features and the surrounding matrix. Adequate information about
community layout is essential to the interpretation of large, complex
sites, but is nearly impossible to carefully, cost-effectively collect
using traditional excavation techniques alone. Given the complexities
and importance of the Castor Farm site and the surrounding area, geophysical
survey methods are invaluable for site mapping, and will allow future
excavations to be appropriately targeted as research questions are clarified.
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Geophysical Instruments
Purchase of geophysical instruments by the IPFW-AS was funded by the
National Science Foundation. Acquisition of this equipment has
allowed the IPFW-AS to incorporate ground penetrating radar (GPR), magnetometry,
and resistivity instruments into its research and undergraduate education
program. These instruments are used in archaeological research to
collect detailed data about the locations and nature of near-surface archaeological
deposits. These instruments collect qualitatively different kinds
of information about subsurface deposits, and each method has its
strengths and weaknesses. Recent studies have shown the versatility
of these kinds of geophysical data, and have demonstrated the effectiveness
of using several geophysical survey methods in combination (e.g., Ambros
and Larson 2002; Kamei et al. 2002; Marshall 2001; Neubauer et al. 2002;
Walker 2000).
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GPR uses radio waves
to actively probe the earth to depths up to several meters. Radio
waves are reflected by buried objects, structures, and surfaces, and
these reflections can be used to create three-dimensional images of buried
features. In the past, some archaeologists have expressed reservations
about the usefulness of GPR data. Neubauer et al. (2002) argue that
much of this disappointment stems from the application of unsuitable survey
logistics, data processing, visualization and interpretation techniques.
Efforts towards standardization of the collection and presentation of
archaeological GPR survey data are underway (see Neubauer et al. 2002),
and experimental studies are constantly refining our understanding of
geophysical data collection and interpretation (e.g., Hildebrand et al.
2002; Leckebusch and Peikert 2001; Neubauer et al. 2002; Walker 2000).
Additionally, processing software geared specifically towards archaeological
data is now available, making analysis and interpretation of complex GPR
data more practical. The IPFW Archaeological Survey uses GPR Slice
software designed by Dr. Goodman (center in the photograph to the
right).
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A gradiometer (two
magnetometer heads combined in a single instrument) is used to detect
fluctuations in the earth’s natural magnetic field caused by the presence
of cultural materials such as fired clay or heated rock. Magnetometry
data of varying resolution are commonly used to discern broad elements
of site structure as well as the size, shape, contents, and function of
individual features (e.g., Marshall 1999; Martin et al. 1991; von Frese
1984; von Frese and Noble 1984).
In the photograph at the right, Josh Herman collects gradiometer data
across a 10m x 10m grid while controlling sensor height. Because the gradiometer
is a passive instrument, the distance of the sensors from the earth's surface
affects the intensity of magnetic anomalies caused by archaeological features.
This experimental study was part of the 2005 REU.
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A resistivity meter
measures how well sediment conducts an electrical current between two
points on the surface. Areas of varying moisture content, such as
filled ditches or culturally compacted areas like house floors, can be
detected using a resistivity meter. Areas of compaction hold
less moisture, and appear as high resistivity areas. Sediment disturbances
(features, etc.) hold more moisture, and appear as low resistivity areas.
Resistivity survey has been used successfully at the Castor Farm site as
well as at the Strawtown Enclosure and 12-H-1052.
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The resolution of all these
methods is influenced by a number of variables, including instrument
capabilities, survey grid/spacing, environmental factors, and data processing.
Data from these instruments are collected and processed digitally, and
can be overlain and interpreted with the aid of software applications such
as Surfer or a Geographic Information System (GIS). By “ground-truthing”
the geophysical data through limited excavation, it is possible to generate
empirical models of site layout.
References
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