Geophysics
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Archaeo-Physics, LLC was contracted
by the Midwest Archaeological Center (MWAC)
to perform a geophysical investigation at
the Garden Coulee Site (32WI18). The objective
of the investigation was to identify subsurface
cultural features associated with the 19th
century occupation of the site. The investigation
consisted of electrical resistance and magnetic
field gradient surveys. These data were
to be used in conjunction with pedestrian
survey data to address questions regarding
site integrity, site limits, and intra-site
patterning. The investigation was successful
in identifying a number of geophysical anomalies
thought to be caused by archaeological features.
SURVEY DESIGN AND TECHNICAL PARAMETERS
The survey area was divided into a series
of squares, or survey “grids,”
measuring 20-x-20m. The survey grid had
been staked at 40m intervals by MWAC personnel
prior to geophysical survey, and tapes were
used to place the intermediate stakes forming
the 20-x-20m survey grids.
Magnetic field gradient survey was performed
using a Geoscan Research FM36 fluxgate gradiometer.
The magnetic survey was performed over 154
contiguous survey grids totaling 6.16ha.
The instrument was operated in the 0.1 nT
sensitivity range. Transect spacing was
0.5 m with 4 samples taken per meter along
each transect (8 samples per m2). A small
number (15) of survey grids (see Figure
5 for location) were surveyed at a 1m
transect interval, with 8 samples taken
per meter (8 samples per m2). The transect
interval was changed to maximize the area
that could be surveyed before the predicted
arrival of several days of inclement weather.
This sampling strategy increases survey
speed at the expense of some loss of resolution
in the x (east-west) direction. It is not
thought that the interpretability of these
data was significantly compromised.
A Geoscan Research RM15 resistance meter
was used to perform resistance survey. It
was combined with a PA5 probe array and
operated in twin probe mode using a mobile
probe spacing of 0.5m. Resistance survey
was performed 84 contiguous survey grids
totaling 3.36ha. Transect spacing was 1m
with 2 samples taken per meter along each
transect (2 samples per m2).
Click
here for a general background on electrical
resistance and magnetic field gradient survey
methods.
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Garden Coulee Site (32W118) Magnetic
field gradient survey.
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Garden Coulee Site (32W118) Electrical
Resistance Survey.
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RESULTS AND INTERPRETATIONS
The results of the geophysical surveys
are presented graphically. This is
done so that anomalies of cultural
and natural origins can be generally
recognized by their pattern. When
rendered graphically, the patterns
resulting from the cultural and natural
physical phenomena causing the detected
anomalies can be visualized and interpreted.
Ground truthing of the geophysical
data sets (i.e. archeological test
excavations) will greatly inform on
the interpretation of the geophysical
data. Verification (or refutation)
of preliminary interpretations and
insights into feature composition
and geology can allow us to revise
or elaborate our interpretations,
and to do so with greater confidence.
The results of the geophysical surveys
and ground truthing should be used
in conjunction with other available
sources of information such as surface
artifact distribution and historic
documents to understand the site patterning,
to locate features for excavation,
and to develop a fundamental grasp
of the overall Garden Coulee site
context.
The data from the magnetic and resistance
surveys are presented in Figures 1
and 2 (see above), respectively. Figures
3 and 4 (see below) show the combined
survey results and surface artifact
density data overlaid on geophysical
survey data. Graphics highlighting
areas interpreted as cultural and
natural features are overlaid on the
geophysical survey results in Figures
5 and Figure
6 are keyed to the accompanying
text.
Preliminary interpretations are intended
as a starting point for hypothesis
generation and testing. Interpretation
of geophysical survey results should
be an ongoing and iterative process
involving continuing inputs from archaeological
geophysicists and archaeologists.
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Garden Coulee Site (32W118)
Electrical Resistance Survey
with Magnetic Anomalies.
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Garden Coulee Site (32W118)
Electrical Resistance Survey
with Magnetic Anomalies and
Surface Artifact Densities.
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INTERPRETATIONS
Both the magnetic and resistance
surveys were successful in mapping
numerous anomalies thought to
be caused by archaeological features.
It is likely that most of the
anomalies in the magnetic data
set are cultural in origin, and
that many of them are associated
with archaeological features and
artifacts of interest. While resistance
survey results are somewhat more
ambiguous, there are a number
of anomalies thought to be associated
with archaeological features.
Several of these have a strong
correlation with magnetic anomalies.
Correlation between the two geophysical
data sets tends to confirm the
cultural origin of anomalies and
gives greater insight into feature
patterning and composition. Correlation
between the geophysical data and
surface artifact densities is
poor in some areas and stronger
in others. There are a number
of possible causes for this:
- Archaeological features
may have been misidentified
in the geophysical data.
- Some archaeological
features may have been
undetectable by the
methods employed.
- Earth moving and agricultural
activities may have
re-worked archaeological
deposits, displacing
artifacts.
- The distribution of
artifacts within the
site may have been patterned
within the site by cultural
processes, such as dumping
refuse outside the village
or the use of specialized
activity areas.
- Variable surface visibility
at the time of the pedestrian
survey.
Any or all of these may
be causes for the apparent
discrepancy between surface
artifact densities and
geophysical anomalies.
In general, confidence
in the results of the
geophysical survey results
is fairly high, although
interpretations offered
are to be considered preliminary,
and subject to verification
and refinement. NEXT
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