Case Study
Malodorous (24RB2062)
A Prehistoric Campsite in Rosebud County, Montana
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Figure 1. Feature B5-F1,
one of several stone ring features identified using magnetic
survey at the Malodorous Site
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Archaeo-Physics LLC was contracted by GCM Services,
Inc. to perform a magnetic field gradient survey at the Malodorous
Site (24RB2062), a prehistoric campsite in Rosebud County, Montana.
This case study illustrates the use of data processing to overcome
the technical challenge of strong natural magnetic anomalies
at a site. The survey also located buried stone rings (Figure
1), which were not suspected at the site previous to the survey.
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Figure 2. Original survey
data plot
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Sedimentary rocks typically
lack appreciable remanence and are often low in susceptibility,
but may become highly magnetic when thermally altered. This
is, of course, the principle that makes many features detectable,
but the burning of the coal seam beneath the Malodorous site
has resulted in very strong magnetic fields whose gradients
are too steep to be suppressed by the configuration of the instrument.
Figure 2 shows high amplitude variance throughout this data
set (note that range of the scale is over 200 nT). Although
some anomalies that were thought to be caused by features of
interest do appear, they are difficult to distinguish. |
To suppress the geologic response, a statistical filter
was designed to suppress the geologic background and enhance small,
weak anomalies (the results are shown below). This technique, called
highpass filtering, subtracts the local mean from each data point.
The dimensions and weighting of the window used to calculate the
local mean of each point must be adapted to site-specific conditions.
Processed data plotted in Figure 2 shows small local anomalies in
greater contrast against a "flattened" geologic background.
Unfortunately, operator/instrument induced defects exacerbated by
strong local gradients are enhanced as well. While these can generally
be distinguished by their rectilinear patterning, they may obscure
more subtle cultural patterning.
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Figure 3. Survey results with additional
processing. Stone rings identified based on magnetic data
are circled in yellow
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When excavation results (Figure 4, below) are compared
the magnetic data plot, the relationship between many of the features
and associated anomalies can be seen. Although more subtle anomalies
of interest are probably obscured in this magnetically cluttered
data set, many of the features were accurately identified prior
to excavation. Although most of the hearth features proved difficult
to distinguish, all but one of the stone rings recorded at the site
was identified based on the magnetic survey results. Stone rings
were apparent because of cultural patterning of naturally occurring
rock.
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Figure 4. Map of excavated features
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