Archaeo-Physics, LLC - geophysical survey and subsurface mapping

Case Study

Malodorous (24RB2062)

A Prehistoric Campsite in Rosebud County, Montana

Stone ring feature
Figure 1. Feature B5-F1, one of several stone ring features identified using magnetic survey at the Malodorous Site
  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.

malodorous site magnetic survey

Figure 2. Original survey data plot

  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.

Malodorous site magnetic survey processed data
Figure 3. Survey results with additional processing. Stone rings identified based on magnetic data are circled in yellow

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.

Excavated features at the Malodorous site
Figure 4. Map of excavated features

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