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Case Study

Sample Density and Resolution: Cahokia

The data set below was part of a larger methodological study performed at Cahokia Mounds Historic Site. Electrical resistance data were collected at very high sample densities, revealing small-scale detail in the archaeological record. The sample density of this subset of the resistance survey data was experimentally "thinned" by removing alternate lines of data (other processing parameters are identical). The relationship between sample density and feature detection may be readily observed.

Figure 1. Subset of resistance survey data. The survey area is located to the northeast of monks mound. Although other portions of the resistance survey revealed more spectacular large-scale rectilinear patterning, this subset was chosen because of the unexpected detail revealed at very high sample densities (16 s/m). Darker shades represent higher resistance.

Cahokia Mounds resistance / resistivity survey

Figure 2. (below) Major components of (presumed) cultural patterning are indicated by colored lines. Other less distinct patterning has not been marked. The rectilinear or trapezoidal patterning indicated by yellow lines is thought to be associated with wall-trench architecture. The patterning indicated by the dashed yellow line is fainter, but individual postmolds within the wall trench may be visible. Such small-scale features might be better resolved by a narrower mobile probe spacing (a 75cm spacing was used to resolve deeper features). Larger-scale resistance lows thought to be associated with archaeological features are marked in orange. Numerous resistance highs (not marked) may be caused by pits or other features, although their patterning is more dificult to interpret. North-south striations visible throughout are caused by plow scars.

Cahokia Mounds resistance / resistivity survey with interpretations

Figure 3. (below) data sample density has been reduced to 8 s/m. Wall trenches have become largely, if not entirely, unrecognizable. Discrete resistance highs are less distinct. The large-scale resistance lows in the southeastern portion of the plot are still fairly distinct.

Cahokia Mounds resistance / resistivity survey - 8 samples/meter

Figure 4. (below) Data density has been reduced to 2 s/m, very commonly employed on prehistoric and historic sites. Most cultural patterning in this data plot has become unrecognizable, although this sample density has successfully mapped large-scale cultural patterning elsewhere on the site.

Cahokia Mounds resistance / resistivity survey - 2 samples/meter

Comments: While high data densities are relatively time consuming (and therefore costly) to collect, the level of detail obtained may be very rewarding. Sampling strategy should represent a pragmatic compromise between resolution and cost. Very high sample density was rewarding on this site, not because of the ephemeral nature of the features, but to resolve the features in the complex physical context created by long-term intensive occupation.

Sample density and resolution summary page

Sample density and resolution: 3030 Winchester

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