Preservation Matters: Remote Sensing Earth Resistance

Earth resistance survey, also known as soil resistance, is a geophysical data collection technique that uses an electrical current to detect the presence of archeological features below the ground surface. Resistance is the property of the soil (conductor) to oppose the flow of an electric current. This method results in detailed two or three-dimensional images of the subsurface environment. Resistance survey is appropriate for projects that call for remote sensing and nondestructive investigations. This technique collects data on subsurface material without disturbing the soil. The resulting values range depending on the minerals and composition of the soil matrix. A successful survey shows variation in values over the project area. Those contrasted areas may represent features created through past human activity. This technique is useful for identifying buried structural features of the past, such as remnants of walls, building foundations, gravel pathways, or graves.

Components

Knowing your tools is critical for ensuring a successful survey. This geophysical method uses an instrument called a resistance meter. There are many different array configurations for resistance instruments, such as twin probe, Wenner, Schlumberger, pole-pole, dipole-dipole, and more. The twin probe array described in the following paragraph is the most common configuration presently used in archeological investigations. The instrument induces an electrical current through two probes and measures the resulting voltage with an additional two probes. The probes are electrodes at the base of the meter's frame. The electrodes are spaced horizontally along the base of the frame and have a space between them of 50 to 100 centimeters. There is a difference between resistance and resistivity that is critical to planning and understanding the results of a resistance survey. Resistance measurements are sometimes converted to to resistivity values and provide a means to factor in depth. Resistance is a measure of the soil's ability to oppose an electric current. Resistivity is the resistance of soil per unit of depth for a unit of cross section (probe separation). Electrical resistivity is measured in ohm-meters and resistance is measured in ohms. Resistance is determined by the formula R=V/I where R is resistance, V is voltage, and I is current. Electrical resistivity is represented by the formula ρα _= 2πR(a) where ρα _is the apparent resistivity of the soil beneath the mobile probes, "a" is the mobile electrode spacing, and R is the resistance. Before data collection, plan how the data will be presented.

Data Collection

The first step in preparation is determining whether the project area's environmental conditions are suitable for a survey. Salinity and soil moisture are integral to resistance surveys, since an electrical current must have a conductor to record resistance levels. Factors such as ground temperature and soil compaction also affect the efficacy of resistance surveys. Additionally, earth resistance data collection works best in even terrain. Determine the project area and set up a grid for data collection. Use only non-metal tapes and stakes to avoid interference in the data. Data collection should consist of three to four people. There are a few components to data collection that require teamwork. One person will operate the meter by placing the probes in the ground, typically every 50 centimeters. Another person holds the cable and makes sure it is not tangled. One or two additional people move the tape that the meter operator is following at the end of each transect. This survey type works best for large project areas, so allot an appropriate length of time for survey.

Visualizing Data

Consider whether the data will be displayed in resistance or resistivity measurements before visualizing the data. After a plan of action is decided, the data is put into a software program for processing. The results of a resistance survey are three-dimensional, or a “point cloud." The density of points in the cloud depends on the spacing of probes. The visual created from the points is presented in 3D or as a static 2D map. Post-processing of data focuses on visualizing the spatial distribution of the results. This allows the user to understand the range of values and their distribution. One way to visualize data is by associating the values with contrasting properties, such as color and opacity. Set a color scale on the map that defines contrasts between disturbed and undisturbed soil.

Interpreting Results

Interpreting geophysical results is an iterative process that takes practice. There are a couple methods for defining features on the resistance map once you find a good range of contrast. Two popular options are to define the changes in soil resistance based on color and opacity tables, or to use contouring techniques to make the subtle features detected more visible. Interpretation depends on the site's background. If the researcher knows there was a structure once located in the project area, then changes in soil resistance may be attributed to that known past structure. Buried archeological features with varying moisture contents stand out compared to the soil around them and can therefore be detected in the survey. This information informs future investigations.

References

• De Vore, Steven L., and William B. Butler (2021). Initial Geophysical Assessment of the Iliniwek Village Site, Missouri. Missouri Archaeologist 82:103-120.
• Gaffney, Chris and John Gater. (2003). Revealing the Buried Past: Geophysics for Archaeologists. Stroud, United Kingdom: Tempus.
• Johnson, J. K. (2006). Remote Sensing in Archaeology: An Explicitly North American Perspective. The University of Alabama Press, Tuscaloosa.
• Jones, Geoff. (2005) Mapping Unmarked Graves at Layman's Cemetery. Hennepin History64(3):20-26.
• McKinnon, Duncan P. and Bryan S.Haley (editors). (2017). Archaeological Remote Sensing in North America: Innovative Techniques for Anthropological Applications. The University of Alabama Press, Tuscaloosa.
• Schmidt, A. (2013). Earth Resistance for Archaeologists. Altamira, London.

About NCPTT
The National Center for Preservation Technology and Training (NCPTT) is a research, technology and training center within the National Park Service.NCPTT helps preservationists find better tools, better materials, and better approaches to conserving historic buildings and landscapes, archeological sites, and museum collections. It conducts research and testing in its laboratories, provides cutting edge training around the U.S., and supports research and training projects at universities and nonprofits. NCPTT pushes the envelope of current preservation practice by exploring advances in science and technology in other fields and applying them to issues in cultural resource management. NCPTT publishes its Preservation Matters Series to provide easily accessible guidelines for preserving cultural materials.

To obtain more information on this or similar subjects, contact us at:
National Center for Preservation Technology and Training
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Website: www.nps.gov/ncptt
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Series Editor: Kirk A. Cordell, NCPTT Executive Director
Authors: Sadie S. Whitehurst and Tad Britt, NCPTT Archeology.Steven De Vore (retired), Midwest Archeological CenterCover
Photo: Resistance Survey at the Etzanoa Country Club Site, Kansas.