Preservation Matters: Remote Sensing Electromagnetic Induction

Electromagnetic induction (EMI) instruments simultaneously measure both electromagnetic conductivity and magnetic susceptibility. These two different geophysical properties are altered by many human activities. Each one can detect and characterize archaeological features. By measuring different physical properties, the two types of data together can provide greater insight than either one by itself. Features can be mapped by variation in soils, a variety of building materials, burning, and organic enrichment. Metal is also detectable through EMI and can be a source of extraneous clutter or an important part of the archaeological record. As with most geophysical surveys, large areas are the ideal setting for EMI investigations. Large areas provide important context, are more interpretable, and may reveal unexpected features. The conductivity and susceptibility results of EMI instruments are comparable to several other geophysical methods. This educational brief on EMI reviews the components of conductivity instruments, how to prepare for their use, methods for data collection, and ways to visualize results.

Components

Many instruments for measuring conductivity and susceptibility are on the market, but the most common type of configuration is the dual coil (slingram) EMI meter. A transmitter coil emits an EM signal, while receiver coils measure its response to nearby materials. Some instruments have multiple receivers for different depth ranges. The delay of the returned signal is a response to conductivity, while its strength indicates magnetic susceptibility. The components of an EMI meter include:•a long horizontally-oriented probe, held parallel to the ground surface, that emits an EM current and receives readings;•a handle to carry the probe and keep it at a steady distance from the ground;•a transmitter, or control unit, to manage when the probe emits a current.In modern instruments, the control unit acts as a data logger. Data may also be recorded by hand during each reading.

Preparation

The major components of preparation are background research and monitoring the operator's surroundings. A comprehensive assessment of the project area's land use history is useful for deciding where to conduct EMI survey. This type of survey is best used for large scale data collection. Assessing sites such as cemeteries, villages, and farmsteads of the past are ideal. The instrument detects features on the soil, so having an idea of what could possibly be sensed by the EM induction meter is a good way to prepare. EMI meters are highly sensitive to metals. Make sure shoes, phones, personal effects, and ground stakes are not going to interfere with the instruments.

Data Collection

The EMI instrument communicates with a separate data logger during a survey. Personnel for a field survey should consist of at least two people. One person holds the EMI and another person holds the data logger. The team walks together during the survey so that the logger and EMI are parallel. When collecting data, take care to remove metal from clothing and wear shoes with no metal or steel-toe inserts. The EMI is extremely sensitive to metal and it interferes with the data. To start data collection, the team walks slowly along a transect with the EMI maintaining a consistent pace along the line and a steady height above the ground. As the team is walking the transect line, the person holding the data logger takes a number of readings each meter. The sampling rate may be 10 readings every meter along the transect line. The number of readings and the number of transects depends on the size of the survey area and whether the data resolution needs to be high or low.

Visualizing Data

Conductivity (quadrature) values are recorded in milli Siemens per meter (mS/m), while susceptibility (in-phase) measurements are recorded in SI units. These results represent measurements at each interval with different values for different depths. Software designed specifically for processing geophysical data can take this information and display it in the form of a map. A grey-scale or color gradient is used to create a contrast between features of high and low conductivity values. Geophysical researchers look for contrasts in the data that might represent subsurface features, or anomalies. Anomalies of interest can sometimes be very subtle, especially in the presence of metal and other high-contrast materials. Based on background research, the location of anomalies on the map may correspond with evidence of past human activity. This helps guide future research and investigations.

References

• Bevan, B. W. (1983). Electromagnetics for Mapping Buried Earth Features. Journal of Field Archaeology 10(1):47-54. https://doi.org/10.1179/009346983791504354.
• Bevan, B. W. (1998). Geophysical Exploration for Archaeology: An Introduction to Geophysical Exploration. Midwest Archaeological Center Special Report No. 1. U.S. Dept. of the Interior, National Park Service, Lincoln, Nebraska. https://digitalcommons.unl.edu/natlpark/91
• Ernenwein, E. G., Hardgrave, M. L. (2009).Archaeological Geophysics for DoD Field Use: A Guide for New and Novice Users. Environmental Security Technology Certification Program(ESTCP) Project SI-0611. Department of Defense, Washington, DC. https://usace.contentdm.oclc.org/digital/collection/p266001coll1/id/3461/
• Gaffney, C. F., Gater, J. (2003). Revealing the Buried Past: Geophysics for Archaeologists. Tempus Publishing, United Kingdom.
• Johnson, J. K. (ed.) (2006). Remote Sensing in Archaeology: An Explicitly North American Perspective. University of Alabama Press, Tuscaloosa.
• McKinnon, D. P., Haley, B. S. (eds.) (2017).Archaeological Remote Sensing in North America: Innovative Techniques for Anthropological Applications. University of Alabama Press, Tuscaloosa.

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, archaeological 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
645 University ParkwayNatchitoches, LA 71457
Website: www.nps.gov/ncptt
Phone: (318) 356-7444

Series Editor: Kirk A. Cordell, NCPTT Executive Director
Authors: Sadie S. Whitehurst and Tad Britt, NCPTT Archeology.Geoff Jones, Archaeo-Physics, LLC.
Cover Photo: EMI at the Etzanoa Country Club Site, Kansas.