Preservation Matters: Remote Sensing Magnetic Susceptibility

Magnetic susceptibility is an electromagnetic (EM) technique which assesses the ability of soils and the features within it to be temporarily magnetized in the presence of a magnetic field. This effect is called induced magnetism. Certain materials are more susceptible to magnetism than others, and this property is affected by many human activities. Importantly, burning and organic enrichment tend to enhance magnetic susceptibility. Disturbed or organically depleted soils, building materials, industrial wastes and other materials associated with human occupation can also be detected. Magnetic susceptibility surveys are often used to assess large areas at low resolution to study large scale patterning, even in agricultural areas where sediments are mixed by cultivation. Susceptibility measurements can even be collected at multiple depths with a subsurface probe or measurement of individual soil samples, but these labor-intensive techniques are generally limited to small scale studies. Susceptibility meters map this variance and can reveal archaeological patterning. Researchers interpret these anomalies as target areas for further investigation, or for preservation on culturally sensitive sites.

Components

There are several types of instruments that measure magnetic susceptibility. The contents of this brief apply to the following instruments:•‘Search loop’ or ‘field coil' instruments: take measurements at the surface of a volume of earth beneath the coil;•hand-helds: can be used on exposed surfaces such as excavation walls and floors,•down-hole instruments: take both vertical and horizontal readings to create a 3-dimensional representation of soil susceptibility;•and bench-top meters (generally small and portable): measure the susceptibility of cores or bulk soil samples, either in the field or laboratory.

All of the above use a coil to transmit an EM field and measure the induced magnetic response. The main differences between them are the dimensions of the coil and the configuration of the system for different tasks, such as field coil vs. down-hole data collection.

Preparation

Preparation for susceptibility survey involves review of the project area's history, soil composition, and the instruments being used.Find historic aerial photographs and topographic maps that might show structures that were once present in the project area. Large-scale magnetic susceptibility works best when detecting features effected or caused by events such as settlement, earthwork construction, or industrial and agricultural use. Have an idea of what your data might look like after the survey by gathering information on the project area's soil stratigraphy. Different material like clay, sand, gravel, and bedrock all have different susceptibility to magnetism. Look for artifacts on the surface of your project area. Magnetic susceptibility readings may be affected by metal artifacts, although the effect is very localized. This should be considered in data processing and visualization. High concentrations of surface metal debris may preclude susceptibility survey and these conditions would be detrimental to most geophysical surveys.

Data Collection

There are several different methods and instruments used for to measuring susceptibility, but all follow similar principles.

Make sure to plan the intervals where data collection will occur. Spacing between readings will depend on how much time is allotted for data collection, the project area size, and what features are anticipated. For large scale reconnaissance survey, readings are taken at intervals of several meters or more. For small scale, high resolution survey, data is collected in transects typically spaced at 50 cm apart.

Although the techniques of data collection differ, similar principles ensure good data quality:

• Test instruments before starting a survey to make sure data is being recorded properly.
• Configure the instrument for the type of survey you are conducting. Longer sampling times give more accurate readings, but slow down the survey.
• Always calibrate the sensor before you start a reading. For high resolution surveys, calibration may be done at the beginning and end of each transect. For some instruments, you can take a reading of a known material to ensure your data output is what is expected. Instruments are often calibrated by taking a reading in the air since it has a value of 0.
• Be consistent in how measurements are taken. With field coil surveys, try to ensure that the coil is in good contact with flat ground surfaces during measurement.
• Monitor data for unusual readings that may indicate metal beneath the coil, or the need to calibrate.
• Log data each time a reading is taken and make sure the data is recorded and stored.

Visualizing Data

Processing the data collected in the field is the first step towards data visualization. Magnetic susceptibility is a dimensionless quantity, χm = M/H, measured in SI units (M is magnetization and H is applied magnetic field strength). A software program geo-references the data to create a map. Data processing usually involves interpolating between samples, filtering of data noise and invalid readings, and enhancements based on the range or scale of anomalies. Rendering effective maps is an iterative process. Data are most often rendered as image maps, but contour maps may be effective for low resolution surveys or overlays. Data may be re-processed to emphasize features of interest or smooth small scale variance where it is a distraction from large scale patterning. Adjusting color scales and contrast emphasizes features of interest, making them more visible. Color gradients display greater ranges of data, while grey-scale imaging can be more intuitive and subtle.

References

• Dalan, Rinita A., 2001, A Magnetic Susceptibility Logger for Archaeological Application. Geoarchaeology 16(3):263-273.
• Gaffney, Chris and John Gater, 2003, Revealing the Buried Past: Geophysics for Archaeologists. Stroud, United Kingdom: Tempus.
• 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.

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 71457Website: www.nps.gov/ncptt
Phone: [phone]

Series Editor: Kirk A. Cordell, NCPTT Executive Director
Authors: Sadie S. Whitehurst and Tad Britt, NCPTT Archeology. Geoff Jones, Archaeo-Physics, LLC.
Cover Photo: Field coil magnetic susceptibility at Marksville, Louisiana.