OUR GEOPHYSICAL SURVEYS ARE CONDUCTED BY

DR. JARROD BURKS

The use of geophysical survey in archaeological investigations was largely pioneered in Europe and has been around for over 50 years. Archaeologists in the United States have been slow to integrate the use of geophysics in their research. However, in the past 20 years American archaeologists have slowly begun to integrate this important technique into a wide range of archaeological endeavors. Despite its ability to reliably identify important cultural features below ground, geophysics is still not a regular part of Section 106 archaeological compliance work in the U.S.

At Ohio Valley Archaeological Consultants we regularly use geophysical survey as an integral part of our Phase II archaeological assessment studies. We also use geophysics during Phase I assessments and Phase III mitigations, where it plays a key role in project planning.

When used in conjunction with traditional excavation methods, our geophysical surveys allow us to more cost effectively evaluate the integrity of archaeological sites. We frequently use geophysical survey, combined with strategically placed hand excavation blocks, in lieu of large scale earth removal with mechanized equipment. Large scale earth removal is far more time consuming than hand excavation and, when done properly, requires large field crews. Furthermore, large scale earth removal techniques are very damaging to archaeological sites. Using geophysical surveys, the OVAC archaeological crew can identify important cultural resources below ground with a minimum amount of ground disturbance. This technique not only saves time, and money, it also helps preserve our important cultural resources for the future.

INSTRUMENTATION, SURVEY DESIGN,
AND DATA PROCESSING

Geophysical testing instruments passively and actively measure a wide range of physical properties of near-surface sediments by detecting subtle differences in electrical conductivity, electrical resistivity, and magnetic susceptibility/remanent magnetization, among many other observable properties (Gaffney and Gater 2003; Weymouth 1986).   Ohio Valley Archaeological Consultants, Ltd., uses the Geoscan Research series of geophysical instruments for archaeological applications, including the FM 36 fluxgate gradiometer and the RM 15 electrical resistance meter. Both instruments were specially designed for archaeological applications and are ideal for detecting subtle variations in the earth’s geophysical properties in near-surface contexts.

Magnetometers, such as the FM 36 fluxgate gradiometer, are very sensitive to ferromagnetic materials (e.g., iron objects). Iron objects, such as historic nails, farm machinery parts, and many other structural and mechanical components, have very strong, unmistakable magnetic signatures. However, what sets magnetometers apart from more common metal detectors, besides only being sensitive to ferromagnetic materials, is their extreme sensitivity—magnetometers can detect very subtle changes in sediments. Most magnetometers react to two kinds of magnetization on archaeological sites: thermoremanent magnetization and magnetic susceptibility (Clark 2000). When sediments and rocks are heated above a certain temperature, known as the ferromagnetic Curie temperature (ca. 500-700C; Lowrie 1997), their magnetization is in effect zeroed and realigned to the local magnetic field, producing a permanent remanent magnetization. Campfires can produce more than enough heat to reach the Curie point. Upon cooling, magnetic compounds, such as magnetite and hematite, recrystalize and are fixed with a common orientation toward magnetic north. Intense heating can make an otherwise magnetically neutral (i.e., random) patch of ground highly magnetic by producing magnetic wood ash and by altering minerals in the ground itself (Linford and Canti 2001).

Soils and ferromagnetic substances that have high magnetic susceptibility react to induced magnetic fields, which in most archaeological cases is the earth’s own magnetic field. Certain soil horizons, such as topsoil (A horizons), and components of soil are more susceptible to induced magnetic fields than other kinds of soil. If a hole dug a few feet into the ground is backfilled with mixed up sediments, the backfilled hole will likely have a different magnetic susceptibility than the surrounding, intact soils—especially if the topsoil ends up in the bottom of the hole adjacent to clay-rich subsoil. Human occupation of an area is known to enhance the underlying soil’s magnetic susceptibility (Tite and Mullins 1971). While the mechanisms behind the enhancement of a soil’s magnetic susceptibility are not totally understood, bacteria that use and produce small magnetic particles are known to be at least part of the process (Fassbinder et al. 1990).

The FM 36 fluxgate gradiometer simultaneously detects both kinds of magnetism, remnant magnetism and magnetic susceptibility, and cannot differentiate the two. This instrument contains two fluxgate detectors in a gradiometer array—that is, the two detectors are arranged one atop the other. Detector separation in the FM 36 is fixed at 50 cm. The uppermost detector senses the earth’s background magnetic field, which in the Midwest U.S. measures approximately 50,000-60,000 nanotesla and can vary as much as a few hundred nanotesla from morning to evening in one day (Breiner 1973). The lower detector senses the earth’s background magnetic field and changes in it caused by objects or soils on the surface or up to about two to three feet (60-90 cm) beneath the surface. Fired earth in prehistoric hearths and organic-rich soil in buried trash pits tend to concentrate the earth’s magnetic field in measurable amounts of approximately 3-20 nanotesla in Ohio, for example. The instrument’s onboard computer subtracts the reading of the top detector (earth’s varying background magnetism) from the reading of the bottom detector (earth’s varying background magnetism plus local magnetic variability), leaving the local, vertical magnetic gradient caused by surface and buried phenomena. This number is stored in the instrument until a data dump is performed.

Geophysical surveys are typically conducted by taking numerous readings along parallel lines (a.k.a. transects) in a rectilinear block (a.k.a. block). Typically, we use 20 x 20-meter blocks. For most projects, data collection transects are spaced 50 cm apart with 8 magnetic readings logged per meter along each transect, producing a total of 6,400 data readings per complete 20 x 20-meter block.

Once the data are dumped from the fluxgate gradiometer to a laptop computer, Geoscan Research’s Geoplot 3.0p software is used for data processing. Such processing is fairly common and involves applying complex mathematical algorithms to the data to reduce background noise and accentuate the potential, buried archaeological phenomena.

The following table considers a range of archaeological features, historic and prehistoric, and their probability of detection using the FM 36 fluxgate gradiometer. This table represents the probability of detection in situations with typical Ohio soils. Detection probabilities change with changing soil conditions.

Known Target Anomalies- FM 36 Geophysical survey

References

Bevan, B.
1998 Geophysical Exploration for Archaeology: An Introduction to Geophysical Exploration. Special Report No. 1. Midwest Archaeological Center, Lincoln, Nebraska.

Breiner, S.
1973 Applications Manual for Portable Magnetometers. Geometrics, San Jose, California.

Clark, A.
2000 Seeing Beneath the Soil: Prospecting Methods in Archaeology. Revised Edition. Routledge, New York.

Fassbinder, J. W. E., H. Stanjek, and H. Vali
1990 Occurrence of Magnetic Bacteria in Soil. Nature 343:161-163.

Gaffney, Chris, and John Gater

2003      Revealing the Buried Past: Geophysics for Archaeologists. Tempus, Stroud, England.

Linford, N., and M. Canti
2001 Geophysical Evidence for Fires in Antiquity: Preliminary Results from an Experimental Study. Archaeological Prospection 8:211-225.

Lowrie, W.
1997 Fundamentals of Geophysics. Cambridge University Press, Cambridge, Great Britain.

Tite, M. S., and C. Mullins
1971 Enhancement of the Magnetic Susceptibility of Soils on Archaeological Sites. Archaeometry 13:209-219.

Weymouth, John W.
1986 Geophysical Methods of Archaeological Site Surveying. In Advances in Archaeological Method and Theory 9:311-395.