Magnetic "Pollution" of Archaeological Sites
Magnetometers used in archaeological geophysics generally use very sensitive fluxgate and cesium sensors that are capable of mapping very subtle anomalies due to soil disturbance, organic enrichment, and other phenomena having very low magnetic contrast with their surroundings. Unfortunately, on all but the most remote archaeological sites, some degree of magnetic "clutter" from modern cultural materials such as metal and brick is almost inevitable. Because of the very strong remanent fields associated with these materials, they can completley obscure more subtle features of interest and complicate survey data processing. A prehistoric feature may be evidenced by an anomaly of only a few tenths of a nanoTesla, whereas a metal object as small as a nail can create an anomaly several meters in diameter and hundreds of nanoTeslas in strength.Unfortunately, many sites are cluttered with the metal artifacts of archaeologists past and present.
The images above show the effect that two metal objects - a steel grid stake and a pin flag wire - might have on a magnetic field gradient data set. The pale blue areas represent blank values where the local gradient exceeds the range of the instrument (+/-204.7). Fortunately, the gradiometer configuration minimizes the radius of response to metal objects (compared to a 'total field' configuration). This particular illustration is a synthesis of data from different sites. It was constructed to allow us to show a data set both with and without modern clutter. It should be noted that the burned Mississippian house feature appearing in this dataset is less subtle than most features encountered on prehistoric sites in North America, and that the amount of metal is not great. Similar (and worse) effects from metal objects are routinely found on real-world surveys.
To a lesser extent, any metal object (ferrous or non-ferrous) can afffect electromagnetic methods such as conductivity and GPR. While this should be considered, it is less critical, because the radius of response is more localized, and therefore less disruptive to the integrity of the dataset.
Steel pinflags, nails, datums, fences, etc. that are deliberately or accidentally left on sites can have a very detrimental effect on magnetic data. Whenever possible, Plastic, wood, or aluminum substitutes should be used for these items. Steel pinflag stakes are particularly problematic. Ubiquitously used, often lost, and difficult to find after the plastic flag has degraded and fallen off, the wire stake creates a surprisingly large magnetic anomaly. The slight expense of fiberglass-stake pinflags will be found to be well worth the benefits. Unlike larger diameter plastic flag stakes, very thin, rigid fiberglass-stake flags are available that are as convenient to use as metal-stake flags. It is hoped that these considerations will be reflected in standard archaeological practices in the near future.
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