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9 Magnetometry: Nature’s Gift to Archaeology KennethL.Kvamme Magnetometry is one of the most productive prospecting methods employed in archaeology . It is a method that responds particularly well to the archaeological record because a variety of natural and cultural processes combine to generate numerous magnetic variations that point to subsurface features. It is almost as if nature designed the components of archaeological sites to be made visible by the magnetic variations they exhibit. Additionally, the rapid data acquisition rates of current instrumentation allow large areas to be surveyed in relatively small amounts of time. High spatial sampling densities are also achieved, well into the sub-meter range, offering good spatial detail. This combination of large-area coverage and high spatial resolution surpasses the capabilities of all other ground-based geophysical survey methods, and this is not a trivial factor . The imaging of large contiguous areas increases the likelihood that whole cultural features with regular, interpretable geometric shapes will be encountered and recognized . This phenomenon forms the basis of a fundamental recognition element in air photo interpretation and satellite remote sensing (Avery and Berlin 1992:52): circles, ellipses, squares, rectangles, and straight lines in imagery are generally of human origin (such geometric forms occur much less frequently as products of nature). Surveys of small areas might reveal cultural features, but without seeing them in their entirety and context it is difficult or impossible to interpret what they might represent or to ascribe 206 ~ Kenneth L. Kvamme significance to them. A small linear feature could represent part of a room, a house wall, a ditch edge, pavement, a trail, or a road, for example. Surveys of large contiguous areas increase the likelihood that sense can be made of patterns in cultural landscapes, and magnetometry is one of the best ways to obtain such surveys in archaeological geophysics. The combination of high spatial resolution, wide-area coverage, the increased pattern recognition that large-area surveys allow, and the many characteristics of archaeological deposits that cause magnetic variations gives magnetometry one of the highest detection probabilities in remote sensing for small and varied cultural features over broad landscapes. The net result is that magnetometry has become the workhorse of archaeological geophysics. Successful case studies number in the thousands from around the globe, and entire villages and even urban areas of the past have been surveyed and mapped within areas approaching a square kilometer (e.g., Gaffney et al. 2000; Summers et al. 1996). Magnetometry is a prospecting method that maps local variations of the earth’s magnetic field in the near-surface. It is a rare passive method of remote sensing because it employs the earth’s magnetic field, rather than generating its own artificial one (the active technologies of radar, resistivity, and electromagnetic induction all introduce artificial fields to measure responses in the ground). Useful results in any form of remote sensing are obtained from contrasts between archaeological features and the natural background. In other words, if archaeological deposits or features possess physical properties different from those of the surrounding matrix, a distinction may be noticed between them. A buried stone foundation, for example, might be more magnetic than the surrounding earth. Such contrasts are referred to as “anomalies” until they can be identified, a task that often requires excavation. Yet, anomalies frequently can illustrate a sufficiently clear pattern for direct interpretation, as when the rectangle of a house foundation is unambiguously expressed. This phenomenon forms the basis of the pattern-recognition approach, which becomes increasingly possible with surveys of large areas because whole features are better understood than partially revealed ones. These ideas are more fully illustrated below. The basic sources for archaeological magnetometry include Aitken (1974), Bevan (1998), Clark (2000), Tite (1972), and Weymouth (1986), with more advanced material presented by Scollar et al. (1990). History of Archaeological Magnetometry Magnetometry is concerned with the strength of the earth’s magnetic field. Although a common compass is designed to measure the direction of this field, it also responds to the field’s strength by its oscillation period. This phenomenon may be employed as a crude detector of large anomalies. Bevan (1998:20) reports that the National Park Service used this method to locate a sunken Civil War ironclad and illustrates changes in a simple compass needle by as much as two degrees in the vicinity of an iron-filled well at a Civil War fort in Petersburg, Virginia. Magnetometers are instruments speci...

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