Archaeological features such as pits, privies, cisterns, domestic architecture, etc., represent localized disturbances to soils that would otherwise comprise relatively homogeneous deposits (at the spatial scale relevant to archaeological sites). Features frequently contain organically enriched fill that is darker in color or different in texture than the surrounding soils. It is often this visual (and, to some extent, textural) contrast that permits archaeologists to detect features during excavation. Similarly, geophysical techniques can detect subsurface archaeological features that contrast with the surrounding soils in terms of electrical resistance, magnetic, or other properties. Factors that can create a geophysical contrast include soil compaction, particle size, organic content, artifact content, burning, and moisture retention. Remnant magnetism and magnetic susceptibility are particularly relevant for magnetic feature detection. Heating iron oxides (present in many soils) above ca. 400 degrees Centigrade results in a permanent change (remnant magnetism) in the object’s magnetic field. Human occupation often introduces burned and organic materials to the local soils and increases magnetic susceptibility. In general, any human action that involves the localized disturbance of the soil is potentially detectable by geophysical techniques. Localized disturbances associated with tree roots, rodents, and other natural phenomena, as well as recent cultural activities (vehicle ruts, plow furrows, etc.) are also often detectable.
In a geophysical map, cultural features (as well as other discrete disturbances) may appear as anomalies, i.e., spatially discrete areas characterized by geophysical values that differ from those of the surrounding area. Prehistoric features such as pits and hearths are typically characterized by a very low contrast with the surrounding soil matrix. Historic features frequently contain metal artifacts and architectural debris (brick, mortar, stone footings, etc.) and thus typically exhibit a stronger contrast with their surroundings.
Several other factors can make it difficult to identify anomalies associated with low contrast features. All geophysical surveys are to some extent affected by noise, a seemingly random component in the data attributable to the instrument itself, the operator’s field technique, or variability in the site’s soil, rocks, etc. Clutter refers to non-archaeological, non-random, discrete phenomena that complicate feature detection. Clutter can include plow furrows, rocks, tree roots, rodent burrows, and modern metallic debris. At some sites, anomalies associated with clutter can be stronger and more numerous than anomalies related to cultural features.
-- An excerpt from Dr. Michael Hargrave's report of the 2004-2006 Geophysical Surveys at New Philadelphia.
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