Geophysical survey is used to create maps of subsurface archaeological features. Features are the non-portable part of the archaeological record, whether standing structures or traces of human activities left in the soil. Geophysical instruments can detect buried features when their physical properties contrast measurably with their surroundings. In some cases individual artifacts, especially metal, may be detected as well. Readings taken in a systematic pattern become a data set that can be rendered as image maps. Survey results can be used to guide excavation and to give archaeologists insight into the patterning of non-excavated parts of the site. Unlike other archaeological methods, geophysical survey is neither invasive nor destructive. For this reason, it is often used where preservation (rather than excavation) is the goal, and to avoid disturbance of culturally sensitive sites such as cemeteries.
Magnetometers, electrical resistance meters, ground-penetrating radar (GPR) and electromagnetic (EM) conductivity meters are most commonly applied to archaeology . These methods can resolve many types of archaeological features, are capable of high sample density surveys of very large areas, and of operating under a wide range of conditions. Geophysical methods used in archaeology are largely adapted from those used in mineral exploration, engineering, and geology. Archaeological mapping presents unique challenges, however, which have spurred a separate development of methods and equipment. In general, geological applications are concerned with detecting relatively large structures, often as deeply as possible. In contrast, most archaeological sites are relatively near the surface, often within the top meter of earth. Instruments are often configured to limit the depth of response to better resolve the near-surface phenomena that are likely to be of interest. Another challenge is to detect subtle and often very small features – which may be as ephemeral as organic staining from decayed wooden posts – and distinguish them from rocks, roots, and other natural “clutter.” To accomplish this , it usually requires at least one and sometimes dozens of readings per square meter.
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