Magnetic surveys are probably the most productive prospecting methods employed in archaeology. It is as if nature designed archaeological sites to be made visible by the many magnetic variations they exhibit.

Seven principal phenomena contribute to magnetic anomalies within archaeological sites.

Different deposits can vary in the type and density of magnetic iron compounds.

Intensive firing of the soil (beyond the Curie point, about 600^o C depending on the material) greatly elevates apparent magnetism, whether purposeful (hearths, kilns) or accidental (a burned house).

Thanks to natural processes working at the surface that include weathering and a fermentation process that changes certain iron compounds to more magnetic forms, topsoils (and paleosols) are magnetically enhanced. There is even a bacterium that concentrates certain magnetic compounds. Extended human occupations that introduce organic and fired materials to the topsoil further enhance soil magnetism within settlements.

The removal of magnetically enriched topsoil during the construction of ditches, house pits, or other depressions, causes a local lowering of the magnetic field over these features.

Accumulations of topsoil, such as occur in mound or sod constructions, berms adjacent to excavated ditches, or when storage or other pit features are filled with topsoil after abandonment create local increases in the magnetic field.

Rocks employed in the construction of buildings or pavements might be more magnetic than surrounding soil (e.g., igneous rocks), while others might be less magnetic (e.g., certain limestones), producing a local decrease. A table of common magnetic susceptibilities reveals these differences.

Iron or steel artifacts produce relatively large magnetic measurements that are readily detected and commonly expressed as dipole anomalies, consisting of paired positive and negative extremes.

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