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Geonics EM38-MK2 Conductivity Meter

 

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The EM38-MK2 is an electromagnetic induction meter, often referred to as a conductivity meter. The instrument measures both magnetic susceptibility and conductivity at two depths (maximum depth is about 1.5 meters) in the ground by inducing an electromagnetic field and then measuring the response of the ground (the secondary electromagnetic field). In this way, electrical conductivity is measured without the use of probes using the electrical component (quadrature) of the secondary electromagnetic field. At the same time, the magnetic susceptibility of the ground is measured from the magnetic component (in phase) of the secondary electromagnetic field. Both of these data types are very useful in archaeology and other near-surface geophysical applications including soil and contamination studies.

Induced Electrical Conductivity

Conductivity is a measure of the ability of a material to conduct an electrical current. Variation in conductivity is almost entirely dictated by the amount of moisture in the soil. Coarse graind, well-drained soils (gravels, sands) exhibit relatively low conductivity, whereas fine grained soils (clays, silts) that hold more moisture exhibit higher conductivity. Conductivity is useful at archaeological sites because cultural features represent localized disturbances to natural soil strata, and often include concentrations of organic materials, rocks, and other artifacts. These disruptions to the natural soils and sediments are associated with a localized contrast in moisture retention and therefore electrical resistance. Often pit features, ditches, earthworks, and other soil moisture-related features are readily detected with conductivity.

Magnetic Susceptibility

Magnetic Susceptibility (MS) is a measure of the ability of a material to become magnetized in the presence of a magnetic field. All atoms have their own magnetic fields owing to their orbiting electrons, so all substances react (are susceptible) to nearby magnetic fields to some degree. The MS of materials can be enhanced in a variety of ways. The natural formation of soils involves the conversion of weakly magnetic oxides and hydroxides to more strongly magnetic forms, so topsoil is much more magnetic than subsoil layers. In addition, human activities further enhance topsoil so that soils at archaeological sites are often more magnetic than those in surrounding non-cultural areas. Humans enhance pedogenic processes and therefore susceptibility contrasts by increasing organic matter and altering the porosity of soils, and also by increasing the soil temperature through firing. They may also integrate burned or high-susceptibility materials into the ground. Burning, whether natural or human-caused, also enhances MS. The principle types of features that can be detected with an MS survey are therefore burned features, and those involving the displacement of soil. Human magnetic enhancement of topsoil often increases the susceptibility contrast at archaeological sites, thereby increasing the ability to detect human-created features.