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8 Magnetic Susceptibility RinitaA.Dalan Magnetic susceptibility surveys occupy a unique niche in archaeological research distinct from other near-surface geophysical methods. Historically, susceptibility surveys have not been as widely employed as magnetometry, resistivity, or ground-penetrating radar surveys; however, recent field and laboratory-based applications together with advances in instrumentation have resulted in an increase in interest in the application of susceptibility techniques. A susceptibility study can provide information that is unavailable through more traditional geophysical methods. For example, when applied in conjunction with laboratory soil magnetic techniques, not only may archaeological features be located and defined but also questions regarding formation and postdepositional processes may be approached. Susceptibility studies are applicable to a broad range of archaeological sites, features, and environments and, through the use of various sensors, are capable of resolving contrasts in susceptibility over large and small scales. Though they can be time consuming compared with other geophysical methods, the complementary information they provide and the ease with which these techniques can be mastered render them worth consideration for archaeological research. Overview of the Technique Basic Principles Magnetic susceptibility provides a measure of a material’s ability to be magnetized. As its full name, low field magnetic susceptibility, suggests, this property quantifies the response of a material to a weak magnetic field (i.e., one on the order of the earth’s 162 ~ Rinita A. Dalan field). It is defined as the ratio of the magnetization induced in a sample to the inducing (magnetizing) field. Magnetic susceptibility is measured in the presence of the magnetizing field and thus is distinct from a magnetic remanence, or “permanent” magnetization, that is measurable in the absence of a magnetic field. Magnetometer surveys record spatial variations or anomalies in the earth’s magnetic field caused by local changes in magnetization. These magnetic anomalies may arise from localized deposits of materials with a higher or lower susceptibility or from materials possessing a magnetic remanence. Magnetometer surveys make no distinction between these; the observed anomaly expresses only the net effect of any induced and remanent magnetizations. Magnetic susceptibility surveys differ from magnetometer surveys in that they measure only the induced component of this signal. As magnetic susceptibility surveys can be used to distinguish features resulting from susceptibility contrasts from those carrying a magnetic remanence, they can serve as a useful complement to magnetometer surveys. Magnetic susceptibility can be expressed either as a susceptibility per unit volume (κ) or as a mass normalized susceptibility (χ ). Volume susceptibility (κ) is a ratio of the volume magnetization induced in a material of susceptibility κ by an applied weak magnetic field. In the SI system of units (i.e., International System of Units, abbreviated from the French Le Système International d’Unités), this is a dimensionless quantity . Mass susceptibility is equal to the volume susceptibility divided by density and has units of cubic meters per kilogram in the SI system. As part of soil development, surface soil layers typically become magnetically “enhanced ” in comparison to subsoil layers. This results in greater values of magnetic susceptibility for topsoil as opposed to subsoil layers. The general process that produces these susceptibility contrasts is the conversion of weakly magnetic oxides and hydroxides to more strongly magnetic forms within the surface soil layers. Figure 8.1 presents susceptibility profiles measured from two cores at the Cahokia Mounds State Historic Site, Illinois. In both profiles, enhancement of topsoil over subsoil layers is apparent, with relatively high mass magnetic susceptibility values (χ, second column) at the surface, which decrease with depth and then stabilize in the underlying subsoil. Surface soils for both cores also show increases in another magnetic property, anhysteretic remanent magnetization (ARM), as indicated in the first column of Figure 8.1. ARM is an artificial remanence given to the sample in the laboratory. Like low field susceptibility, it increases with an increasing concentration of magnetic grains. Mechanisms responsible for this process of magnetic enhancement include naturally occurring or human-generated fires, as well as pedogenic enhancement through various inorganic and organic pathways. When explanations for magnetic enhancement were first proposed (LeBorgne 1955, 1960a, 1960b), firing was suggested as the primary mechanism. Since that time, however, pedogenic enhancement has been documented as a widespread phenomenon (Dearing et al. 1996; Maher and Taylor 1988) that is equally important in producing the enhanced magnetic properties of topsoil. Pedogenic enhancement occurs as part of soil development through low-temperature [18.223.106.232] Project MUSE (2024-04-25...

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