Magnetic gradiometer or magnetometer (sometimes referred to as mag) is used to map the local variations within the survey areas’ magnetic field. Magnetic variations can be indicative of past cultural activities. These variations are produced by the presence of ferrous metals (typically present within historic sites), materials heated beyond their Curie point (including kilns, bricks, burned buildings, and hearths), soils modified through anthropogenic activity (such as trenches, middens, palisades, and roads).
Magnetometer data are acquired through passively measuring the local variations in the earth’s magnetic field. These subtle variations are measured most easily using a gradiometer, which eliminates the earth’s primary magnetic field through the use of two vertical sensors. The difference between the measurements taken at the top and bottom sensors removes the constant variation allowing the examination of local changes. Magnetic data are collected in transects, which are part of a larger grid. Transect data are lined up within the grid and space between each transect is interpolated to form a set of two dimensional data were it is often possible to see patterning indicative of archaeological features. From this point, interpretations can usually be made about possible subsurface features.
Magnetometer can be used on both historic and prehistoric sites. It is typically best suited to sites without a lot of metal present, both on the surface and below the surface. Large amounts of metal can “hide” more subtle anomalies. For this reason, magnetometer is not typically recommended for urban settings, sites where there is a great deal of modern trash, or historic sites with components dating after the 1940’s, where large amounts of metal are present. New South Associates has used magnetic gradiometer to map Civil War era trenches, prehistoric villages, and pre-revolutionary war structures. Data collection does not require that the sensors have direct contact with the ground surface, but dense vegetation (that cannot be cleared) and steep topography can still be problematic. New South Associates currently owns a Bartington Grad-601 Dual Sensor Fluxgate Gradiometer.