Gradiometry (also known as magnetometry or magnetic gradient survey) is a passive geophysical method that detects local variation in the strength of the earth’s magnetic field. These variations can be caused by a variety of natural and cultural features that alter the magnetic field emanating from the earth. The difference between the strength of the earth’s magnetic field (about 30,000 to 60,000 nanoteslas [nT]) and the strength of anomalies of archaeological interest (typically 1 to 100 nT) is great, requiring a very sensitive instrument to detect (see Bevan 1998; Burks 2004b; Kvamme 2006).
A gradiometer is an instrument that contains two sensors separated vertically. The top sensor measures the strength of the earth’s magnetic field, while the bottom sensor measures the strength of the earth’s field as altered by any near-surface anomalies. By subtracting the top measurement from the bottom measurement, the instrument “corrects” for the strength of the earth’s magnetic field and “reads” only the local deviation.
Two kinds of magnetism are important to archaeological geophysics: themoremanent magnetism and magnetic susceptibility. During heating, magnetic particles in the material are realigned to the local magnetic field, producing a permanent remanent magnetization (Burks 2004b:8). This thermoremanent magnetism is most pronounced above the Curie temperature (about 600 degrees C), but occurs at lower temperatures as well (Kvamme 2006:207-208). Thermoremanent magnets are permanent magnets that exhibit magnetism in the absence of a magnetic field (Kvamme 2006:207; see also Clark 2000:65).
Materials (such as soil, rocks, etc.) that are magnetically susceptible become magnetized in the presence of the earth’s magnetic field (Clark 2000:65; Kvamme 2006:208). The organic materials in topsoil typically have a greater magnetic susceptibility than the subsoil (Clark 200:100; Kvamme 2006:208), allowing magnetic survey to detect localized variations in the magnetic field caused by disturbances that extend below the surface of the earth: holes that have been excavated into the subsoil and re-filled with more organic sediment (i.e., prehistoric pit features, graves, historic cellars, fence posts) can often be detected by gradiometry if the magnetic contrast between the fill and the surrounding soil is great enough.
The size, shape, strength, and magnetic characteristics of a magnetic anomaly can be used to make inferences about the feature causing the anomaly. Magnetic anomalies are frequently classified as either dipolar or monopolar, which can be used as a basis for describing various anomaly classes (Burks 2004a). Dipolar anomalies have distinct negative and positive poles which are visible on gradiometric maps as localized, adjacent areas of increased (positive) and decreased (negative) magnetic field strength. Strong dipolar anomalies are often caused by the presence of ferrous metal objects, such as historic debris. Dipolar anomalies can also be caused by the presence of ferrous sediment and/or rocks that have been heated in place to a high temperature.
Monopolar anomalies have only a single “pole,” discernable in a magnetic map as a localized area of increased or decreased magnetic strength. These anomalies are typically caused by variations in the amount of magnetically susceptible material and may be interpreted as storage/refuse pits, basin structures, or hearths.
Typical gradiometers used for archaeological survey allow relatively rapid collection of data (Kvamme 2006). The gradiometer is carried by an operator along a series of transects at a constant speed, collecting data automatically. Data are downloaded from the instrument and used to create magnetic maps of the surveyed area. These maps are processed with software to bring out the anomalies of interest and aid in interpretation.
Bevan, Bruce W.
1998 Geophysical Exploration for Archaeology: An Introduction to Geophysical Exploration. Midwest Archaeological Center, Special Report No. 1. United States Department of the Interioir Natioanl Park Service Midwest Archaeological Center, Lincoln Nebraska.
2004a Searching for Fort Pickawillany: Large-Area Geophysical Survey at a Mid-Eighteenth Century English Trading Post in Miami County, Ohio. Contract Report 2004-24. Ohio Valley Archaeological Consultants, Columbus, Ohio.
2004b Using Geophysical Survey in Cultural Resource Management in Ohio: From Projects Large to Small and Historic to Prehistoric…Recent Results from the Field. Presented at the Ohio Archaeological Council Fall membership meeting, Boonshoft Museum, Dayton, Ohio.
2000 Seeing Beneath the Soil: Prospection Methods in Archaeology. Reprint of 1990 edition. B.T. Batsford, London.
Kvamme, Kenneth L.
2006 Magnetometry: Nature’s Gift to Archaeology. In Remote Sensing in Archaeology, edited by Jay K. Johnson, pp. 206-233. University of Alabama Press, Tuscaloosa.