Magnetic Exploration of Archaeological Sites

Abstract

Many archaeological structures, features, and artifacts can be detected with a magnetometer because the iron that they contain has been changed to a very magnetic form by some cultural process. Examples include the conversion of nonmagnetic iron ores into metallic iron, the modification of nonmagnetic clay into magnetic brick or ceramic inside a kiln or fireplace, or the creation of magnetic soil by biological action within organic soil. Additionally, some objects are detectable because they have been moved from their natural location by humans; this movement can result in an artificial concentration or depletion of magnetic materials: Examples include igneous stones (magnetic) that form the foundation of a building or nonmagnetic stones, such as limestone, in the foundation of a building where the soil is rather magnetic. A ditch that was dug into nonmagnetic subsoil, later to be refilled with magnetic topsoil, is another example. A magnetic survey locates features that are either more magnetic or less magnetic than their surroundings. Some practices for doing magnetic surveys are summarized here, along with information about understanding the magnetic maps that are created. Most of this information is for the person who is starting to do geophysical surveys. This section is written for the archaeologist, rather than the physicist. It is assumed here that either you will be doing this magnetic survey, or that you want to learn how to ask someone to do the survey for you. This discussion will emphasize Roman and Greek archaeology in the Mediterranean area. For a thorough review of the application of magnetic surveys to archaeological exploration, see the book by Aspinall et al. (Magnetometry for archaeologists. Altamira Press, Lanham, 2008).