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Courtesy of Archaeological Services University of Durham (ASUD). Copyright ASUD.
 
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Magnetometry

Magnetometry is the technique of measuring and mapping patterns of magnetism in the soil. Ancient activity, particularly burning, leaves magnetic traces that show up even today when detected with the right equipment. Buried features such as ditches or pits, when they are filled with burnt or partly burnt materials, can show up clearly and give us an image of sub-surface archaeology.

Soil is a complex build-up of material that is affected by many types of past activity. One of the most common is burning: this can take place for a variety of reasons - deliberately or accidentally; in one location or spread across an area of crop or woodland. Burning permanently changes the magnetic properties of the surrounding soil by altering the magnetism of tiny iron particles. After this soil or stone has been moved, such as by ploughing, earthwork construction or ditch infill, this activity can be traced by looking for variations in soil magnetism against the general background of the earth's magnetic field. An area in which the soil has a slightly different magnetic orientation to the surrounding earth can indicate the presence of sub-surface archaeology. Archaeological features show up as higher or lower readings: deposits containing much burnt material (such as ditch fills) are usually higher; stone walls usually lower. Magnetometry can normally penetrate up to one to two metres in depth.

In some cases, the magnetic properties of the soil can be altered by bacterial action. This is most common in wet soil conditions, and therefore can also be useful archaeologically in locating old river channels, for example. Magnetometry only works when past activity has produced a measurable pattern of magnetic contrast. Graves, for example, rarely show up magnetically because they involve putting the same soil back into the hole very quickly after it was dug out (but cremations are very magnetic). Magnetometry is also not possible where there is non-archaeological magnetic contamination, such as metal fencing, iron refuse or traces of extensive modern burning.

An instrument known as a 'magnetometer' is required to measure and map traces of soil magnetism. There are various types of magnetometer, 'proton' magnetometers were the first to be widely used, 'fluxgate gradiometers' are most common at present; the latest 'caesium vapour' magnetometers are now making an impact.

The survey area is laid out in a regular grid, with each metre point marked on string lines. To avoid contaminating the readings, surveyors must be free of magnetic materials, so watches, rings and credit cards must be left away from the survey area. Their clothing must not contain metallic zips, buttons, studs or other such fastenings. Readings are usually taken every metre or half-metre and then down-loaded from the magnetometer onto a computer and plotted using a special graphics programme. The results are usually plotted in squares representing the survey grid, with magnetic variations being represented by darker or lighter colours. When all the grid squares are joined together, they give a full area picture of patterns of magnetism, which can then be interpreted archaeologically.

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PREHISTORIC BURIALROMAN PERIOD FARMANGLO-SAXON ROYAL PALACEMEDIEVAL VILLAGEMEDIEVAL CASTLEPOST-MEDIEVAL LEAD WORKINGTWENTIETH CENTURY COAL MINE