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
Return to Geophysics.