Archaeological geophysics, gives us a group of techniques that can be used to detect buried archaeological remains. Some of the more common ones include magnetometry, magnetic susceptibility mapping, resistivity and ground-penetrating radar (GPR). These are known as remote-sensing or prospection techniques. They help archaeologists locate buried structures and deposits and are very useful for helping to target excavation areas.
From experimental beginnings in the 1950’s and 1960’s, geophysical techniques have grown into standard archaeological field practice, both on research and development led projects. Still one of the fastest-developing aspects of field survey, it relies heavily on computer technology to translate electrical and electronic measurements taken in the field into maps and plots that can be interpreted archaeologically. The areas which can be surveyed effectively using geophysics have grown from a few square metres in the 1950s to many thousands of square metres today, making it an important tool for investigating landscapes as well as individual features and sites.
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.
In some cases, the magnetic properties of the soil can be altered by bacterial action. 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.
Resistivity is a form of geophysical survey where electrical current is passed through the ground. Electrical resistance in the soil varies, and is affected by the presence of archaeological features. The patterns of resistance in the soil are recorded, plotted and interpreted.
This method uses a resistivity meter, which is a square frame with two or four metal points (electrodes) sticking downwards, 50 or 100 centimetres apart along the base of the frame. These are inserted into the soil a brief pulse of weak electrical current is passed between them; the level of resistance (measured in ohms) is recorded on a data-logger; these readings are later downloaded onto a computer which uses a graphics programme to produce a map or plot of the resistance patterns across the survey area.
All electrical currents are subject to some level of resistance – this can be very low in the case of copper wire or water, or very high in the case of air, pottery or stone. Electrical currents are conducted through the soil by mineral salts contained in water – the moisture content is therefore essential to the level of resistance in the soil.
Buried archaeological features with varying moisture content such as ditches and pits (often higher moisture) and stone walls (often lower moisture) stand out compared to the soil around them – and can therefore be detected in the survey. Once the patterns of resistance are plotted, it is often very clear where walls or ditches exist, even though they are invisible from above ground. This information can be used to complement other geophysical data, for example from magnetometry, or to target the location of excavation trenches.
Ground penetrating radar (GPR) is a variation on conventional radar, which sends a radio signal into the soil which is reflected by variations in density below ground. It is very good at detecting the structures of buried masonry structures, and can effectively show holes or voids below ground – it is therefore valuable in finding buried parts of buildings such as crypts, tombs or cellars, and showing up building foundations.
GPR is very useful to archaeology where there are voids and substantial deeply-stratified masonry remains, hence it is particularly valuable on urban sites, ditches, cave structures, ancient mines or large landscape features such as dry river channels, but it is rather less use in the softer and more finely-differentiated types of deposit encountered on many rural archaeological sites.
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2009 Magnetometry for archaeologists, Lanham
Conyers L.B., Goodman D.
1997 Ground Penetrating Radar, An Introduction for Archaeologists, London
2006 Geofizyka archeologiczna, Warszawa