Geophysical surveying techniques detect anomalies in the soil caused by the presence of buried features. Using these methods it is possible to determine the location and depth of such features and, by interpretation of the data, what they might be. If a detected anomaly is considered to be the result of human activity the information can be used to decide where or where not to excavate.
The two most commonly used geophysical surveying methods are electrical resistivity and magnetometer surveying.
Electrical Resistivity Surveying
An electrical current is passed through the soil between electrical probes and the resistance measured. This technique is based on the principle that different types of soil conduct electricity depending on how much moisture is present. The greater the amount of water present in the soil the better the conductivity. Buried features such as pits and ditches tend to retain more water than the surrounding undisturbed soil and therefore will show less resistance to the electrical current.
Conversely, ‘dry’ features such as buried walls will record a high resistance. An electrical resistivity survey is best suited for detecting ditches and pits in relatively dry soils such as chalk and gravel and masonry in wetter soils such as clay. In both cases there will be a marked difference in the conductivity readings for the features when compared with the surrounding soil.
The disadvantages of this method is that it can be slow given the need to set up a number of stationary and mobile electrical probes. It is not so effective for detecting anomalies that are deeply buried and results can be adversely affected if the soil is particularly hard or dry.
Magnetometer Surveying
This type of survey is undertaken using a mobile instrument called a magnetometer. It works by detecting small and local distortions in the magnetic field of the earth that can result from previous human activity. When soil is heated by the construction of hearths or pottery kilns for example, the iron particles that are present in the soil realign themselves from the normal random pattern to the direction of the magnetic field of the Earth. This realignment remain fixed even after cooling.
Anomalies such as pits and ditches can be detected because the disturbed soil will give a larger magnetic reading when compared to the surrounding soil. Buried walls and stone foundations will contain less magnetic particles than their surroundings and can be located on that basis.
Magnetometer surveying is good for detecting fired clay structures such as hearths, and pottery kilns. It does not require fixed probes in the ground and therefore is quicker than electrical resistivity survey, but it does not work well in urban environments where there are power lines or large amounts of metal present.
Comparison of the Two Methods
Both of these archaeological survey techniques have their strengths and weaknesses and each method is better at detecting certain types of buried feature. The best strategy is to employ both but this is dependent on time, cost and the location of the site.
Sources:
Colin Renfrew & Paul Bahn, Archaeology: Theories, Methods and Practice, fourth edition (Thames & Hudson, 2004)
Kevin Greene, Archaeology: An Introduction, fourth edition (Routledge, 2002)
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