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Geophysical Prospection Techniques

Geophysical Prospection Techniques

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There are a number of non-destructive (i.e. without excavating) methods to study archaeological sub-soil features, labelled geophysical prospection techniques. Some of them are quite commonly used in archaeological survey projects, because –just as field-walking– they are a fast, non-invasive and relatively cheap way to collect archaeological data and to improve our knowledge of mapped surface artefact scatters.

This section is a short introduction into the three most frequently used techniques, starting with electrical resistivity/conductivity . This method is based on the principle that different sub-soil features (i.e. filled trenches, pits, structures, etc.) conduct electricity on different levels, mainly because of their respective degrees of moisture. Hence, a soggy, clay soil will conduct electricity perfectly, whereas a solid stone wall will hardly act as a conduit at all. A variety of instruments can be employed to send an electric current into the ground, receive it and measure either the resistivity or the conductivity encountered. Depending on the instrument and method of choice, these measurements can be plotted as a horizontal plan (most commonly), a vertical section, or combined into a three-dimensional model.

The second, frequently used geo-prospection technique is magnetometry. Its principles are rather more complex, but the basis is that subsoil features can form anomalies in the Earth’s magnetic field, as the magnetic properties of different materials are highly variable. Magnetometers can measure these properties and produce data, which can, again, be plotted on a map.

Ground-Penetrating Radar (GPR ) is a technique which uses the principles of radar technology. By transmitting radio waves through the ground surface and re-collecting the signal, it is possible to record the time it takes the waves to bounce on different subsoil features, and therefore their depth and basic characteristics. However, GPR results are notoriously hard to read and it requires highly specialized skills in geophysics and computer modelling to understand all variability and possible methodological biases.

The choice for any of these techniques depends on the character of the survey area (for instance, electrical resistivity requires a certain humidity; magnetometry can be highly biased by the presence of large metal objects (e.g. electricity poles), etc.), as well as the availability and costs of geophysical instruments and specialists. It is absolutely vital to have a proper understanding of the potentials and limitations of the different methods before applying them in the field, or interpreting the data they have gathered.

Bibliography and further reading

Clark, A., 1996, Seeing Beneath the Soil: Prospecting Methods in Archaeology (2nd ed.), London.

Sarris, A. & R. Jones, 2000, ‘Geophysical and Related Techniques Applied to Archaeological Survey in the Mediterranean: A Review’, in: JMedA 13.1, pp. 3-75.

Research topics: Survey Methodology