Geophysical methods in archaeological prospection

Magnetic map, Ullastret (Girona, Spain)

Magnetic prospection

The magnetic prospection is the most widespread geophysical method for archaeological investigation. Using multi-sensor arrays of fluxgate gradiometers large areas can be surveyed in short time. Diverse types of objects and structures are detectable: pits, ditches, walls and foundations, fireplaces, ovens and furnaces. The contrast of the magnetic susceptibility of the searched material in relation to the surrounding soil or rock is decisive for the detectability. Flexible instrumentation allows the use of magnetic prospection in open and flat regions as well as in hilly and mountainous areas. The arrays can be equipped with 4 to 10 or even more probes and can either be moved on wheels or carried by hand.

GPR depth slice, Tejada La Vieja (Huelva, Spain)

GPR prospection

The GPR (Ground-Penetrating Radar) or georadar is based on the propagation of high-frequency electromagnetic waves into the ground. The georadar waves are reflected and refracted at layer boundaries. Thus, walls and foundations, pipelines, cavities and other inhomogeinities can be detected. The register of travel-time differences and amplitudes of the electromagnetic waves provides information on the position, depth and specific properties of buried objects and layers. Spherical resolution and depth of penetration depend on both the GPR antenna’s frequency and the electromagnetic properties of the ground. Broadly speaking, the higher the frequency, the better the spherical resolution, but with diminishing depth of penetration.

ERT profiles

Electrical resistivity tomography (ERT)

Resistivity or geoelectrical surveys result in an image of the subsurface resistivity distribution. Knowing the resistivity of different material types, it is possible to convert the resistivity image into an image of the ground allowing the identification of geological and archaeological structures. Resistivity measurements are conducted by injecting an electrical current (I) into the ground via two current electrodes. The resulting voltage difference (U or ∆V) is measured at two potential electrodes. From the current (I) and the voltage differences (∆V) the apparent resistivity ρa is calculated using Ohm's Law and the geometric factor K which describes the electrode configuration. Resitivity surveys are recommended if structures of greater depth (> 3 m) are to be explored. The method also is suitable for surveys in complex urban environment.