Ground Penetrating Radar (GPR) investigates reflection characteristics of a given material being under the influence of electromagnetic energy waves. The fundamental properties of the respective medium – in archaeology: the underlying soil and all objects included – are the electrical conductivity and the magnetic permeability. When properties change i. e. at the border of two bodies of different material the GPR energy is reflected partially.
A sufficient contrast regarding to the electrical properties of the different materials allows the reflections of electromagnetic waves to be received by an antenna.
To do a GPR survey two antennas - housed in one case - have to be moved across the area with a more or less continuous speed. One antenna transmits electromagnetic pulses into the ground, the other one receives reflected energy. Usually measurements are taken along profiles the data is sampled to measurements of some few centimeters of distance (e. g. 0.02 m). For archaeological purposes profile distance should be 0.5 m. When electromagnetic waves enter a material part of their energy is reflected at its interface and collected by a receiving aerial. Measurements are taken in a dense horizontal raster (0,02 x 0,5 m). GPR Data show a high resolution vertically and horizontally. While the soil can be surveyed down to several metres, resolution of the data decreases with increasing penetration depth.
GPR traces every kind of material occuring in archaeological features (generally soil and stone), but its use depends on the proper electrical properties of the soil (depending on season, hydrology etc.). It is especially useful for surveys at sealed areas like pavement and in the close vicinity of areas heavily disturbed by recent human activity.