GPR / Georadar GPR (Ground Penetrating Radar) is a non-destructive geophysical method used to detect subsurface structures and objects of archaeological or modern origin. Transmitting antennas radiate pulses of electromagnetic waves into the ground which are reflected and/or diffracted when changes in the electrical material properties, especially changes of permittivity (ε), occur. This mainly appears along layer boundaries, stones, walls or foundations, but also along anthropogenic features like cable lines and pipes that cause strong reflections. All reflections are registered by a receiver mounted on the antenna. Travel times of waves are mostly less than 100ns (10-7s), therefore the device can be moved relatively quickly whilst recording. There must be good surface contact between the antenna and the ground to ensure good penetration of the transmitted waves. Information about depth and character of anomalies are obtained from travel time and amplitude of signals. Skilled data processing necessitates special software and a profound theoretical knowledge about geophysical wave theories and methods.
Penetration depth and resolution of the GPR measurements depend on wavelength and frequency of the electromagnetic waves. Non-destructive testing (NDT) requires high-frequency antennas, whereas for geological and geotechnical investigations 100 MHz antennas are best suitable. Best results for archaeological prospection are achieved using 200 to 500 MHz antennas.
In some cases archaeological features and surrounding material have very little variances in their geophysical properties. To achieve clear-cut datasets it is important to clean the investigation sites as good as possible and assure surface contact of the antenna. Furthermore dense measuring grids and large connected areas increase data quality and the success of the measurement. Generally GPR is a good and reliable method to detect walls, stone settings, foundations, cavities or similar structures. Results (reflections) are displayed in radargrams, which are vertical sections of the ground, obtained for each profile measured. Because they display the broad archaeological situation much better, horizontal sections are chosen for visualization more often. Therefore all radargrams (best is a dense set of parallel GPR profiles) are transformed into time slices and displayed as horizontal sections of the ground. Simply said a set of vertical sections with physical information about travel times of electromagnetic waves is processed to give horizontal information incl. depth specifications of buried objects and structures.
GPR is a dependable tool for archaeological prospection that enables optimal excavation planning or even spares the latter. Furthermore it is possible to use high quality results (which always depend on the individual surface and soil conditions of the investigated area) for 3D models of virtual reconstructions.
GPR applications and data examples: