Georadar - Principles and applications

The georadar (GPR for Ground Penetrating Radar ) is a non-destructive geophysical prospecting technique based on the analysis of propagation phenomena (refraction, reflection and diffraction) of high-frequency electromagnetic waves (10 MHz to 2 GHz) in the subsurface. Georadar, initially of a pulse nature, is based on the excitation of the subsurface by a train of short-duration pulses (1 to 50 ns) from a transmitting antenna, in order to detect, with the aid of a receiving antenna, successive echoes associated with contrasts in permittivity or conductivity encountered by electromagnetic waves during their propagation. These contrasts indicate the presence of buried targets or subsoil stratifications. The use of frequency georadar is much more recent, due to the instrumental constraints associated with it, and is the subject of a significant amount of current research.

It is the movement of the radar on the surface or in the ground that makes it possible to acquire traces (radar slices or "scans") over a time window, and to form radargrams (or radar images) of the structure of the subsoil. A distinction is made between applications designed to detect objects or anomalies, and those aimed at determining the intrinsic properties of the subsoil. The applications are numerous: geology, hydrology, glaciology, environment, mining prospecting, neotectonics, archaeology, civil engineering... These applications include locating buried metallic or non-metallic objects such as cables, pipes, foundations, reinforcement, cavities, weathered areas and mines, and characterizing the intrinsic properties of geological materials (soils, rocks) or artificial materials (concrete, asphalt or wood). Each type of application requires a specific experimental set-up (reflection or transmission acquisition, spatial sampling, 2D or 3D mapping, nominal excitation frequency, etc.) and associated processing of the raw signals (filtering, migration, data inversion) in order to reconstitute a model of the subsurface. Improving detection with a ground-penetrating radar system currently involves the development of new signal processing and tomography techniques. We present here the various steps involved in defining the optimum acquisition parameters for georadar prospecting.