The GPR surveys are designed to return information on the material layering underlying the surface investigated and its condition indirectly. The tool consists in a radar system that allows to investigate repeating electromagnetic pulses waves (20 MHz – 3 GHz) that last a few nanoseconds. These electromagnetic pulses reach the reflective discontinuity surfaces (positioned at a depth of some centimetres up to a few dozen meters) and are reflected therein. A wideband antenna receives the signal and the time taken to reach every reflective surface is recorded. By performing the transmission and moving the antenna along the surface of the investigating area, the response represents the continuous profile of the reflecting surface, and so you have the substrate arrangement too. The operating mechanism is based on the fact that the GPR is a video system wideband radar which has a Gaussian shaped pulse whose duration can be expressed by the transform frequency spectrum. The reflection of each spectral component is determined by the constant and the conductivity of the crossed medium. So the impulse, which crosses various layers, is partly reflected and partly transmitted to every discontinuity. Each medium included between two interfaces can be identified according to the way it is crossed by these pulses.
WALL-COATING SURFACES ANALYSIS
If GPR analysis is applied to the wall-coating surfaces (regardless of the construction system) you can have two levels of deepening depending on the needs. The first level consists in simply identifying walling cavities or detachment zones between overlapping parameters. The second level, more expensive, leads to the reconstruction of complete sections of the walls along all the detected sections, identifying the thicknesses of the inner walling, the vacuum areas, fillings with heterogeneous materials and any discontinuity.
TUNNEL WALL COATING ANALYSIS
The GPR survey allows to examine:
- Coatings thickness;
- The presence of any cavities in the extrados;
- Position, depth and type of centrings;
- Coating discontinuities
The adopted methodology is generally like the one described above, except the use of different power stations, set on self-propelled vehicles suitable gallery size. The result of these investigations is of prime importance because it provides informations needful to any consolidation and extraordinary maintenance operation.