GNGTS 2015 - Atti del 34° Convegno Nazionale

while the other in the 1.6-2.3 GHz one. In the tow R.Ex., the two antennas are aligned with the truck: at the head the medium frequency antenna is positioned, while on the back the higher frequency one, distant from each other 0.61 m. Depending on the objectives of the investigations and the depth of the targets, we tested that the 2.3 GHz with the 800 MHz couple is used for very high resolution investigations and shallow objectives (determination of the centimetric thickness of aggregate layers), reaching a maximum depth of about 30 cm for the 2.3 GHz and 1m for the 800 MHz. The combination of 1.6 GHz with 500 MHz antennas is generally used to detect structures reaching a depth up to 2 m, like the base and subgrade strata for the 500 MHz antenna, and about 50 cm for the antenna of 1.6 GHz. This is not a standard rule since, as for any practical GPR application, the performance must be tested in situ in order to verify the actual penetration depth and resolution. One clear advantage of the ground coupled antennas system is the better signal penetration compared with that of the air-coupled ones; another advantage is the better focusing respect air- coupled antennas, that allows an improved pavement cracks or cables detection (Saarenketo, 2009). Laboratory analysis performed on asphalts cores during the R.Ex. system implementation, let us to determine the maximum elevation of the antennas from the ground to obtain the best tradeoff between GPR performances and logistic effectiveness. The accurate positioning of the GPR data is the most important factor during the data collection phase. Data with incorrect spatial referencing are worthless to the customer and damaging for engineering applications. Positioning can be done using an encoder, that controls the trace interval, or/and using GPS devices; in addition markers at known reference points can be fixed during the data acquisition. The R.Ex. system allows a combination of the previous methods allowing a better control and location of the GPR traces. We install a RTK mode GPS with centimetric precision connected with the GPR control unit, so for each trace of both antennas, we have the coordinates(X, Y and Z). This procedure allows us to geographically locate some defects of the roads, locate specific points of relevant thickness variations, and to associate the position of a drill core to a specific trace, which is useful and necessary for an accurate calibration of the EM velocities. Processing and data production. The processing sequence is aimed at enhance the signal to noise ratio by reducing ringing and low frequency components (DC), as well as the high frequency ones, after the determination of the actual time zero (drift removal). In the following case histories we adopted a standard processing sequence encompassing: background removal, high-pass filtering, true amplitude recovery and depth conversion. A static correction has not been applied due to the sub-horizontal topographic surface and the sub parallel nature of the imaged horizons. In relation to the evaluation of the reliability and accuracy of the obtained results, one of the most critical points is the degree of confidence of the thickness values of the different Fig. 1 – R.Ex. GPR system and its components connected to a van during a survey. 108 GNGTS 2015 S essione 3.2