## 3 Results
Figure 4: Examples of signal improvement thanks to off-line stacking: (**A**) example traces after processing and before any stacking; (**B**) traces after off-line stacking of 5 laterally adjacent traces. The zone of the traces in which the improvement is discussed is reported with a shaded yellow area and with a black dashed line. In addition, traces reported in Figure 4 also partially explain why the in-line stacking appears to be not as good as the off-line stacking in improving the trace quality and increasing the SNR. It can be indeed noted that the reflection event at around 20 ns (black dashed line) appears with a peak amplitude, which is slightly varying in the profile direction. Therefore, the in-line stacking of these traces would not result in a trace as clear as that of the off-line stacking, as previously commented with respect to Figure 3C. After assessing the goodness of off-line stacking on single example traces, we repeat the same comparison on whole radargrams. Example radargrams in different portions of the investigated GPR volume are reported in this respect in Figures 5-7. Following the same presentation approach as before, in all the figures, radargrams after processing and before any stacking (panels A) are compared with the off-line stacked ones (panels B), the in-line stacked ones (panels C), and radargrams after both off-line and in-line stacking (panels D). Figure 5: Examples of radargram improvement thanks to stacking within progressive 70 and 100 of the whole GPR volume: (**A**) radargrams after processing and before any stacking, (**B**) radargrams after off-line stacking, (**C**) radargrams after in-line stacking, and (**D**) radargrams after both off-line and in-line stacking. The area in which the improvement is discussed is evidenced with a green rectangle. We can observe that the whole radargram highly benefits from off-line stacking in the 15 to 25 ns time range (green rectangles) and that the improvement of the quality of the radargrams is significantly better with the use of the off-line stacking with respect to the other stacking approaches. Moreover, the random noise starting at about 10 ns is also greatly reduced. Especially, specific reflecting features are more visible in the off-line stacked radargrams than in the others, e.g., around progressive 90 m in Figure 5, 515 and 525 m in Figure 6, and between progressive 550 and 570 m in Figure 7, confirming the increase in the potential investigation depth. In all the figures, it can also be further evidenced that the main problem of in-line stacking is that, reducing the number of traces, it easily worsens the clarity of 2D radargrams (as an example, see comparisons of Figure 5B,C around progressive 90 m and of Figure 6B,C around progressive 525 m). Comparing in-line stacking and off-line stacking  
Figure 6: Examples of radargram improvement thanks to stacking within progressive 500 and 530 of the whole GPR volume: (**A**) radargrams after processing and before any stacking, (**B**) radargrams after off-line stacking, (**C**) radargrams after in-line stacking, and (**D**) radargrams after both off-line and in-line stacking. The area in which the improvement is discussed is evidenced with a green rectangle. along radargrams again suggests that the latter is the better improvement option. The combination of both stacking options (panels D) is better than just in-line stacking, but the resolution loss is still problematic to accurately detect buried structures and specific reflection features. The different stacking approaches are finally evaluated on time-slices, obtained by interpolating all the 2D radargrams coming from the different stacking options into different 3D data volumes. Example time-slices of the different approaches are presented at two different times: in Figure 8 at 14 ns and in Figure 9 at 18 ns.