## 3 Results  
Results of the performed surveys and of the different stacking attempts are reported in the following in order to understand which is the better approach to improve data quality. First, we show how the in-line and off-line stacking improves the GPR signals of single example traces. Four example traces are shown in Figure 3 to this aim: Figure 3A reports an example GPR trace after processing; Figure 3B reports an example GPR trace in the same position of the previous, obtained with off-line stacking of five laterally adjacent traces; Figure 3C reports an example GPR trace in the same position, obtained with in-line stacking of five adjacent acquired traces in the profile direction; and Figure 3D reports an example trace after both off-line and in-line stackings. Figure 3: Examples of signal improvement thanks to in-line and off-line stacking: (**A**) example trace after processing and before any stacking; (**B**) trace after off-line stacking of 5 laterally adjacent traces; (**C**) trace after in-line stacking of 5 adjacent acquired traces in the profile direction; (**D**): trace after both in-line and off-line stackings. The zone of the traces in which the improvement is discussed is reported with a shaded yellow area. It can be observed that in the non-stacked trace after processing, the noise after about 15 ns is of the same order of magnitude as the signal (shaded yellow area). This strongly compromises the investigation depth and, as mentioned, is caused by the presence of highly attenuating clay materials at the site [19]. Both off-line (Figure 3B) and in-line (Figure 3C) stackings manage to partially reduce this noise, and the related traces show useful signal reflections where in the non-stacked trace they were barely visible, consequentially improving the investigation depth. Nevertheless, off-line stacking (Figure 3B) appears to offer an improved quality of the example trace in the same time range, with an increased attenuation of the high frequency noise and general better visibility. In order to provide a quantitative estimation of this improvement, the signal-to-noise ratio (SNR) of the different data reported in Figure 3 was computed. The SNR is the ratio of the signal summed squared magnitude to that of the noise. Coherently with the qualitative considerations performed, an increased SNR was observed in the off-line stacked traces (SNR = 9 dB) with respect to both the non-stacked traces after processing (SNR = 6 dB) and the in-line stacked ones (SNR = 8 dB). Further increase in the SNR can also be obtained by applying both in-line and off-line stacking approaches (SNR = 11 dB); however, this would compromise the resolution, as will be discussed later. Similar considerations have been observed along different portions of the acquired GPR volume and in different example traces. We judge that the off-line stacking appears to be more suited for the suppression of the noise and to increase the investigation depth. Indeed, apparently the stacking of laterally adjacent traces results in an increased SNR and more coherence of the finally obtained trace with respect to the stacking of consecutive acquired traces, merging the advantage of using different acquisition channels in the same portion of the investigated volume. Another indubitable advantage of the off-line stacking with respect to in-line stacking is related to the different resolution in the profile direction that the two approaches can offer. This is represented in Figure 4 where a comparison of some consecutive GPR traces after processing (Figure 4A) and the same after off-line stacking (Figure 4B) is reported. Again, an improvement in the quality of the traces can be observed in a similar time range than before (shaded yellow area). Apart from increasing the investigation depth, this also allows for better evaluation of the continuity of reflections among different traces. The same result cannot be obtained with in-line stacking because, within the distance analyzed in Figure 4, all the observed traces would collapse in a single one.