## 4 Discussion  
In all the comparisons performed within the paper (example traces, radargrams, and time-slices), we noticed an improvement, thanks to off-line stacking, in the quality of the results obtained. For the specific test site of Tindari, this improvement is especially evident in the 15-25 ns time range. Compared with the traditional in-line stacking, while also this last can be partially helpful to improve the general quality of the results, often it negatively affects the radargram quality. In fact, stacking n adjacent traces in the profile direction (five in this case) reduces the number of traces by a factor of n. Fewer traces consequently mean fewer pixels in the radargram, reducing its resolution. One possible solution to overcome this in-line stacking limitation would be acquiring more traces per meter to collect more data to be stacked. However, this is often not convenient with multichannel systems, which have to handle a large data flow and limit the acquisition speed according to the selected spacing between traces. In this sense, the off-line stacking seems to be, therefore, an excellent alternative to improve the radargram quality, especially at depth, without compromising acquisition times. It has to be considered that also off-line stacking is performed at the expense of the resolution, given that stacking n adjacent traces in the lateral direction (five in this case) reduces the number of traces by a factor of n and consequently worsens the lateral resolution. While this loss in resolution does not affect the radargram quality, it can potentially deteriorate the quality of time-slices depending on the dimensions of the desired targets to be imaged. Therefore, while suppressing noise, the off-line stacking can smear small, discrete targets or reduce lateral resolution, especially if the group size or channel  
Figure 9: Examples of time-slices improvement thanks to stacking at t = 18 ns: (**A**) time-slices after processing and before any stacking, (**B**) time-slices after off-line stacking, (**C**) time-slices after in-line stacking, and (**D**) time-slices after both off-line and in-line stacking. The area in which the improvement is discussed is evidenced with a green rectangle. spacing is large relative to the target size. However, in the specific situation in which the survey aim is the identification of archaeological remains of relevant dimensions (i.e., walls or structures), we noticed that off-line stacked time-slices are instead very clear and informative. In this respect, off-line stacking has the advantage of suppressing possible small anomalies and scattering and indeed improves the final image instead of deteriorating it. This can be considered true until the number of laterally stacked traces is limited such as to maintain at least a reasonable lateral resolution (i.e., generally <0.25 to 0.5 m for archaeological investigations). One problem worth mentioning is that off-line stacking can, nevertheless, worsen the data quality at shallow depths. Stacking five adjacent traces separated about 4 cm each means stacking something situated in the range of about 16 cm. Therefore, shallow targets can be misplaced or decrease in quality when applying off-line stacking (this problem also happens with in-line stacking). This problem is not clearly noticeable in the data reported in the paper since at the test site the buried features lie deeper, often at least at 30 cm of depth. In this sense, one has to be conscious that, for example, above 50 cm of depth, it is better to consider the non-stacked data, while below 50 cm of depth, it is better to consider the stacked data. The choice of this depth is arbitrary and seems valid for the Tindari test site, but may be reconsidered for other sites according to site-specific soil characteristics. From the results obtained in this study, we can therefore conclude that off-line stacking is a simple and viable option to reduce noise in multichannel GPR data, especially in challenging environments.