## 5 Discussion  
Electrical DC and electromagnetic (GPR) techniques were applied to an archaeological framework reconstructed in a laboratory to test the capability of geophysics to identify some objects characterized by different materials (stone, bricks, mortar, etc.) and shapes (rectangular, circular, irregular, etc.) placed in lacustrine and very wet scenarios. The aim of the experiments was to analyze the limits of the geophysical techniques, as GPR and ERT, when the archaeological structures are below the water level or close to the water table (high water content). The experiment was realized in two distinct phases: in the first one (WP1), ERT and GPR analyses were performed in different scenarios characterized by an increasing water content (WL1-WL2-WL3). The surveys were carried out adopting 3D GPR and ERT. The results have demonstrated the limits of the two methodologies for archaeological issues when the remains are below the water table (high saturation degree). Regarding GPR surveys, in all the analyzed cases, the influence of the water on the propagation of the em pulses was evident. In Figure 15, the results obtained at the frequency of 400 MHz in the three different scenarios analyzed in WP1 are compared. A strong decay of the radar amplitude signals with the increasing of the groundwater level is clearly recorded. Figure 14: 2D ERTs obtained on S1 and S2 with floating (**a**,**b**) and underwater (**c**,**d**) electrodes using the dipole–dipole Array (the inversions are performed with ResIPy). Further in the first two cases (WL2 and WL3), characterized by a lower water content in the vadose zone, the electromagnetic reflections allow to identify all the buried structures (Figure 15c-f) while in the full saturated case, the GPR anomalies are less evident (15g-h). Figure 16 shows the frequency spectrum of the mean trace for the radargram acquired above the wall and road (S2). In WL2 and WL3 (Figure 16a,b) the signal spectra are almost identical with a limited downshift for the wetter case (Figure 16b). On the contrary, the spectrum in WL1 (Figure 16c) suffers a substantial variation towards the lower frequency where the biggest variation is evident, and the archaeological structures presents are more difficulty to be identified. The frequency centroids estimated for the three cases were equals to 484 MHz (WL3), 406 MHz (WL2) and 348 (WL1) in agreement with the expected signal attenuation in a lossy medium which is affected by absorption and dispersion phenomena able to cause a decay of their amplitude [100]. However, the 3D reconstructions obtained with the GPR data have demonstrated a great capability to identify all the structures in WL2 and WL3; in WL1, where the attenuation is greater, it was impossible to detect those objects placed at the greater depths or not perpendicular to the adopted acquisition grid. It is worth noting that some difficulties are also encountered with the drier scenario (WL3) where the results are noisier because the inadequate compaction of the soil and the scarce permittivity contrast did not allow to reconstruct the real shape of the archaeological structure, that are better focused in WL2 where the water content is higher; therefore, the expected geophysical contrast is higher. Figure 15: Comparison of the radargrams acquired in correspondence of the S1 (**a**) and S2 (**b**) that represent the main sections of the test site (**c**). The radargrams acquired in WL3 (**c**,**d**), WL2 (**e**,**f**), and WL1 (**g**,**h**) allow to detect the different structures (anomalies in red). In WP2 (underwater scenario), GPR showed encouraging results in presence of non-ionic solutions giving optimal information about the distribution and geometrical shape of the structures despite the attenuation limits (strong variation of the spectrum frequency characterized by a centroid value of 287 MHz). Thus, GPR confirmed its usefulness for the archaeological research also in lacustrine contexts as support of the well-established sound acoustic techniques allowing to obtain a higher resolution.