## 3 Materials and Methods
### Geophysical Surveys  
Geophysical surveys were performed with the use of the SIR-3000 TerraSurveyor (GSSI System) ground penetrating radar (Figure 4a) and Syscal Pro Switch 96 (Iris Instruments) georesistivimeter (Figure 4b). Data acquisition and processing were set considering the two different scenarios related to WP1 (Figure 4a,b) and WP2 (Figure 4c,d). In detail, GPR surveys were carried out in WP1 using a reference grid where the distance from each line was equal to 0.20 m and the investigations were made in both the main directions (red arrows in Figure 5a). In WP2, the presence of the water did not allow the 3D acquisition and only the two sections S1 and S2 were investigated (Figure 5c). In this case, the antenna was placed on a small boat dragged with pulleys fixed at the concrete wall of the pool to allow for the collection of the data without the use of an odometer. A marker every 0.075 m was imposed for assigning the real position to the acquired traces. All the data were acquired in continuous and reflection mode with the two-time window of 70 ns at the frequency of 400 MHz. The scan samples were set at 512 with a resolution of 16 bits and a transmission rate of 100 KHz. Figure 3: Two WPs were investigated: (**a**) WP1 with the water table placed at three different depths (respectively fixed to 0.20 m (WL1), 1.00 m (WL2) and 1.50 m (WL3); and (**b**) WP2, with the archaeological site covered by a water column of 0.3 m. The numbers indicate the distances in meters. GPR data processing consisted of the use of some basic operations, including band-pass filters, gain function removal, amplitude compensation, background removal filter, and Kirchhoff migration. To migrate the data, the em velocity evaluation was performed, studying the diffraction hyperbolas generated by the archaeological features that was constantly varied due to the raising of the water table. In order to detect and reconstruct the archaeological remains, after the examination of the individual radargrams, 3D data volumes were created and some significative time slices, converted in depth slices, were extracted to identify the buried structures. In WP2, it has been necessary to add a further step for the data editing, including the association of the real position to the traces acquired without the use of the odometer. Regarding the ERT acquisitions in WP1, 3D ERTs were performed with the use of two different types of 3D arrays. In detail, a 3D acquisition based on a grid of 96 steel electrodes (pink circles in Figure 5a) distributed on an area of 7 \(\times\) 4 m was adopted. First, 3D pseudo resistivity data were collected using dipole-dipole array with in-line, parallel-line and diagonal-line dipoles; the electrode spacing was 0.60 m in both the directions. Further, a 3D non-conventional array with a loop shaped (loop-loop array) was used: both current (C+ and C\(-\)) and potential (V+ and V\(-\)) electrodes were distributed along the entire site surface according to four concentric rings (Figure 5b). In the outer ring (red circles), the electrodes were placed at a reciprocal distance of 1.00 m; in the second ring (magenta circles), the mutual distance was 0.75 m; for the last two inner rings, the distances were 0.50 (green circles) and 0.25 m (blue circles), respectively. In WP2, where underwater scenarios have been simulated, 2D ERTs were carried out with electrodes placed both on the underwater floor and on the water surface by a  
Figure 4: GPR Sir 3000 and 400 Mhz antenna with survey wheel: (**a**) loop–loop ERT acquisition system (**b**) performed in WP1 (unsaturated scenarios), 400 Mhz GPR antenna on a small boat, and (**c**) a 2D ERT profile with floating electrode (**d**) in WP2 (underwater scenario). floating system, which was realized ad hoc. In addition, only the S1 and S2 sections were investigated (Figure 5c).