## 4 Survey Plan  
The instrument used to collect the data was an ADD-01 (see Acknowledgments) which is composed of two lightweight, portable boxes connected by a wireless radio frequency device. The boxes include a current generator and a measurement and control unit, respectively. It is distinguished by a 50 W low-power generator and was designed mainly for archaeological research down to a few tens of meters of depth at most. The frequency at which a current sine wave is generated can be chosen from 8 to 33 Hz. On the control unit, the exciting current intensity can be adjusted between 1 and 400 mA. The acquisition board has a bandpass filter to remove unwanted noise. The yellow polygon in Figure 2a and the green polygon in Figure 3 outline the investigated area at Saepinum (2014-2016 field campaigns) and the Villa of Neratii (2008-2011 field campaigns). Figure 5 shows the electrode array used for the tensorial resistivity measurements. The current electrode distance was fixed to 10 m and two perpendicular directions were used (A1-B1 and A2-B2). Potential electrodes M1-N1 and M2-N2 (with an equidistant space of \(\Delta_{\mathrm{x}}=\Delta_{\mathrm{y}}=50\) cm) were placed in the central rectangular area between the current electrodes. Once a grid was completed, the pattern was moved into the investigation area without overlapping with the previous one. In total, 258 tensors were obtained for each grid. The white polygon in Figure 3 indicates the sample grid moved in the investigation area in order to cover the surface bordered by the green polygon at the Villa of Neratii. Figure 5: The electrode layout with current electrodes A1-B1 and A2-B2 and potential electrodes M1-N1 and M2-N2. Once the grid is completed (black rectangle), the electrode pattern is moved to the adjacent grids (grey rectangles) in the investigation area without overflow between the previous one. The major axis is perpendicular to the decumanus and the theater scene with which it has similar dimensions. The structure is located to the northeast of the theater and the porticus following the same orientation. The resistive anomaly is characterized by two interruptions along the external perimeter and a highly conductive internal area. The interruptions visible in Figure 7b-d are located both on the long side (northwest) and on the short side (southeast), suggesting the presence of two distinct accesses. Internally (about 394 m\({}^{2}\)) the area does not appear to show a paved floor but a homogeneous ground, a possible beaten surface. Considering the size of the structure, its location, and its orientation, two hypotheses are proposed on the possible function of the structure:  
* It could indicate the presence of a theatre/odeion, oriented northeast/southwest, preceding the one currently visible oriented northwest/southeast (Figure 8a). This supposition could be justified by the need to create a new structure, which could contain the increase in the number of users following a demographic increase in Saepinum that the previous theatre could no longer accommodate. * It could represent a single monumental complex belonging to the same phase as the theater and the porticus. In this regard, it is appropriate to analyze the result taking into consideration an inscription reused as a paving stone or as an entrance threshold of the proscenium of the theater which contains the following text: [.H]erentinius M(arci) (filius) Obellianus/[ca]mpum piscinam porticum (sua) p(ecunia) f(ecit) [51]. The information reports the existence in Saepinum of a large complex including a campus, a swimming pool, and a porticus, presumably located behind the theater building [47]. The campus-swimming pool-porticus complex therefore appears to have been designed at the same time as the theater and includes a space where gymnastic exercises and gladiatorial games took place; a U-shaped portico that hosted theater spectators during intervals and in the case of bad weather; and a garden in which a fountain made with recycled materials was placed in the late imperial era.