# Exploring Ephemeral Features with Ground-Penetrating Radar: An Approach to Roman Military Camps
## 3 Results
### Sige of Cerro de Castareno  
Since the magnetometer did not give good results in the Villa Maria sector, the GPR survey was vital in defining the archaeological structures here located. This situation resembled our first geophysical survey in Carrecastro in 2017, where magnetometry could not document the perimeter defences of the camp [31]. In Villa Maria, the defensive ditches are visible in the GPR data at a shallow depth (ca. 50-60 cm below the ground level) (Figure 11). The surrounding ground matrix, excavated in the hard limestone rock, contrasts with the backfilled ditches. Therefore, the latter is visible until ca. 190 cm below the ground level. This sedimentation process is supposed to have taken place quickly after the defeat of the inhabitants of the Late Iron Age _oppidum_ of Cerro de Castarreno. One of the most exciting results of the GPR survey was discovering a ditch section that was not visible in aerial photography. Initially, we thought that the small military camp in the area of Villa Maria was attached to the inner structures of the _contravallatio_, using these defensive lines to protect its north-western flank. Nevertheless, the camp defences continue towards the northwest, intersecting the double ditch. The morphology of the two structures is very similar, and both were detected at similar depths, so we cannot determine which one was built first at this stage. The synchronicity of the two elements seems unlikely (Figures 11 and 12). One possible explanation for this phenomenon is that the camp was built before the siege lines and that the latter reduced the size of the former during a second phase. Although possible, the option of an extension of the camp seems unlikely. The second hypothesis is that the siege lines were erased and backfilled in a second phase corresponding to the construction or enlargement of the camp. In this case, we should be able to appreciate differences in the deposits clogging the different sections of the ditches in future excavations. Results were also good in the Los Andinales sector (Figures 13 and 14). Once again, we initially assumed that the siege lines would connect with the camp ditch. The upper section of the ditches starts to be visible at 50 cm below the plugughed horizon, and these are detectable until ca. 120 cm deep. Interestingly, the GPR slices show a 10 m long gap between the two elements under analysis. In theory, the interruption may not make any defensive sense, but it could be related to the presence of a gate. Given the enormous effort to build the siege complex, it seems unlikely that the Romans considered soil hardness a limiting factor. Figure 10: Voxler visualisation of the GPR survey at Sueros de Cepeda. Arrows point to the camp’s defensive ditch. Figure 11: Selection of depth slices at Villa Maria, with the indication of the interpreted lines of the siege system. Red lines mark the location of the excavation. EPSG: 25830. Figure 12: Voxler visualisation using Face Render of the double ditch system and the camp’s ditch at Villa Maria. Figure 14: Voxler visualisation of the double ditch and the camp trench. Arrows point to the low amplitude values that indicate ditches. Figure 13: Selection of depth slices at Los Andinales camp. Note the double ditch and the black lines resulting from the interpretation of the siege scenario in aerial photography. Red lines mark the location of excavation trenches. EPSG: 25830. The visualisation of the volumetric data (Figure 14) also provides good information to detect the different characteristics of the trench fillings. The volumetric visualisation cannot provide accurate information about the trench profiles, but the excavation shows characteristic V-shaped solutions [17] (see the location of the trench in Figure 13, and excavation in Figure 15d. In this study case, the survey aimed to spot features that, in some cases, appear parallel to the path followed by the GPR sensor, which is an obstacle to achieving the highest data quality.