# Exploring Ephemeral Features with Ground-Penetrating Radar: An Approach to Roman Military Camps
## 2 Materials and Methods
### GPR Survey Methodology  
GPR is a well-known geophysical technique in archaeological research [19; 20; 28; 29; 30]. When combined with other geophysical methods (i.e., magnetometer, electric resistivity, or electromagnetic induction), it has been proven helpful in studying Roman military sites [13; 26; 27; 31; 32; 33]. However, the spectacular results obtained in permanent military bases tend to make us forget that most archaeological sites related to the Roman army were ephemeral installations briefly occupied, i.e., marching camps, structures related to siege scenarios, construction camps, etc., [34]. In our case, solid stone structures or other elements designed to last longer periods were not expected. In the Iberian Peninsula, the application of GPR for studying sites linked with the Roman military expansion is rather restricted, although some specific study cases highlight the capabilities of this technique [26]. The implementation of GPR surveys in the study cases described in Section 2.2 took place after initial site plans were produced by other means. Therefore, the application of GPR was oriented to the investigation of specific features, aiming to assess the morphological characteristics of ephemeral structures preserved at those sites and to maximise our success in capturing information coming from archaeological excavations conducted right after the geophysics, either in the same area or as close as possible to the GPR surveyed area. To this end, we used a Noggin 250 MHz system from Sensors and Software mounted on a Smart Car by the same company (Figure 3 and Table 2). This system was first tested and systematically used in surveying the Roman city of _Segisamo_ (Sasamon). Although this  
Figure 2: Location of GPR surveys within case studies presented in the text: (**a**). Alto da Raia; (**b**). Sueros de Cepeda; (**c**). Los Andinales; (**d**). Villa Maria (background image: Google Earth). Black lines indicate the proposed outline of the Roman camps. Red areas delimit the GPR survey areas. EPSG: 25830.  
site will not be discussed in this paper, the technique has provided good results in a similar geological and pedological context to the Roman army siege scenario described below in Section 2.2.3[35]. The 250 MHz antenna provides a good equilibrium between resolution and penetration depth. Besides our first-hand experiments in the Sasamon area, [PERSON] et al. [36] offer a comparative perspective of different antenna frequencies and providers in which this GPR system is well-positioned. The survey was organised following georeferenced grids of various sizes set up on the ground with RTK GNSS instruments. These grids were surveyed only in the direction of the X-axis with a 50 cm separation between lines. The GPR raw data was processed in Ekko Project v.5 (Sensors and Software): wave velocity was calibrated using hyperbola fitting, and depth slices with a maximum resolution of 5 cm per pixel were eventually interpolated. The data filtering process mirrors the work performed successfully by other scholars using similar equipment [37; 38], including Dewow filter, gain correction, background removal, and bandpass filtering. Both visualisation and interpretation were made in QGIS 3.22. Another aspect that could be considered for future geophysical surveys of Roman camps is the possibility of visualising volumetric data using software such as Voxler (Goldersoftware) [39]. On this occasion, different visualisations were experimented with, namely slices cut through volumetric renders. Secondly, Face Render was used to isolate values related to the ditches' fillings, which appear as low amplitude reflections in the GPR data. Thanks to the Hilbert transform processing, the visualisation of subtle differences in