# Optimised Extraction of Archaeological Features from Full 3-D GPR Data
## 3 Methods  
As a test for the extraction of archaeological features, we used a full 3-D GPR collected in April 2018 with a 3-D MiniMIRA array GPR (Mala Geoscience) equipped with 5 transmitting and 4 receiving 400 MHz shielded antennas arranged in a configuration allowing for the \"simultaneous\" collection of 8 parallel profiles with a constant distance equal to 8 cm within a swath 56 cm wide. To optimize the spatial resolution, we set a trace spacing also equal to 8 cm, thus obtaining a constant inline and crossline spatial sampling. The system was connected with an electromechanical odometer for triggering and with an RTK GPS allowing for absolute positioning with centimetric accuracy. During GPR data acquisition, a photogrammetric survey was carried out using a DJI Phantom 4 Pro V.2 quadcopter unmanned aerial vehicle (UAV) equipped with a camera with a Sony Exmor-R sensor of 1\" and 20 Mpixel of resolution. The flight for the acquisition of the frames was performed in waypoints mode using the DJI Ground Station Pro software and was carried out at an altitude of 35 m AGL (above ground level). To ensure high overlap between frames, fundamental for photogrammetric reconstruction with a structure from the motion algorithm (SfM), the flight was planned with more than 80% of sidelap and overlap. Overall, 2.35 ha of terrain were surveyed and 347 high resolution RGB images were captured. A fundamental step of the work was the positioning of a large number of ground control points (GCPs) to achieve high accuracy in the georeferencing photogrammetric model. Seventeen GCPs were set and surveyed with a GNSS Stonex S9 III NRTK receiver connected to a Hexagon Smartnet reference stations network for real time GNSS correction. The accuracy of the root mean square error (RMSE XYZ) achieved on the GCPs was about 3 cm. The coordinate system used for georeferencing the data was ETRS89/UTM zone 33 Nord (EPSG:25833) and for the conversion of ellipsoidal heights into orthometric ones, referring to the IGM42 datum, the special GK2 IGMI grids were used. A second photogrammetric survey was carried out after archaeological excavation (with the same setting described) so that all the available data (i.e., photogrammetric, geophysical, and archaeological) could be integrated in a GIS environment, thus providing an easy tool for the archaeologists and end-users to visualize and integrate all the information; this is discussed further later in the paper. The resulting 3-D GPR data volume formed by more than 300 swaths had an extension of about 160 \(\times\) 90 m. The dataset was not regular due to the already excavated areas with outcropping walls and pavements and isolated trees, both making data acquisition therein logistically impossible (Figure 1). At first, we applied a standard processing flow including drift removal, background removal, bandpass filtering (corner frequencies were 50-150-600-800 MHz, with a typical non-symmetrical trapezoidal shape to limit the Gibbs phenomena), spherical divergence correction, amplitude recovery, migration (Stolt), and depth conversion using rSlicer software (DECO Geophysical) specifically developed for volume data processing and analysis. In-house algorithms implemented in Matlab were applied to solve specific data issues related in particular to the time misalignment between adjacent traces. Swaths were then interpolated within rectangular grids (chunks), which were manually selected by the operator. A crucial point for any GPR survey and in particular for the ones focused on archaeological targets is the EM velocity field reconstruction. In fact, knowledge of the EM velocity is essential for depth conversion, but even more important for imaging (migration) algorithms and recovery of physical parameters that are strictly linked to the subsurface material's characteristics [38]. The EM velocity can be estimated with different techniques, allowing for various accuracy levels.