# Optimised Extraction of Archaeological Features from Full 3-D GPR Data
## 1 Introduction  
Ground penetrating radar (GPR) is probably the most popular geophysical technique for archaeological exploration [1] mainly due to its resolution, which is higher than that of the other available techniques, and its high versatility [2]. There are several other available geophysical techniques used for archaeology, in particular electrical resistivity tomography (ERT) [3], magnetometry [4], infrared thermography [5], and multi-spectral surveys [6], but GPR has demonstrated its wide versatility since it has been applied at different steps of archaeological prospecting and at various scale levels, assuring resolution higher than that of the other methods. In fact, there are examples of surveys on previously unexplored wide areas [7] and detailed studies of already excavated or exposed single structures and features (e.g., [8, 9]). Some decades ago, GPR surveys encompassed just a few single separated common offset (CO) profiles [10], later, GPR was improved with mono- or bi-directional grids of CO profiles. In the latter acquisition scheme, there is a higher spatial resolution and some data redundancy, in turn allowing in some cases for the gathering of information even when complex and superimposed structures are present. While single profiles (i.e., B-scans) only allow for the interpretation and correlation of structures in 1-D (i.e., in the profile direction or \"along track\"), acquisitions along regular (or irregular) grids make the combining of collected profiles (crosslines) possible, producing pseudo 3-D data volumes (i.e., C-scans) that can be sliced not only along the profiles' directions buttypically at constant times or depths (time- and depth-slices, respectively) [11]. Time- or depth-slices represent a rapid and efficient way to provide planar comprehensive views of the anomalies' patterns. They are essential, especially for the analysis of large areas, but they have intrinsic constraints in the case of great subsurface complexity such as when there are different superimposed structures or non-regular features. This is often the case for archaeological surveys due to the presence of collapses, refilling materials, restorations, and so on. When grids are established, typically with a crossline spacing equal to 100 or 50 cm [12], the spatial data sampling is about one order of magnitude higher along-track than across-track, with a trace distance typically in the range of 2-10 cm. As a consequence, data interpolation is neither uniform nor trivial, making it sometimes difficult to identify and accurately locate structures and anomalies. Such a problem was analysed in detail by the authors of [13] who remarked on the importance of accurately recording not only reflections but even diffractions, the latter being very common in some archaeological surveys. Therefore, to obtain spatially analiased data, quarter-wavelength spatial sampling is considered a minimum requirement for full-resolution GPR recording both inline and crossline [14; 15]. For archaeological surveys, a very dense spatial sampling is almost impossible to obtain with any single-channel bistatic GPR equipment due to the following: the logistical complexity when trying to collect regularly spaced close profiles; the very long acquisition time; and the physical dimensions of the antennas, which are typically several decimetres long and wide. Therefore, even though it is possible to combine a series of parallel or intersecting 2-D profiles to create a 3-D volume, it is not a real \"full resolution\" volume. As a matter of fact, present standards of 3-D GPR in archaeological prospection are therefore based on \"pseudo 3-D\" volumes obtained by combining a series of 2-D profiles [16; 17], so the expression \"2.5-D\" seems to be more appropriate than the \"3-D\" for that specific acquisition and processing approach [18]. In order to improve the performance of GPR, different acquisition strategies have been proposed and tested for archaeological applications.