# Optimised Extraction of Archaeological Features from Full 3-D GPR Data
## 1 Introduction
In particular, multi-offset (MO) or multi-fold applications offer some relevant advantages. The difference in the MO method is that each subsurface point is imaged by multiple wavefronts, while in the standard CO only a single wavefront is exploited. MO techniques have been the standard for reflection seismic data acquisition and processing for many years [19], but they are seldom applied on GPR data due to their long acquisition time and the differences between seismic and electromagnetic (EM) waves. A comprehensive review of the MO method can be found, for instance, in [20]. In any case, besides the longer acquisition and processing time required, there are interesting examples of MO GPR surveys in different archaeological environments that demonstrate their improved performance, especially when irregular and superimposed targets are present and when the velocity field has strong vertical and lateral variations [21; 22]. In recent years an extraordinary advancement in the GPR field took place thanks to the implementation of new multi-channel (also known as multi-array) systems that became available on the market. The first and obvious step ahead is related to determining the productivity rate because this new equipment can \"simultaneously\" collect a series of parallel CO profiles. Another, even more important improvement is that the full resolution can be easily obtained; in other words, equal (or similar) sampling rates can be fixed for both inline and crossline distances, thus obtaining true 3-D full resolution data volumes. Most of the equipment can be moved by cars or all-terrain vehicles (when logistical conditions are appropriate), further improving the overall productivity rate. Besides engineering applications (e.g., [23; 24]), archaeological and cultural heritage studies have become another typical application because requests to investigate large unexplored or only partially investigated areas to accurately estimate their archaeological potential are common. Examples of applications in various conditions and with slightly different objectives are reported in [25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35]. The obtained results are generally of high quality and resolution, but often they are not homogeneous within the investigated areas. In fact, the applied processing and especially some parameters used in the algorithms are neither easy to set nor constant. One of the most important parameters is the EM velocity, which can vary due to the different buildings and surrounding materials but also due to different water content. In this paper we use a full 3-D GPR survey collected in the Aquileia archaeological park (Northeast Italy), because it is exemplary for several problems, limitations, and constraints often occurring in the interpretation of large GPR archaeological surveys. Our specific aims were:  
1. Achieve optimized information that is directly useful for archaeologists for planning excavation, often in areas where irregular and already excavated areas are present. 2. Image complex structures in which many superimposed and irregular structures are present. 3. Obtain full georeferenced results that are easy to understand and manage even by end-users and those who are not trained in geophysics. 4. Validate geophysical results with excavations and evaluate the intrinsic resolution limits of GPR by comparing its results with excavations. In order to achieve these objectives, we focused on data processing, on improved data analysis by means of GPR attributes, and on automated picking/interpretation strategies to limit the subjectivity of the interpretation process while minimizing its temporal length. In the next sections we describe some characteristics of the site used as the test site, then we detail the methodologies we exploited, and finally we address some methodological issues and discuss the obtained results in a wider and more general context.