**2. Monitoring micro-discontinuities in monumental statues**

coating or vapour barriers, and compatibility with substrate. However, cultural heritage requires maintenance not only for its walls, but also its infrastructures and security of foun‐ dations. Therefore, safety and maintenance management of such sites must include imaging of infrastructures and potential discontinuities. The same importance should be given to archaeological sites, especially in urban areas. Therefore, there is also a need for improved non-invasive methods of visualization in evaluating the progress of the buried infrastructures

108 Imaging and Radioanalytical Techniques in Interdisciplinary Research - Fundamentals and Cutting Edge Applications

Ground penetrating radar (GPR), which is also called surface penetrating radar, is a timedependent, high frequency electromagnetic geophysical technique that can provide a 3D pseudo image of the subsurface, including the fourth dimension of colour, and can also provide accurate depth estimates for many common subsurface objects [1, 2]. GPR uses the scattered wave field of high frequency electromagnetic (EM) waves. The EM waves travel at a specific velocity that is determined primarily by the permittivity of the material. The principles of GPR have been explained extensively in the literature [3], especially for fault and fracture imaging [4-9]; in assisting contaminated sites by locating buried features of interest such as under‐ ground storage tanks, pipes [10-11], unexploded ordnance (UXO) and clutter [12-14]; and in

Ground penetrating radar (GPR) provides more detailed results than other geophysical methods, because it can image the position and the depth of targets within very complex and restricted areas. The method is non-destructive and can be applied on a surface, a wall, or a monument [21-23]. The method can also be used in urban areas or in archaeological structures and, depending on the antennas and the particular situation, can achieve a resolution of the order of several centimetres 24-26]. Therefore, it has been the most commonly used method for defining cultural heritage and buried remains at archaeological sites. Furthermore, detailed imaging has become an important area of interest [1, 24- 28]. Generally, parallel 2D profile data are acquired in the archaeological site. 3D data imaging, obtained by aligning parallel 2D profile data sets, is used to identify temporal changes at a constant depth. The locations and the depth of the remains in the study area can be determined via slices of the 3D data volume. Therefore, the GPR method gives more precise results than other geophysical methods. However, the obtained results and their interpretation can be further improved when the data set is visualized as a volumetric rendering of the remains. This method allows anyone to

The aim of this chapter is to show transparent 3D GPR data visualization with the most suitable viewing angle into this 3D data volume including buried objects, which is called a transpar‐ ent 3D half bird's-eye view of the GPR data volume or its sub-volumes. Therefore, first, we introduce the study areas and data acquisition,followed by general data processing steps ofthe gathered 2D GPR profile data set. Third, we show a revised colour range of the amplitude scale, representing the fourth dimension of the hybrid 2D/3D visualization. Fourth, we attempted to realize a new amplitude–colour-balancing approximation, according to the travel time range ordepthrange,asanalternativeapproximationofgaininordertopreventexaggeratedamplitude gain,whichaffects transparencyandobscuresburiedinfrastructures.Fifth,we appointedanew opaque function, which must support amplitude–colour scale in order to supply transparen‐

the mapping of shallow stratigraphy and discontinuities [3, 15-20].

imagine how an area looked by looking into the 3D image.

of archaeological heritage.
