## 1 Introduction  
In recent years, the use of non-invasive geophysical and geomatic techniques has assumed an increasingly important role in the field of cultural heritage, especially by supporting conservation and restoration projects. Geophysical methods are very useful for assessing the presence of underground structures in preventive archeology at different scales [1, 2, 3, 4, 5, 6]; addressing conservation and stability issues of architectural monuments by inspecting soil foundations; assessing the mechanical properties of structural elements [7, 8, 9, 10, 11, 12, 13]; and exploring internal and superficial structures of precious and delicate targets, such as statues, wall paintings and mosaics [7, 14, 15, 16, 17, 18]. Among these techniques, ground-penetrating radar (GPR) is widely used thanks to the miniaturization of the instrumentation, the high investigation resolution and the minimal impact on the analyzed surfaces. For example, [PERSON] et al. [7] presented GPR prospecting on three different constructive elements that are typical of historical buildings (a wall, a masonry pillar and a marble column), allowing for the characterization of the masonry, the detection of cracks and the imaging of metallic reinforcement bars. The obtained information was relevant for providing better knowledge of the history of the monuments and their current internal state to be evaluated for in any possible restoration intervention. [PERSON] et al. [14] used GPR combined with seismic transmission tomography to provide important results in an investigation of columns and walls of a 14 th-century UNESCO monument by giving information on the quality and spatial distribution of the materials used in the construction of the monument. [PERSON] et al. [15] applied different GPR systems with frequencies of 1600 MHz, 500 MHz and 250 MHz to study the mosaics of Delos island, which holds a significant body of ancient Greek art of the Hellenstic period that is on UNESCO's World Heritage List. The 1600 MHz system allowed for identifying the boundaries of the mosaic layers, as well as problematic areas, such as bulges and high levels of moisture that may cause deterioration. For the same topics, geophysical methods can also be applied jointly with geomatic techniques that, thanks to the advances in data collection, processing and visualization, represent an important source of knowledge regarding diagnostics and documentation [19; 20; 21; 22; 23; 24; 25; 26]. [PERSON] et al. [20] conducted a 3D metric survey through photogrammetry and ground-penetrating radar (GPR) tests applied to the study of the trapezophoros with two griffins attacking a doe of [PERSON], which is a masterpiece of ancient art that needs to be protected. The work provided information on both visible and hidden defects, such as numerous cracks that affect the sculpture. [PERSON] et al. [21] showed the usefulness of a multidisciplinary approach to heritage documentation involving close-range photogrammetry and ground-penetrating radar techniques, as well as the development of finite-element-based structural models. The study was focused on the documentation of a medieval bridge concerning the geometric shape, the building material, and the current damage and its causes. [PERSON] et al. [25] showed a spatial-analysis-based protocol for the interpretation of data coming from different non-invasive tests to improve the extraction process of the pattern's decay. The case study was a frescoed wall of a Gymnasium in Pompeii, which was investigated with the following non-invasive techniques: structure-from-motion photogrammetry (SfM), ground-penetrating radar and multitemporal infrared thermography. This approach enabled the extraction of decay patterns to construct a 3D model that constituted the deformation map of the painting analysis methodology and to establish the first step for the restoration of an important multilevel characterization of a fresco that is useful for the protection and mitigation of its deterioration risk.