GPR prospecting of cylindrical structures in cultural heritage applications: a review of geometric issues

Cultural heritage diagnostics often involves the application of geophysical methods like groundpenetrating radar (GPR) along non-planar surfaces. Standard processing algorithms are based on the hypothesis of half-space geometry. In the investigation of cylindrical structures like columns with a small radius, this basic assumption is strongly violated and the application of conventional algorithms may be inappropriate. Although frequently overlooked, this fact implies that appropriate algorithms should be developed to take into account the additional data complexity arising from the unusual acquisition setting and exceptional care should be used in interpreting the radargrams. The first part of this paper offers a systematic review of problems related to geometric issues (air waves, multiple reflections, shape of the various traveltime curves) performing a theoretical analysis of major differences between cylindrical and half-space models. The second part shows, through numerical simulations and real data examples, the inappropriateness of the half-space model in the processing and interpretation of GPR surveys along cylindrical structures. The numerical simulations help visualizing the differences in reflection/diffraction features from buried and above-surface objects and errors arising from the use of inappropriate models. The real data examples offer a tangible demonstration of the benefits obtained from exploiting the correct cylindrical model and the additional information gained from later arrivals, such as multiple reflections, in constraining the estimation of electrical parameters (propagation velocity) related to constitutive and health states of the structure.