Ground-penetrating radar for observing tree trunks and other cylindrical objects

• Analysis of radar wave propagation in tree trunk.
• Numerical, laboratory and real tree radar data.
• Significant effects of the total internal reflection.
• Visualisation of the electric field distribution in the trunk using Finite-Difference Time-Domain (gprMax2D).

To improve forest management and to prevent collapses of trees, it is necessary to investigate the internal part of tree trunks. In order to do it non-invasively, ground-penetrating radar (GPR) appears as a promising inspection device. The objective of this paper is to investigate particularities of tree trunks radar images, considering the circumferential data acquisition geometry, as a function of the radar configuration and trunk section structures. In order to better understand this kind of data, a target reflection curve was analytically described, then, the total internal reflection (TIR) phenomenon was explained and illustrated. Subsequently, classical radar measurements were compared with an application of differently shaped (planar and circular) metal shields acting as perfect electrical conductors (PEC). For comparing the methods, three experiments were performed: (1) numerical simulations using the software gprMax2D, based on Finite-Difference Time-Domain method, (2) GPR investigation of a laboratory model of a tree trunk, (3) real tree trunk measurements. The use of a planar or circular PEC increased the visibility of the medium edges, so, these GPR images were considered of a better quality. Internal object reflection curve and TIR detection were essential for general description of a GPR image. All experiments showed satisfactorily the internal inhomogeneity and the information will be useful for future tomographic reconstruction.