Employing ADI-FDTD subgrids for GPR numerical modelling and their application to study ring separation in brick masonry arch bridges

In realistic numerical modelling of ground-penetrating radar (GPR) and when parts of the computational domain need to be modelled in detail, the implementation of subgrids into the conventional finite-difference time-domain (FDTD) mesh could greatly economize on computational resources. A novel alternating-direction implicit FDTD subgridding scheme is used to numerically simulate the GPR responses from delaminations located in brick masonry arches. The heterogeneity of these structures renders electromagnetic signals, which originate from the interaction between the GPR system and the bridge, often complex and hence hard to interpret. Therefore, GPR numerical models were created in order to study the attributes of reflected signals from various targets within the structure of the bridge. Results from a range of modelling scenarios are presented. The effect of varying the thickness of faults, their location in brickwork, as well as the effect of water ingress in hairline delaminations on GPR signals, are examined. Moreover, GPR vertical resolution and the presence of lossy brickwork are studied through various numerical models.