FDTD modeling of borehole georadar data

High-frequency electromagnetic wave phenomena associated with borehole georadar experiments are complex. To improve our understanding of the governing physical processes, we have developed an advanced finite-difference time-domain (FDTD) solution of Maxwell¿s equations in cylindrical coordinates. The computational domain is bounded by cylindrical symmetry conditions along the left model edge and by suitably adapted uniaxial perfectly matched layer (UPML) absorbing boundary conditions along the top, bottom and right model edges. An important feature of this algorithm is the use of a powerful grid-refinement technique that enables us to account for detailed design aspects of borehole georadar antenna systems. This type of modeling allows us to estimate the radiative properties of typical borehole georadar antennas under realistic operating conditions, which represents the basis for improving conventional ray-based amplitude inversions as well as for the eventual development of waveform inversion algorithms. The algorithm is validated with respect to analytical solutions for wire-type dipole antennas and then applied to model crosshole georadar data acquired under well-controlled conditions in the NAGRA (Swiss Cooperative for the Storage of Nuclear Waste) Grimsel rock laboratory in the central Swiss Alps.