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.