Analyses of traveltimes and amplitudes of crosshole georadar data
provide estimates of the electromagnetic velocity and attenuation
structures of the probed media. Ray-based tomographic inversion of
georadar traveltimes is identical to the well-established crosshole
seismic approach and considered to be rather robust. In contrast,
ray-based inversions of crosshole gaeoradar amplitudes depend critically
on the assumption that the radiative properties of the transmitter
and receiver antennas correspond to those prevailing in a homogeneous
full-space. The validity of this a priori assumption as well as the
potential implications of its violation are unknown. In particular,
it is not known to what extent the radiation patterns are affected
by coupling of dipole-type georadar antennas to local variations
of the electric material properties. To explore this problem, we,generate
synthetic crosshole georadar data for a suite of stochastic models
using a finite-difference time domain (FDTD) solution of Maxwell's
equations in cylindrical coordinates. Analyses of radiation patterns
extracted from the synthetic data indicate that radiation pattern
distortions are primarily due to scattering and attenuation effects
along the wave propagation path and not to changes in antenna coupling
to local variations in the electrical material properties. In principle,
such "path effects" are the target of tomographic amplitude inversions.
This interpretation is supported by the fact that increasing heterogeneity
enhances the amplitudes of "artefacts" in ray-based tomographic reconstructions
of the crosshole georadar amplitude data, but does not systematically
degrade the quality of the tomograms. Ray-based amplitude tomography
is strictly valid only for homogeneous media and cannot account for
scattering effects. As a consequence, artefacts related to scattering
effects show a rather vague spatial relation to the corresponding
structural features and their magnitude scales with increasing standard
deviation of the heterogeneity. This suggests that ray-based amplitude
tomography is quite useful for detecting prominent conductivity anomalies,
whereas the structural and petrophysical details of such tomographic
reconstructions should be regarded a, rather qualitative in nature.